CryoSat-2 (Earth Explorer Opportunity Mission-2)
CryoSat-2 is the follow-on Earth Explorer Opportunity Mission in ESA's Living Planet Program. It replaces CryoSat, which was selected for development in 1999 and lost as a result of launch failure on October 8, 2005. CryoSat-2 will have the same mission objectives as the original CryoSat mission; it will monitor the thickness of land ice and sea ice and help explain the connection between the melting of the polar ice and the rise in sea levels and how this is contributing to climate change.
The original CryoSat mission was proposed by Duncan Wingham of the University College London (UCL) and an international science team. Duncan Wingham is also the mission PI. A nominal mission duration of three years is planned (excluding the commissioning and validation phases, which may last up to six months).
In Feb. 2006, ESA received the green light from its Member States to build and launch a CryoSat recovery mission, CryoSat-2, based on the same objectives as the original CryoSat mission. However, the design of the CryoSat-2 spacecrasft is being updated. The changes required to the design of CryoSat-2 were scrutinized from December 2006 to January 2007. The Δ-CDR (Critical Design Review) was completed on Feb. 1, 2007. 1) 2) 3) 4) 5)
A total of 85 improvements/modifications have been approved in the design of CryoSat 2 (of which 30-40% have been small software changes that make the satellite much easier to operate). The new key features of CryoSat-2 include the following items:
• The SIRAL-2 (SAR/Interferometric Radar Altimeter-2) design includes a full backup SIRAL system, in case the primary payload malfunctions. Once in orbit, a special algorithm will be used to convert data collected by the CryoSat-2 satellite to create more accurate ice maps. - As a result of the dual SIRAL payload and associated interfaces, and other improvements to reliability, there has been a knock-on effect to the design of the satellite. For example, the backup SIRAL system has to be kept warm while it is switched off - the additional heater power is provided by increasing the size of the satellite's battery.
• A heat-radiating panel is being added. The path of CryoSat-2's orbit means it will face extremes of temperature. The panel ensures the electronics are protected
• Solar panels on the satellite's back are being added to account for the additional power requirements. Unlike many spacecraft, CryoSat 2 does not have solar wings.
In addition to building the new satellite, a number of field experiments to support the CryoSat-2 mission, were conducted or are getting underway in the Arctic. First is the Arctic Arc Expedition, part of the IPY (International Polar Year) 2007-2008. 6) 7)
- Antarctic 2008/9 CroVEx campaign in the blue ice region (see Figure 1) in December 2008: German scientists from the Technical University of Dresden and the AWI (Alfred Wegener Institute) are spending up to four months venturing out onto the vast frozen reaches of what is known as the 'blue ice' region near the Russian Novo airbase in Dronning Maud Land in Antarctica. The aim is to take very accurate measurements of the surface topography, both from the air and on the ground to contribute to the validation program for CryoSat-2. 8)
In parallel to the efforts on the ground, the Alfred Wegner Institute (AWI) will be flying their POLAR5 aircraft across the blue ice site – starting just before Christmas and finishing before the New Year. From the plane, the AWI team will collect laser and radar height measurements along the very same tracks as the ground team. To do this they are using ESA's ASIRAS (Airborne Synthetic Aperture and Interferometric Radar Altimeter System), which simulates the measurements CryoSat.
- CryoVEx (CryoSat Validation Experiment) 2008 (3-week campaign in May 2008 in the far north of Greenland and Canada). CryoVEx 2008 is a continuation of a number of earlier campaigns that focus on collecting data on the properties snow and ice over land and sea. This year's campaign is a huge logistical undertaking as airborne, helicopter and ground measurements are being taken simultaneously in three different locations - out on the floating sea-ice north of the Canadian Forces Station Alert, on the Devon ice cap in Canada and on the vast Greenland ice cap. A Twin Otter is carrying two key instruments: ASIRAS (Airborne SAR/Interferometric Radar System), a radar altimeter that mimics the radar altimeter onboard CryoSat-2, and a laser scanner which maps the surface beneath the plan, and a helicopter with an on-board sensor that measures sea-ice thickness. 9)
- In the spring of 2007, an international team of scientists stationed in Svalbard, Norway and two polar explorers are crossing the North Pole on foot. Both teams are part of a common effort to collect vital data on the ground and from the air in support of ESA's ice mission CryoSat-2. The expedition's two Belgian explorers, Alain Hubert and Dixie Dansercoer, 'stepped' onto the sea ice off the coast of Siberia on March 1, 2007 each pulling a 130 kg sledge holding supplies and equipment. A parallel campaign by scientists from Germany, Norway and the UK is unfolding in the extreme northern archipelago of Svalbard, Norway. As part of the CryoVEx 2007 campaign (CryoSat-2 Validation Experiment), they are spending one month making measurements of snow and ice properties along long transects that crisscross the ice sheet surface.
- As the ground experiments are carried out, measurements are also being taken from the air by the Alfred Wegner Institute (AWI), Bremerhafen, Germany. The Dornier-228 aircraft carries the ASIRAS (Airborne SAR/Interferometric Altimeter System) instrument, which is an airborne version of the radar altimeter instrument onboard CryoSat-2. By comparing the airborne data with ground measurements scientists will test and verify novel methods for retrieving ice-thickness change from the CryoSat-2 satellite mission ahead of the launch. - ASIRAS was built by Radar Systemtechnik (RST) of Switzerland with the support of the AWI and Optimare for the implementation and operation on an aircraft. It was test flown in March 2004 over the snow and ice expanses of Svalbard.
- The CryoVEx 2006 campaign took place April/May, 2006 and consisted of coordinated airborne and ground activities in support of CryoSat-2 validation goals over three land validation sites (Devon Island in Canada, Central Greenland and Svalbard, Spitzbergen, Norway) and a series of ice experiments over Alert / Ellesmere Island, Canada.
- LaRA (Laser and Radar Altimeter) campaign in the Arctic region of Greenland and Svalbard: The D2P (Delay-Doppler Phase-monopulse Radar) instrument of JHU/APL participated in this campaign which took place in May 2002 under joint NASA/ESA sponsorship to support calibration and validation activities, and science investigations in advance of the CryoSat and ICESat missions. The D2P radar altimeter was flown aboard the NASA-P3 aircraft along with the ATM (Airborne Topographic Mapper) laser altimeters to collect simultaneous laser and radar altimeter (hence, the LaRA campaign) measurements over land and sea ice.
- CryoVEx (CryoSat Validation Experiment) campaign: As a follow-on to the LaRA campaign, the D2P system was flown again in 2003 under joint NASA/ESA sponsorship as part of the CryoVEx field campaign. As in 2002, simultaneous laser and radar altimeter measurements were collected in the Arctic.
Such painstaking ground work is necessary to be able of measuring ice thickness down to centimeter level (1-3 cm average) from space. This in turn may lead to a better understanding of the impact that changing climate is having on the polar ice fields. See also the D2P and ASIRAS descriptions on the eoPortal (along with the campaigns for the validation of the SIRAL instrument).
The CryoSat-2 spacecraft is being built and integrated by EADS Astrium GmbH of Friedrichshafen, Germany, as prime contractor of a consortium. The spacecraft structure consists of a long rectangular main platform body, surmounted by fixed solar arrays in the form of a tent. The spacecraft has neither deployable appendages nor any other moving parts except for thruster valves. The lower surface of this structure is permanently earth facing. All electronics are mounted on the nadir plate acting as radiator. The antennas used for radio communication, and the Laser Retroreflector, are mounted on this surface; an emergency antenna for command and monitoring is also fitted on top of the satellite between the solar arrays. The two SIRAL instrument antenna dishes are mounted on a separate rigid bench in the forward section of the S/C. In addition, a dedicated SIRAL radiator is mounted at the nose tip. 10)
Figure 2: Illustration of the CryoSat-2 spacecraft with thermal covers on the SIRAL antennas (image credit: ESA)
Figure 3: Alternate view of the CryoSat-2 spacecraft (image credit: EADS Astrium)
The spacecraft is 3-axis stabilized. A slight nose-down attitude of the S/C (6º) is chosen (using magnetorquers and 10 mN cold-gas thrusters) to ensure minimum attitude correction due to gravity-gradient disturbances. The S/C has dimensions of 4.6 m x 2.34 m x 2.20 m. The S/C mass is about 720 kg (including 36 kg propellant), the design life is 3.5 years (goal of 5.5 years). Spacecraft power generation is provided by two triple-junction GaAs solar arrays with an efficiency of 27.5% (two oriented solar panels), each panel provides power of 850 W at normal solar incidence. A PCDU (Power Control and Distribution Unit) provides onboard distribution. The energy is stored by a lithium-ion battery (60 Ah capacity).
The pointing requirements are the main design drivers for the AOCS: 11)
• High precision cross-track pointing knowledge of < 10 arcsec for SARIn mode (SARIn refers to the SAR interferometric mode).
• S/C attitude maintenance with a pointing accuracy of < 0.2º per axis and a pointing stability of < 0.005º for 0.5 s in the nominal Earth-pointing phase of the mission
• Provision of very low disturbances due to thruster activity to meet the very high precise orbit determination (POD) accuracy of the CryoSat orbit.
• The AOCS (Attitude Orbit and Control Subsystem) comprises the following elements:
- A cold gas system (RCS) for attitude control and orbit transfer and maintenance maneuvers, 16 attitude control thrusters (10 mN) and 4 orbit control thrusters (40 mN). Nitrogen is used as propellant (132 l tank). 12)
- A set of 3 magnetorquers is used for compensation of environmental disturbance torques in support of RCS. The MT30-2-GRC, originally developed and qualified for the GRACE mission by ZARM Technik GmbH, has been selected for CryoSat.
- A set of three star tracker heads (also a part of the payload) providing autonomous inertial attitude determination for the spacecraft. The multiple configuration makes the sensor system one-failure tolerant, except for the rare occurrence of simultaneous sun and moon blinding of two heads, to which the system software is tolerant. Consequently, two camera heads are operated in parallel at all times to cope with sun-blinding. In its acquisition mode, which takes 2-3 seconds, the star tracker calculates a coarse attitude by matching triangle patterns of stars with patterns stored in its catalog. Subsequently, in attitude update mode it calculates precise attitude at a rate of 1.7 Hz.
The star tracker attitude serves also as reference for determining the orientation of the SIRAL interferometric baseline. The orientation of the interferometric baseline needs to be very accurately measured in-flight: small errors in knowledge of the roll-angle translate into substantial errors in the elevation of off-nadir points. The HE-5AS star tracker of Terma A/S, originally developed and qualified for the NEMO (Navy EarthMap Observer) and FCT (Foreign Comparative Test) projects, is selected for CryoSat. It is a fully autonomous star tracker capable of delivering high-accuracy inertial attitude measurements from a lost-in-space condition with no external attitude information. The EOL performance of the star tracker is < 3.2 arcsec in the lateral axes and < 16 arcsec about the roll axis under worst-case conditions. 13)
The star tracker baffle has been designed to meet the required sun exclusion angle of 30º and the moon exclusion angle of 25º. These exclusion angles ensure together with the star tracker accommodation on the antenna bench that during the whole mission sun and moon can only blind one star tracker head at any time.
Figure 4: Photo of one star tracker camera head unit (image credit: ESA)
• A DORIS receiver is part of an overall system, for real-time measurements of satellite position, velocity and time. DORIS measures the Doppler frequency shifts of UHF and S-band signals transmitted by ground beacons. Its measurement accuracy is < 0.5 mm/s in radial velocity allowing an absolute determination of the orbit position with an accuracy of 2-6 cm (see DORIS and LLR description under sensor complement). - The DORIS system comprises a network of more than 50 ground beacons, a number of receivers on several satellites in orbit and in development, and ground segment facilities. It is part of the International DORIS Service, the IDS, which also offers the possibility of precise localization of user beacons.
• CESS (Coarse Earth-Sun Sensor) of CHAMP and GRACE heritage (a patented design of Astrium GmbH). Provides attitude measurements (<5º) with respect to the sun and Earth for initial acquisition and coarse pointing. The FOV of CESS is a full spherical one, i.e. no special search maneuvers are necessary to find the Earth or the sun. Its measurement principle is based The concept is based on temperature differences measured by 6 omnidirectional arranged sensor heads (PT1000 thermistors).
• A set of three three-axis fluxgate magnetometers are used for magnetorquer control and as rate sensors. They provide a measurement range of at least ± 60.000 nT with an accuracy of better than 0.5 % full scale.
The AOCS provides high pointing accuracy (a few tens of an arcsecond), knowledge and stability in nominal Earth-pointing. It also has to perform the orbit changes between the science and validation phase orbits. The AOCS uses inertial attitude measurements from the set of 3 star tracker camera head units and DORIS real-time navigation to convert the inertial attitude into Earth referenced attitude (star sensor FOV of 22º x 22º, ). - The RCS (Reaction Control Subsystem), developed at PolyFlex Space Ltd. (a Marotta UK Ltd. company), is a cold gas propulsion system for auxiliary attitude control (in which it provides deadband protection around the axis defined by the instantaneous geomagnetic field) and for orbit transfer and maintenance maneuvers. It has 16 attitude control thrusters of 10 mN each and 4 orbit control thrusters of nominally 40 mN each. A single high-pressure tank stores 36.2 kg of nitrogen gas at 278.6 bar. 14) 15)
The CDMU (Control and Data Management Unit), consisting of a processor and a hardware-based fault detection system, handles all on-board command and control functions including telecommand decoding and the AOCS processing functions (the OBC is based on the ERC-32 microprocessor). A MIL-STD-1553B communications bus is used as payload interface (for SIRAL and DORIS). The on-board solid-state memory has a capacity of 2 x 128 Gbit.
Figure 5: Block diagram of the major elements od the CryoSat-2 spacecraft (image credit: ESA)
• Experimental Rate Sensor: CryoSat-2 carries a small technology experiment as a passenger. This device is an attitude rate sensor based on MEMS (Micro-Electro-Mechanical-Systems) technology in which microelectronics and mechanical devices (in this case a sensor) are fabricated on the same substrate. The MEMS sensor detects attitude rate to provide the same function as a more traditional gyro and is based on a device widely used in in-car navigation systems. Three orthogonal MEMS sensors are mounted in the experiment, to measure 3-axis attitude rates. The unit is called MRS (MEMS Rate Sensor) in the CryoSat context. The goal is to provide a low-cost rate-sensor or gyro. The device is provided free of charge to CryoSat-2 in exchange for the flight opportunity (Ref. 10).
The measurement data are not used on-board and only sent in housekeeping telemetry to the flight control centre. Here they will be used as an additional data type in monitoring satellite dynamics during attitude transitions.
In the context of the technology program in which the MRS has been developed, it is called SiREUS-FExp, for European Silicon Rate Sensor Flight Experiment. - SiREUS is a compact and lightweight solid-state MEMS rate sensor which was developed in the context of an ESA technology technology program. The UK development team consisted of the following partner organizations: AIS (Atlantic Inertial Systems - formerly BAE Systems of Plymouth), SEA (Systems Engineering & Assessment Ltd. of Bristol), and SELEX-GALILEO a Finemeccanica owned company (formerly BAE Systems of Edinburgh). This development is based on the established BAE SYSTEMS automotive MEMS detector, however significant developments were required to meet the performance requirements while achieving compatibility of the electronics to the space environment and ensuring low recurring price. The partnership with a significant commercial provider such as AIS should be emphasized as a critical aspect of the program. 16) 17) 18)
Table 1: MRS (MEMS Rate Sensor) key requirements and current (2008) status
The SiREUS unit has met or exceeded the key performance requirements set at the start of the program. The unit does not contain any software; all control loops are implemented digitally in an FPGA. The SiREUS unit is fairly compact, but its size is currently dominated by analog electronics, not the MEMS. It may be cost effective to achieve a significant reduction in mass and volume, if this results in a match with many more customer requirements.
SiREUS has demonstrated that it is possible to construct multi-lateral programs to spin-in technology from non space industry organizations and to make significant improvements in the performance of the 'spin-in' technology. There are positive signs for the wider application of this technology in 'spin-off' programs. The instrument has a size of 100 mm x 100 mm x 70 mm.
Figure 6: Top view of MRS FExp front end PCBs (left) and view of the MRS Exp unit on the CryoSat-2 nadir panel (right), image credit: SEA
Table 2: Overview of spacecraft parameters
Launch: The CryoSat-2 spacecraft was launched on April 8, 2010 on a Dnepr vehicle from the Baikonur Cosmodrome, Kazakhstan.. The launch provider was ISC (International Space Company) Kosmotras. 19) 20) 21)
Note: The technical issue with the second stage of the Dnepr rocket that delayed the launch of ESA's Earth Explorer CryoSat-2 satellite in February 2010 has now been resolved – and the new launch date of 8 April has been set. The fuel reserve problem of the second stage surfaced a week before the scheduled launch date and after the 'space head module', encasing the CryoSat-2 satellite, had been mated to the rest of the rocket in the launch silo. Consequently, the space head was returned to the integration facilities pending an investigation and new launch date. 22)
The delay, from the planned launch date of Dec. 2009, is due to the limited availability of facilities at the Baikonur launch site in Kazakhstan, which is particularly busy at the moment. 23)
Satellite Orbit: Non sun-synchronous circular LEO orbit, mean altitude = 717 km, inclination = 92º, nodal regression of 0.25º per day. Ground track repeat cycle: 369 days (with 30 day pseudo subcycles). This configuration allows a sufficient coverage for the polar regions. The CryoSat mission requirements include:
• An orbit change is required during the mission with the objective to visit at least twice a validation orbit, approximately 6 km lower in altitude than the science phase orbit
• The payload must be operated in various modes, as a function of geographical region, such that the orbital operations, and data sets collected, on successive orbits are dissimilar
• The payload utilization demands very precise orbit and attitude restitution. Minimum operations of three years are required.
The CryoSat mission is aimed in part at gaining coincident coverage with the GLAS laser altimeter of the NASA ICESat mission. The following support phases are defined:
• Commissioning phase: The nominal duration is two months. During this phase the satellite and its payload are brought into a fully operating condition in its nominal orbit.
• Science phase: This includes a long-repeat cycle [a 369-day orbit (5344 revolutions) repeat phase will be used, with a 30-day subcycle]. The science phase is the nominal operational support mode of the mission. This orbit is designed to provide very dense orbit cross-overs above 72º of latitude, for use over the ice sheets. With coverage to 88º of latitude, all but a very small area of the land and marine ice fields will be within the coverage of the satellite. In addition, the 30 day subcycle provides approximately monthly coverage of the sea ice fluctuations.
• Validation phases: The objective is to conduct calibration or validation experiments that are at a fixed locations on Earth. In these phases the satellite may be placed into a 2-day repeat orbit. A validation phase may have a duration of up to 1 month, and there may be more than one during the mission lifetime. The measurements made by the satellite mission will need to be verified by ground-based experiments.
RF communications: The S-band link is used for all TT&C communications (2 kbit/s uplink and 8 kbit/s downlink). The physical downlink operates at 16 kbit/s but carries an overhead of error correction coding. The X-band downlink (center frequency of 8.100 GHz) provides a payload transfer rate of 100 Mbit/s. All onboard data are stored in the MMFU (Mass Memory and Formatting Unit) of 2 x 128 Gbit (EOL) capacity. Data arrive at the MMFU directly from the SIRAL instrument on a pair of fast IEEE 1355 standard serial links (SpaceWire for the two high-rate interferometric data channels) and via the MIL-STD-1553 bus for the low rate data channels. Data are also transferred from the CDMU and the DORIS over the MIL-STD-1553 bus. About 320 Gbit/day of onboard source data are being generated and transmitted to the ground.
Figure 7: The CryoSat-2 spacecraft and its instruments (image credit: ESA)
• December 16, 2016: Although not designed to deliver information on ice, ESA's Earth Explorer SMOS satellite can detect thin sea-ice. Since its cousin, CryoSat-2, is better at measuring thicker ice scientists have found a way of using these missions together to yield an even clearer picture of the changing Arctic. 24)
- Carrying a radiometer, SMOS was designed capture images of brightness temperature. While these images can be turned into information on soil moisture and ocean salinity to improve our understanding of the water cycle, it turns out that these data can also be used to measure sea ice.
- In contrast, CryoSat carries a radar altimeter that measures freeboard of sea ice, which is the distance between the waterline and the top of the ice.
- This is being used to work out how the thickness of sea ice is changing and, in addition, how the volume of Earth's ice is being affected by the climate.
- Despite the two missions being very different, scientists from the University of Hamburg and the AWI (Alfred Wegener Institute) in Bremerhaven, Germany, who are involved in both Earth Explorer missions, have found a way of combining data from both satellites to gain a more complete picture of changes in the thickness of ice floating in Arctic waters. — While the accuracy of measurements from CryoSat-2 increases with increasing ice thickness, SMOS data are more accurate when the sea ice is relatively thin, less than about a meter.
Figure 8: The animation shows how data from CryoSat-2 and SMOS have been combined to yield a more accurate and comprehensive view of sea-ice thickness in the Arctic (image credit: AWI)
Figure 9: Although not designed to deliver information on ice, ESA's Earth Explorer SMOS satellite can detect thin sea-ice. By combining measurements from SMOS with measurements from CryoSat-2 the two different satellites missions are yielding an even clearer picture of the changing Arctic. SMOS is also helping to improve the accuracy of sea-ice forecasts, which could help marine traffic operators determine the safest and most economic routes through waters such as the Northwest Passage and the Northern Sea Route as the ice becomes thinner owing to climate change (image credit: ESA, M. Drusch)
- CryoSat measurements yield high-spatial resolution information and cover the Arctic every month. While SMOS offers daily images, they are a much coarser resolution than those of CryoSat-2. Robert Ricker from AWI said, "By combining ice-thickness estimates from CryoSat-2 and SMOS, we obtain a more accurate and comprehensive view on the actual state of Arctic sea ice. Users need timely information across the entire Arctic and we can meet their needs by combing information from these two different, but complementary satellite missions."
- The University of Hamburg is already using SMOS to provide daily maps of Arctic sea-ice thickness during the winter. These maps are produced within 24 hours of the measurements being taken in space. SMOS is also helping to improve the accuracy of sea-ice forecasts, which could help marine traffic operators to determine the safest and most economic routes through waters such as the Northwest Passage and the Northern Sea Route as the ice becomes thinner owing to climate change.
- In addition, both missions' archived data have been merged to generate information on thin sea-ice going back to 2010.
Figure 10: Sea-ice change from SMOS: Based on measurements from the SMOS mission, the animation shows changes in sea-ice thickness during November between 2010 and 2016. Although designed to improve our understanding of Earth's water cycle, SMOS is now being used to provide accurate measurements of thin sea-ice, complementing the CryoSat mission (image credit: University of Hamburg)
- This will make an important contribution to studies into the fragile component of the Earth system and help to understand annual variations and climate change. Lars Kaleschke, from the University of Hamburg, emphasized, "It is good see how information from two different types of measurements can be combined into one product to advance science and improve operational applications. It has now been demonstrated that using ice thickness information from SMOS improves the model computations and forecasts. It will be interesting to see how ocean current and air temperature models will benefit from a better understanding of the sea-ice fields."
• November 30, 2016: ESA's CryoSat-2 satellite has found that the Arctic has one of the lowest volumes of sea ice of any November, matching record lows in 2011 and 2012. Early winter growth of ice in the Arctic has been about 10% lower than usual. - CryoSat carries a radar altimeter that can measure the surface height variation of ice in fine detail, allowing scientists to record changes in its volume with unprecedented accuracy. These observations are vital for tracking climate change and are an essential resource for maritime operators who increasingly navigate the icy waters of Earth's polar regions. 25)
Figure 11: November Arctic sea-ice thickness as observed by CryoSat. Although November 2016 saw ice thicker than usual north of Canada, there is less ice overall in southerly regions such as the Beaufort, East Siberian and Kara Seas (image credit: CPOM/ESA)
- The US NSIDC (National Snow and Ice Data Center) reported that the area of the Arctic covered by sea ice fell to 4.1 million km2 in September this year – slightly less than the sea-ice extent in September 2011. - But CryoSat-2 shows that the ice was thicker at the end of summer than in most other years, at 116 cm on average. This means there was substantially more ice this year than in 2011.
- Thicker ice can occur if melting is lower, or if snowfall or ice compaction is higher. However, the Arctic usually gains about 161 km3 of ice per day in November, but this year's growth has been about 10% lower, at 139 km3 per day, with a total ice volume estimated to have accumulated to 10,500 km3 by the end of the month.
- This would essentially tie with conditions in the Novembers of 2011, when levels were at their lowest on record for this time of the year. Although sea ice in the central Arctic is currently thicker than it was in 2011, there is far less ice in more southerly regions such as the Beaufort, East Siberian and Kara Seas.
- "Because CryoSat can measure Arctic sea ice thickness in autumn, it gives us a much clearer picture of how it has fared during summer," said Rachel Tilling, at the UK's CPOM (Center for Polar Observation and Modelling), who came to these conclusions. "Although sea ice usually grows rapidly after the minimum extent each September, this year's growth has been far slower than we'd expect – probably because this winter has been warmer than usual in the Arctic."
- As demand for information on Arctic conditions increases, CryoSat-2 has become an essential source of information for polar stakeholders, ranging from ice forecasting services to scientists studying the effects of climate change. "In its short, six years of life, we have learnt more about Arctic sea ice from CryoSat-2 than from any other satellite mission," commented CPOM Director and principal scientific advisor to the CryoSat mission, Professor Andrew Shepherd. "To understand the role that sea ice plays in the climate system, and the restrictions it places on maritime operations, we must ensure that its measurements are continued into the future."
- CPOM plans to release a complete assessment of 2016 sea ice conditions in the coming weeks.
Figure 12: 2011–16 November Arctic sea-ice volume. Early-winter Arctic sea-ice volume as observed by CryoSat-2. Sea-ice growth in November 2016 has been about 10% lower than usual, and ties with November 2011 and 2012 as a record low (image credit: CPOM/ESA)
• July 26, 2016: Trying to measure sea levels around rugged coastlines is not always an easy task. ESA's CryoSat-2 satellite is making a difference with its radar altimeter. Sea level is a very sensitive indicator of climate change, reflecting components of the climate system such as heat, glaciers and the melting of ice-sheets. Precisely monitoring changes in the average level of oceans is vitally important for understanding not only climate but also the social and economic consequences of any rise in sea level, especially in coastal zones. Previous radar altimeters have been aimed at measuring oceans and land, but CryoSat's is the first sensor of its kind designed for ice, and able to map sea levels with unprecedented accuracy. 26)
- Scientists from the Norwegian University of Life Sciences also discovered that CryoSat had the potential to map sea level closer to the coast. Using satellite altimeters in coastal zones is notoriously difficult. Norway boasts the world's second longest coastline of some 100,000 km, comprising many islands, steep mountains and deep, narrow fjords. The rugged coastline means that other altimeters produce confused readings close to the coast, showing differences of 10 cm or more. By contrast, CryoSat's results compare favorably with the Stavanger tide gauge in southwestern Norway, provided by the Norwegian Mapping Authority.
- While classical altimetry offers a few tens of observations over a five-year period, more or less near a tide gauge, some 7000 measurements very close to the gauge are obtainable with CryoSat-2. The result is a better affinity with the Stavanger data, to within 7 cm for CryoSat-2, contrasting with the 10–15 cm for classical altimetry. This demonstrates the superior accuracy of CryoSat-2 in the coastal zone, where the regional impact of sea level rise is more important for humans.
- "Conventional altimeters on satellites like Envisat and Jason-3 typically have 10–30 times larger footprint than the new altimeters on CryoSat and Sentinel-3," comments Ole Baltazar Andersen, senior scientist at the National Space Institute and DTU Space of Denmark. "Hence, the radar pulse used to measure the sea-surface height is more frequently disturbed in the coastal zone. Therefore, you have to go further from the coast to obtain accurate observations with conventional altimeters. "Consequently, you are not measuring the sea-surface height right at the coast, as we now can with CryoSat and Sentinel-3." CryoSat-2 also gives favorable results along the remaining Norwegian coast. In comparison with the Kabelvåg tide gauge in the Lofoten area in northern Norway, differences as low as 5.4 cm were obtained. There are now great expectations from the more recent Sentinel-3 mission, which carries a similar altimeter.
• July 21, 2016: After six years in orbit, the status of the satellite remains very good. Funds for operating the mission have been approved until the end of February 2017 and a further extension is foreseen into the next phase of the EOEP-5 (Earth Observation Envelope Program). With the exception of the power subsystem, which switched to its redundant side in October 2013, all other subsystems on the satellite remain in their prime. The onboard consumables are sufficient to operate the satellite until 2025. Recently, the onboard startracker software has been upgraded to improve robustness and performance ready for the next mission extension phase. 27)
- Since launch, the CryoSat ground segment has evolved significantly and has included new products in response to requirements coming from various scientific communities, mainly from areas such as oceanography, marine gravity and hydrology. Two major versions of CryoSat product portfolio have been released to users since launch and a new one is expected by the end of 2017. - The number of CryoSat users has more than tripled during its first six years in orbit, confirming the high level of interest of the worldwide scientific community in this mission.
• April 21, 2016: Sea ice physicists from the AWI (Alfred Wegener Institute) / Helmholtz Center for Polar and Marine Research (Bremerhaven, Germany), are anticipating that the sea ice cover in the Arctic Ocean this summer may shrink to the record low of 2012. The scientists made this projection after evaluating current satellite data about the thickness of the ice cover. The data show that the arctic sea ice was already extraordinarily thin in the summer of 2015. Comparably little new ice formed during the past winter. Today Marcel Nicolaus, expert on sea ice, has presented these findings at a press conference during the annual General Assembly of the European Geosciences Union in Vienna. 28)
- Predicting the summer extent of the arctic sea ice several months in advance is one of the great challenges facing contemporary polar research. The reason: until the end of the melting season the fate of the ice is ultimately determined by the wind conditions and air and water temperatures during the summer months. Foundations are laid during the preceding winter, however. This spring, they are as disheartening as they were in the negative record year of 2012. Back then, the sea ice surface of the Arctic shrunk to a record low of 3.4 million km2.
- "In many regions of the Arctic, new ice only formed very slowly due to the particularly warm winter. If we compare the ice thickness map of the previous winter with that of 2012, we can see that the current ice conditions are similar to those of the spring of 2012 – in some places, the ice is even thinner," Marcel Nicolaus, sea ice physicist at AWI, said today at a press conference during the EGU (European Geosciences Union) General Assembly in Vienna. 29)
- Together with his AWI colleague Stefan Hendricks, they evaluated the sea ice thickness measurements taken over the past five winters by the CryoSat-2 satellite for their sea ice projection. Seven autonomous snow buoys, which the AWI researchers had placed on floes last autumn, supplied additional important clues. In addition to the thickness of the snow cover on top of the sea ice, the buoys also measure the air temperature and air pressure. A comparison of their temperature data with the AWI long-term measurements taken on Spitsbergen, has shown that the temperature in the central Arctic in February 2016 exceeded average temperatures by up to 8ºC.
Buoy data show: the sea ice did not melt during the winter, but it grew only slowly.
- Contrary to a report published by US researchers, this warmth did not result in the thinning of the sea ice cover in some regions over the course of the winter. "According to our buoy data from the spring, the warm winter air was not sufficient to melt the layer of snow covering the sea ice, let alone the ice itself," Marcel Nicolaus explains. During the past winter, the growth of the arctic sea ice was significantly slower than the scientists had expected.
- In previously ice-rich areas such as the Beaufort Gyre off the Alaskan coast or the region south of Spitsbergen, the sea ice is considerably thinner now than it normally is during the spring. "While the landfast ice north of Alaska usually has a thickness of 1.5 meters, our US colleagues are currently reporting measurements of less than one meter. Such thin ice will not survive the summer sun for long," Stefan Hendricks, AWI sea ice physicist, explained.
Large amounts of thick pack ice will be carried away by Arctic sea currents before the autumn.
- Examining the CryoSat-2 sea ice thickness map for this spring, Stefan Hendricks further explained: "The Transpolar Drift Stream, a well-known current in the Arctic Ocean, will be carrying the majority of the thick, perennial ice currently located off the northern coasts of Greenland and Canada through the Fram Strait to the North Atlantic. These thick floes will then be followed by thin ice, which melts faster in the summer. Everything suggests that the overall volume of the arctic sea ice will be decreasing considerably over the course of the coming summer. If the weather conditions turn out to be unfavorable, we might even be facing a new record low," Stefan Hendricks said.
- According to the AWI scientists, the extent of the ice loss will be great enough to undo all growth recorded over the relatively cold winters of 2013 and 2014. AWI researchers observed a considerable decrease in the thickness of the sea ice as early as the late summer of 2015, even though the overall ice covered area of the September minimum ultimately exceeded the record low of 2012 by approximately 1 million km2. The unusually warm winter has thus contributed to the likely continuation of the dramatic decline of the Arctic sea ice throughout 2016.
Figure 13: This map shows in which regions the Arctic sea ice in Feb. 2016 was thinner (blue) or thicker than in Feb. 2012 (image credit: AWI, Stefan Hendricks)
Figure 14: Plot of the CryoSat-2 sea ice thickness data for February 2016 (image credit: AWI, Stefan Hendricks)
• March 2016: Urgent call for a follow-up mission. A group of 179 researchers is concerned the ageing CryoSat-2 mission could die in orbit at any time. They have urged the EC (European Commission) and ESA (European Space Agency) to start planning a replacement. "The mission is now central to international efforts to monitor the state of the cryosphere," they write in a letter to top officials at the EC and ESA. 30)
- CryoSat-2 was launched in 2010 on what was initially supposed to be just a one-off, three-and-half-year observation of marine and land ice - to get a snapshot of any gains and losses. But the performance of the spacecraft's mapping instrument SIRAL ( SAR Interferometer Radar Altimeter) - has exceeded all expectations, and made for some compelling data sets. The satellite has delivered the first complete assessment of Arctic sea-ice thickness and volume, as well as the most precise measurements yet of the volume and mass of the great ice sheets covering Antarctica and Greenland.
- "Over recent years, the ESA CryoSat-2 satellite has significantly improved our understanding of how polar ice sheets - in particular, the WAIS (West Antarctic Ice Sheet) - are changing and contributing to current global sea-level rise," said letter signatory Prof David Vaughan, the director of science at the British Antarctic Survey. "Many of the recent improvements in the models we use to predict the future of the WAIS were driven by the requirement to accurately simulate CryoSat's observations. So maintaining the record of ice-sheet change in future decades will be vital if we are to achieve the most rapid possible improvements in future projections of sea-level rise," he told BBC News.
- How long CryoSat-2 can keep working is anyone's guess. It has enough fuel to sustain itself into the early 2020s but component failure in the harsh environment of its orbit, 720 km above the Earth, is an ever-present risk.
- If there is to be a CryoSat-3, it will not come directly out of the ESA stable. The agency's job is to develop new technologies; its remit does not extend to funding ongoing, repeat missions. This means a successor would fit better within the Copernicus series of satellites - known as the Sentinels - which are currently being rolled out by the European Commission, paid for by EU member states; ESA participates only as the technical advisor.
- One of these new platforms, Sentinel-3, can do some work in polar regions: it has a radar altimeter to sense ice surfaces, too. But the spacecraft's orbit does not reach the same heights, meaning its data contains a 1,860 km wide "hole" at northernmost and southernmost latitudes. This makes it blind to most Arctic sea-ice, for example. Additionally, Sentinel-3's radar does not operate in a so-called interferometric mode. This is the capability that allows CryoSat to measure the slopes and ridges at the edges of the ice sheets, where losses in Antarctica and Greenland have been most pronounced.
- "The Copernicus program is a phenomenal achievement for Europe and Sentinel-3 will be doing vital work, especially over the oceans, but we'd really like to see Copernicus incorporate a proper polar Sentinel," said Prof Andy Shepherd, the principal scientific advisor to the CryoSat mission.
- The scientists are hoping for a swift process with a positive outcome. They want to avoid the gap in observations that would arise if CryoSat-2 fails and a successor is not ready. "A continuation assures a consistency in the estimates of the contribution of ice sheets to sea level change using altimetry," explained Prof Angelika Humbert from the Alfred Wegener Institute for Polar and Marine Research, Germany. "The highly sophisticated processing scheme and the error estimates are established already. In short, we know the sensor well; we've already got quite far with whatever we can squeeze out from its signals and the most benefit, with most efficiency, would come from continuing the mission."
• December 11, 2015: The satellite age has revolutionized our understanding of Earth, giving us accurate information to help critical agreements on climate change such as at the current COP21 (Conference of Parties 21) in Paris , also known as the 2015 Paris Climate Conference within the UNFCCC (UN Framework on Climate Change). Diminishing polar ice is one of the most visible indicators of change, but how much have we learnt over the last decades? 31)
- Spectacular feats of polar exploration actually go back to the 1800's when early expeditions offered a rare glimpse into these icy regions. However, it is only relatively recently that we have understood the importance of ice in the climate system and have evidence that these frozen expanses are becoming a casualty of climate change.
- Arctic sea ice, for example, is particularly sensitive to our warming climate and is often cited as a barometer of global change. Ice that forms and melts in the ocean only has a very tiny effect on sea level – the melting of ice sheets and glaciers that overlie land are the main causes of sea-level rise, along with the thermal expansion of the water.
- However, sea ice does affect how much sunlight is reflected back out to space, it affects global heat transport by insulating the relatively warm ocean from the cold polar atmosphere, and it significantly influences ocean circulation patterns, which play a role in our global climate system.
- Because of the remoteness, extreme cold and hostile conditions of the Arctic, it is impossible to acquire frequent all-weather measurements any other way than from space. Each year, the polar oceans experience the formation and then melting of vast amounts of sea ice. Around the North Pole, an area roughly the size of Europe melts every summer and then freezes again the following year.
- Scientists have been using radar measurements from satellites such as ERS-1, ERS-2 and Envisat for more than 25 years to study this seasonal change in ice extent. They have found that since 2000 the area of the Arctic Ocean covered by ice in the summer has reduced drastically.
- For example, in September 2007, it was discovered that the sea ice had shrunk to its lowest level since satellite measurements began, opening up the Northwest Passage, a long-sought shortcut between Europe and Asia that had been historically impassable.
- The extent of ice reached the lowest on record in September 2012. However, the area of the ocean covered by ice is only part of the story. It is also essential to have measurements of the thickness of the ice to work out how the actual volume is changing.
- Launched in 2010, ESA's CryoSat-2 satellite has shed new light on diminishing polar ice. ESA's CryoSat mission manager, Tommaso Parrinello, said, "By measuring the height of the ice, both of that floating in the polar oceans and of the vast ice sheets covering Greenland and Antarctica, CryoSat is providing essential information on how the ice thickness is changing."
Figure 15: Arctic sea-ice thickness in October–November 2015 as measured by ESA's CryoSat-2 mission (image credit: ESA/CPOM)
• July 20, 2015: Measurements from ESA's CryoSat-2 satellite show that the volume of Arctic sea ice increased by a third following the unusually cool summer of 2013. This new finding suggests that ice in the northern hemisphere is more sensitive to changes in summer melting than it is to winter cooling. 32)
- Scientists at University College London (UCL) and the University of Leeds in the UK used 88 million sea-ice thickness measurements taken by CryoSat between 2010 and 2014. The study, published today in Nature Geoscience, shows a 14% reduction in the volume of summer sea ice between 2010 and 2012, but the volume of ice jumped by 41% in 2013, when the summer was 5% cooler than the previous year. Lead author Rachel Tilling, from CPOM (Centre for Polar Observation and Modelling) at UCL, said, "The summer of 2013 was much cooler than recent years, with temperatures typical of those seen in the late 1990s. "This allowed thick sea ice to persist northwest of Greenland because there were fewer days when it could melt. Although models have suggested that the volume of Arctic sea ice is in long-term decline, we know now that it can recover by a significant amount if the melting season is cut short." 33)
- The team say although the first five years of CryoSat-2 measurements have revealed important information on the state of Arctic sea ice, the record is still short to establish a long-term trend. The team now plans to use the measurements of CryoSat-2 of changing sea-ice thickness to help improve the models that are used to predict future climate change, and also to assist maritime activities in the Arctic region, which can be dangerous and costly to navigate.
Figure 16: Changes in autumn Arctic sea-ice observed by CryoSat-2 during the period 2010-2014 (image credit: UCL/CPOM/University of Leeds)
• May 22, 2015: A recent acceleration in ice loss in a previously stable region of Antarctica has been detected by ESA's ice mission. The latest findings by a team of scientists from the UK's University of Bristol show that with no sign of warning, multiple glaciers along the Southern Antarctic Peninsula suddenly started to shed ice into the ocean starting in 2009 at rate of about 60 km3 each year. 34)
- This makes the region one of the largest contributors to sea-level rise in Antarctica, having added about 300 km3 of water into the ocean in the past six years. Some glaciers along the coastal expanse are currently lowering by as much as four m each year. Prior to 2009, the 750 km-long Southern Antarctic Peninsula showed no signs of change.
- "It appears that sometime around 2009, the ice-shelf thinning and the subsurface melting of the glaciers passed a critical threshold that triggered the sudden ice loss," said Bert Wouters from the University of Bristol, who led the study. "However, compared to other regions in Antarctica, the Southern Peninsula is rather understudied, exactly because it did not show any changes in the past, ironically."
- The study includes five years of measurements from ESA's ice mission, CryoSat-2, which employs an advanced radar altimeter that can measure the surface height variation of ice in fine detail, allowing scientists to record changes in its volume with unprecedented accuracy. The ice loss in the region is so large, that it has even caused small changes in Earth's gravity field, detected by the US/German GRACE mission (Figure 18). — Climate models show that the sudden change cannot be explained by changes in snowfall or air temperature. Instead, the team attributes the rapid ice loss to warming oceans.
Figure 17: Southern Antarctic Peninsula ice loss since 2009 (image credit: University of Bristol)
Legend to Figure 18: Changes in Earth's gravity field resulting from loss of ice from West Antarctica between November 2009 and June 2012 (mE = 10–12 s–2). A combination of data from ESA's GOCE mission and NASA's GRACE satellites shows the ‘vertical gravity gradient change'.
• April 17, 2015: ESA's ice mission has become the first satellite to provide information on Arctic sea-ice thickness in near-real time to aid maritime activities in the polar region. Marking five years in orbit On April 8, 2015, CryoSat-2 is the first mission to deliver complete maps of Arctic sea-ice thickness – a key indicator of global climate change and of the state of the Arctic itself. 35)
- With specialist data processing provided by the UK's CPOM (Center for Polar Observation and Modelling), these measurements can now be delivered within two days of acquisition through a website launched today. 36)
- The rapid data processing is important for managing and planning activities affected by Arctic sea ice, such as shipping, tourism, Arctic exploration and search and rescue.
Figure 19: This image demonstrates the latest 28-day (18 March to 14 April 2015) Arctic sea-ice thickness measurements from CryoSat-2. The interactive map of CPOM allows users to zoom in on various regions of the Arctic for a closer look at ice thickness(Ref. 36), image credit: ESA, CPOM
- "This new capability goes far beyond CryoSat's original purpose, which was to collect measurements for scientific research," said Professor Andy Shepherd, CPOM Director and the CryoSat's principal scientific advisor. "The mission is now an essential tool for a wide range of services operating in areas of the planet where sea ice forms."
- With the rapidly increasing economic growth in the Arctic, timely and routine information on sea-ice thickness will help to ensure that users of the Arctic can plan and carry out their operations safely and with care. The rapid access to data will also ease scientific research in the polar region, improving our understanding of how this sensitive environment is responding to climate change.
- The latest measurements available on the new website also show that sea ice around Norway's Svalbard Archipelago is today only a meter thick – approximately half of what it was in the winter of 2011 just after CryoSat-2 was launched. The thinner ice around Svalbard coincides with a warming of the surrounding Barents Sea (Ref. 35).
• March 18, 2015: Trekking to the far reaches of the Arctic for the sole purpose of collecting snow and ice measurements may seem extreme, but it is thanks to these efforts that scientists will soon have even better satellite information at their fingertips to assess changes in polar ice. - Over the last year, the measurements made by scientists camping on the ice and from various aircraft during a month-long expedition have been painstakingly processed so that they can be used to confirm data from ESA's CryoSat-2 ice mission. 37)
- CryoSat-2 measures the height of ice – both of that floating in the polar oceans and of the vast ice sheets covering Greenland and Antarctica. This information is essential for working out the thickness of the ice and how it is changing and, ultimately, how the volume of Earth's ice is being affected by the climate. Given the crucial nature of understanding the links between ice and climate, it is important to make sure that CryoSat's measurements remain accurate, even if this means heading out to one of the most inhospitable places on Earth.
- Scientists from numerous European and North American organizations volunteered to do just this. Their task was to take measurements of the Arctic sea ice using a suite of sensors on a different aircraft, as well as to measure snow and ice depth meticulously by hand on the ice – all the while keeping an eye out for polar bears. The ground team ranged from hardened polar scientists who wore jeans and took measurements with their bare hands despite temperatures of –30°C to PhD students, who had never experienced the harshness of an Arctic campaign.
- The campaign measurements actually go further than just validating the CryoSat-2 satellite data, they contribute significantly to the continuous improvement in the quality of ice-thickness maps. In fact, this latest Arctic expedition has played an important part in ESA's upcoming CryoSat-2 data product release.
Figure 20: Flying over the boundary between young sea ice (left) and older ice (right) during the 2014 Arctic campaign to validate data from ESA's CryoSat-2 mission (image credit: A. Casey, University of Alberta, released on March 18, 2015)
- From April, scientists will have, for the first time, access to systematic sea-ice ‘freeboard' information with which to generate ice-thickness maps. Freeboard describes the height of sea ice protruding above the water line. Until now, scientists had to process the CryoSat-2 data to calculate this. Tommaso Parrinello, CryoSat-2 Mission Manager, said, "The validation aspect also indicates that the satellite is still in excellent health and providing precision data despite being in orbit for five years." Malcolm Davidson, Head of ESA's Earth Observation Campaigns, said, "We are using CryoSat to measure an extremely dynamic environment" (Ref. 37).
• January 23, 2015: Rapid ice loss in a remote Arctic ice cap has been detected by the Sentinel-1A and CryoSat satellites. Located on Norway's Nordaustlandet island in the Svalbard archipelago, parts of the Austfonna ice cap have thinned by more than 50 m since 2012 – about a sixth of the ice's thickness. 38)
- The research team, led by scientists from the Centre for Polar Observation and Modelling (CPOM) at the University of Leeds in the UK, used satellite observations to document rapid acceleration and ice loss from a formerly slow-flowing, marine-based sector of Austfonna, the largest ice cap in the Eurasian Arctic. During the past two decades, the sector ice discharge has increased 45-fold, the velocity regime has switched from predominantly slow (~ 101 m/yr) to fast (~ 103 m/yr) flow, and rates of ice thinning have exceeded 25 m/yr. At the time of widespread dynamic activation, parts of the terminus may have been near floatation. Subsequently, the imbalance has propagated 50 km inland to within 8 km of the ice cap summit. 39)
- Melting ice caps and glaciers are responsible for about a third of recent global sea-level rise. Although scientists predict that they will continue to lose ice in the future, determining the exact amount is difficult, owing to a lack of observations and the complex nature of their interaction with the surrounding climate. - There is evidence that the surrounding ocean temperature has increased in recent years, which may have been the original trigger for the ice cap thinning. Long-term observations by satellites are crucial for monitoring such climate-related phenomena in the years and decades to come.
Legend to Figure 21: The main figure (top) shows the rate of ice cap elevation change between 2010 and 2014 observed by CryoSat, overlaid on an image acquired by Sentinel-1A (in 2014). Red indicates that the ice surface is lowering. In the southeast region (green box) ice thinning far exceeds the color scale of 2 m per year. - A closer look at the southeast region is shown in the four smaller figures below. These figures show the evolution of ice velocity over the last two decades. Ice velocity in 2014 was mapped using Sentinel-1A and the DLR (German Aerospace Center) TerraSAR-X mission.
• Dec. 15, 2014: CryoSat has delivered this year's map of autumn sea-ice thickness in the Arctic, revealing a small decrease in ice volume. In a new phase for ESA's ice mission, the measurements can now also be used to help vessels navigate through the north coastal waters of Alaska, for example: 40)
- Measurements made during October and November show that the volume of Arctic sea ice now stands at about 10,200 km3 – a small drop compared to last year's 10,900 km3. The volume is the second-highest since measurements began in 2010, and the five-year average is relatively stable. This, however, does not necessarily indicate a turn in the long-term downward trend. — The analysis was made by a team of researchers at CPOM (Centre for Polar Observation and Modelling) at UCL (University College London), UK.
Figure 22: Five years of ice-thickness change in the Arctic (image credit: ESA, CPOM)
• Nov. 2014: The CryoSat-2 mission has recently been extended to the end of 2016. 41)
• October 3, 2014: Although the main task of ESA's CryoSat-2 mission is to measure the elevation of the world's ice but its altimetry measurements acquired over oceans measure sea-surface height — the data of CryoSat-2 has also been used to create a new gravity map, exposing thousands of previously uncharted ‘seamounts', ridges and deep ocean structures. This vivid new picture of the least-explored part of the ocean offers fresh clues about how continents form and breakup (Figures 23 and 24). 42)
Carrying a radar altimeter, CryoSat's main role is to provide detailed measurements of the height of the world's ice. This allows the project to see how the thickness of the ice changes, seasonally and in response to climate change. However, CryoSat-2 works continuously, whether there is ice below or not. This means that the satellite can also measure the height of the surface of the sea. These measurements can be used to create global marine gravity models and, from them, maps of the seafloor.
Although invisible to the eye, the sea surface has ridges and valleys that echo the topography of the ocean floor, but on a greatly reduced scale (i.e., the sea surface mimics the ocean floor). The effect of the slight increase in gravity caused by the mass of rock in an undersea mountain is to attract a mound of water several meters high over the seamount. Deep ocean trenches have the reverse effect (producing a dip on the sea surface). These features can only be detected by using radar altimetry from space. These features can only be detected by using radar altimetry from space.
Scientists from SIO (Scripps Institution of Oceanography) at UCSD (University California San Diego) used altimetry measurements from ESA's CryoSat-2 mission and from the CNES–NASA Jason-1 satellite to create a new marine gravity map mirroring features of the ocean floor– twice as accurate as the previous version produced nearly 20 years ago. They used measurements of CryoSat-2 during the last 4 years as well as the measurements of Jason-1 during the last 12 years. The new maps offer geophysics new tools to investigate little-studied remote ocean basins and processes such as seafloor spreading. 43)
Figure 23: Gravity models are powerful tools for mapping tectonic structures of the deep ocean. The fine grid of altimetry measurements of the sea surface mirror the features of the ocean floor (image credit: SIO)
Previously unseen features in the map include newly exposed continental connections across South America and Africa, and new evidence for seafloor spreading ridges at the Gulf of Mexico that were active 150 million years ago and are now buried by layers of sediment more than a kilometer thick. One of the most important uses of this new marine gravity field will be to improve the estimates of seafloor depth in the 80% of the oceans that remains uncharted or is buried beneath thick sediment.
• August 20, 2014: Measurements from ESA's CryoSat-2 mission have been used to map the height of the huge ice sheets that blanket Greenland and Antarctica and show how they are changing. New results reveal combined ice volume loss at an unprecedented rate of ~500 km3 a year. 44)
- The research was carried out by Germany's Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research. The results were published in The Cryosphere, a European Geosciences Union journal (Ref. 45).
This study focuses on the present-day surface elevation of the Greenland and Antarctic ice sheets. Based on 3 years of CryoSat-2 data acquisition, the team derived new DEMs (Digital Elevation Models) as well as elevation change maps and volume change estimates for both ice sheets. The accuracy of the derived DEMs for Greenland and Antarctica is similar to those of previous DEMs obtained by satellite-based laser and radar altimeters. Comparisons with ICESat data show that 80% of the CryoSat-2 DEMs have an uncertainty of less than 3 m ± 15 m. The surface elevation change rates between January 2011 and January 2014 are presented for both ice sheets. The team compared their results to elevation change rates obtained from ICESat data covering the time period from 2003 to 2009. The comparison reveals that in West Antarctica the volume loss has increased by a factor of 3. It also shows an anomalous thickening in Dronning Maud Land, East Antarctica which represents a known large-scale accumulation event. This anomaly partly compensates for the observed increased volume loss of the Antarctic Peninsula and West Antarctica. - For Greenland the team found a volume loss increased by a factor of 2.5 compared to the ICESat period with large negative elevation changes concentrated at the west and southeast coasts. The combined volume change of Greenland and Antarctica for the observation period is estimated to be -503 ± 107 km3 yr-1. Greenland contributes nearly 75% to the total volume change with -375 ± 24 km3 yr-1. 45)
The new maps, which incorporate 7.5 million elevation measurements of Greenland and 61 million of Antarctica collected by CryoSat-2 in 2012, are the most complete to date from a single satellite mission.
Figure 25: Greenland ice-sheet height (image credit: Alfred Wegener Institute)
Legend to Figure 26: Using 14.3 million measurements collected by ESA's CryoSat-2 mission between January 2011 and January 2014, the research team from the Alfred Wegener Institute has discovered that the Greenland ice sheet is shrinking in volume by 375 km3 a year.
Legend to Figure 27: This new elevation model of Antarctica incorporates 61 million measurements from ESA's CryoSat-2 satellite collected throughout 2012. The edge of the ice sheet is outlined in black.
Legend to Figure 28: Using 200 million measurements collected by ESA's CryoSat-2 mission between January 2011 and January 2014, the research team from the Alfred Wegener Institute has discovered that the Antarctic ice sheet is shrinking in volume by 125 km3 a year.
• May 19, 2014: Three years of observations from ESA's CryoSat-2 satellite show that the Antarctic ice sheet is now losing 159 billion tons of ice each year – twice as much as when it was last surveyed. The polar ice sheets are a major contributor to the rise in global sea levels, and these newly measured losses from Antarctica alone are enough to raise global sea levels by 0.45 mm each year. 46) 47)
- These latest findings by a team of scientists from the UK's CPOM (Centre for Polar Observation and Modelling) show that the pattern of imbalance continues to be dominated by glaciers thinning in the Amundsen Sea sector of West Antarctica. Between 2010 and 2013, West Antarctica, East Antarctica and the Antarctic Peninsula lost 134, 3, and 23 billion tons of ice each year, respectively (note: 1 billion tons of ice, or 1 Gt, is equivalent to ~1 km3 of ice). The average rate of ice thinning in West Antarctica has increased compared to previous measurements, and this area's yearly loss is now one third more than measured over the five years before the launch of CryoSat-2.
- CryoSat surveys almost all – 96% – of the Antarctic continent, reaching to within 215 km of the South Pole. In addition, it has increased coverage over coastal regions, where today's ice losses are concentrated.
Figure 29: Illustration of Antarctica's ice loss regions (image credit: CPOM, Leeds, ESA)
Figure 30: Detail of an ice thinning region in West Antarctica (image credit: CPOM, Leeds, ESA)
- A US article by NASA/JPL and UCI (University of California, Irvine), being published in Geophysical Research Letters of AGU, comes to the same conclusions as their British colleagues. 48) The findings of the study were also presented at a NASA press conference on May 12, 2014. 49)
The study finds a rapidly melting section of the West Antarctic Ice Sheet appears to be in an irreversible state of decline, with nothing to stop the glaciers in this area from melting into the sea. The study presents multiple lines of evidence, incorporating 40 years of observations that indicate the glaciers in the Amundsen Sea sector of West Antarctica "have passed the point of no return," according to glaciologist and lead author Eric Rignot, of UC Irvine and NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. These glaciers already contribute significantly to sea level rise, releasing almost as much ice into the ocean annually as the entire Greenland Ice Sheet. They contain enough ice to raise global sea level by 1.2 m and are melting faster than most scientists had expected. Rignot said these findings will require an upward revision to current predictions of sea level rise.
The team used radar observations captured between 1992 and 2011 by the European Earth Remote Sensing (ERS-1 and -2) satellites to map the grounding lines' retreat inland. The satellites use a technique called radar interferometry, which enables scientists to measure very precisely — within less than a quarter of an inch — how much Earth's surface is moving. Glaciers move horizontally as they flow downstream, but their floating portions also rise and fall vertically with changes in the tides. Rignot and his team mapped how far inland these vertical motions extend to locate the grounding lines. The accelerating flow speeds and retreating grounding lines reinforce each other. As glaciers flow faster, they stretch out and thin, which reduces their weight and lifts them farther off the bedrock. As the grounding line retreats and more of the glacier becomes waterborne, there's less resistance underneath, so the flow accelerates.
Figure 31: Velocity of the Amundsen Sea Embayment sector of West Antarctica derived using ERS-1/2 radar data in winter 1996 with a color coding on a logarithmic scale and overlaid on a MODIS mosaic of Antarctica (image credit: AGU, NASA)
Legend to Figure 31: Interferometrically-derived grounding lines of the glaciers are shown in color code for years 1992, 1994, 1996, 2000 and 2011, with glacier and ice shelf names. Note that for Pine Island and Smith/Kohler, the Figure merges two independent differential interferograms to show a more complete spatial coverage of grounding lines.
• In April 2014, Cryosat-2 achieved four years of operations, covering six-months of commissioning and the follow on routine operations. Overall, the mission has proven to be very successful and the spacecraft overall performance very reliable. As CryoSat-2 completes its fourth year on orbit, unit ageing is being closely monitored by the FCT (Flight Control Team). CryoSat-2 has achieved over 20,000 orbits in flight and with the current power and fuel margins it is highly likely that the satellite would be able to continue well beyond the currently planned end of mission date, in 2016. 50)
- Debris collision monitoring: Space debris has become, and continues to be, a collision risk for orbiting satellites. At the CryoSat-2 operational orbit height of 717km this risk is such that the FCT has put in place a CAM (Collision Avoidance Maneuver) protocol. On reception of close approach conjunction warnings, the CryoSat-2 satellite can be maneuvered out of the way of the piece of debris.
The warnings generated by both the ESA Space Debris Office (SDO) CRASS system and with support from the US JSpOC (Joint Space Operations Center) provide details of the predicted conjunctions between CryoSat-2 and all other tracked LEO objects. These warnings specify the conjunction times, geometries, miss distances, orbit prediction errors and probabilities of the events. This data needs to be analyzed by the FCT, FD (Flight Dynamics) and SDO and a decision on whether to maneuver or not needs to be taken – sometimes at very short notice i.e. within 24 hours.
The conjunction encounters experienced by CS-2 to date have been varied and not always deterministic with debris items including Iridium clouds, COSMOS and the Fengyun-1C debris. Dedicated CAMs have been carried out in a couple of cases interrupting routine operations and collection of Science data. In other cases it has been possible to combine the collision avoidance maneuver with a planned OCM (Orbit Control Maneuver), by slightly changing the timing or parameters of the OCM.
Table 3: Collision avoidance maneuver timings and occurrences (Ref. 50)
- Roll maneuver campaigns over ocean for calibration: A roll maneuver campaign was devised to support the need to characterize the interferometer performance and in addition to determine the extent of thermally induced bending on the SIRAL Antenna bench. The CS-2 roll maneuver campaigns have been supported during commissioning and additionally in Oct. 2011, Sept. 2012, Sept .2013 and with the last campaign in Jan. 2014. The specification for the rolled SARIn measurements is that they take place over ocean for calibration of science data, away from the South Atlantic Anomaly (spacecraft constraint to ensure stable STR behavior), and to be repeated yearly (every 369 days) and for the last campaign one solar cycle apart (480 days) using the same orbit segments. The data takes need to occur during both eclipse and sunlight periods (if possible), covering ascending and descending passes and over different oceans (Pacific and Indian) to ensure a complete set of the varied thermal conditions (Ref. 50).
- Support to IceBridge, CryoVEx Campaigns: As part of the nominal CryoSat-2 support to users the FCT aims to maintain maximum SIRAL availability and minimal loss of science data. In the case of support to NASA's operation IceBridge and ESA's CryoVEx (Cryosat Validation Experiment) type campaigns which occur every one or two years, the FCT aims to avoid possible SIRAL outages over the known underflight periods. These underflights are simply flown to match the time and groundtrack of the CryoSat-2 orbit (Ref. 50).
- Space segment improvements - MMFU Storage Capacity increase: CryoSat-2 operates fairly close to the limits with regard to the on board storage capacity of science data. The current scenario is that with the loss of one badly timed ground station pass, the packet stores can be over written. The CryoSat-2 MMFU (Mass Memory and Formatting Unit ) was originally designed with a spare 32 Gbit, one complete Partition. Analysis was conducted considering the spacecraft and ground segment activities required to increase the MMFU storage capacity to make use of the currently spare 32 Gbit. Simulations were carried out where the extra 32 Gbit were allocated between Packet Store (PS) 1 and PS2 (used to store the SAR and SARIn data) to try to minimize data loss in case of missed ground-station passes before or after a long blind orbit period. The simulations showed that the new PS sizes would allow to cope with a missed pass before or after the blind orbits period without a data loss. For the cases of more than one missed pass even the use of the complete memory cannot prevent data loss but does minimize it. The spacecraft activities needed to implement this change require the powering up of the remaining MMFU partition and to configure the PS1 and PS2 to the new recommended sizes. The SIRAL instrument would need to be Standby throughout this period with an expected outage of at least 2 orbits. Planning for execution of this activity is underway with the dates to be selected outside of the northern hemisphere winter to avoid gaps in science data at the most important ice monitoring times of the year.
- STR Enhanced software for Attitude Update mode: A proposal to update the STR (Star Tracker) Attitude Update mode to account for noisier CCDs at higher temperatures is being considered. As the STR CCDs age, they become more susceptible to thermal effects. CryoSat-2 needs to operate at certain times of the year with warm STRs . To ensure the longevity of the CryoSat-2 operations, the ageing STRs need to continue to work at these warm temperatures. The patches recently received cover enhancements of the IA (Initial Acquisition) mode of the STRs. The idea for further improvements cover possible updates to the AU (Attitude Update) mode of the STRs. One proposal is for the centroiding algorithm to be further enhanced to filter out false stars. Single bright pixels are already filtered out but further improvements could be made. A number of proposals have been put forward and the solution most suited to CryoSat-2 STRs needs to be further assessed.
- Long term performance monitoring: The CryoSat-2 subsystem battery management has been established around the eclipse seasons to optimize the battery capacity as the battery ages. The procedure now in operation was agreed and assessed together with Industry allowing simple rules to be put in place to modify the battery EOC (End of Charge) levels going into and coming out of the longest eclipse periods. The solution chosen was a simple seasonal commanding from ground, based on the observed behavior of the battery voltages. With the minimum battery voltage decreasing (i.e. going into a deeper period of eclipses), as soon as the battery voltage hits a predefined level, the EOC level is increased by one setting. The reverse logic is used as CryoSat-2 emerges from the deeper period of eclipses i.e. with the minimum battery voltage increasing, when the battery minimum voltage is greater than a predefined level for three consecutive days, the EOC levels can be decreased back to the original value. As the mission goes on, these EOC levels will be adjusted to suit the performance seen from the batteries. The management of the EOC levels ensures that the battery is always sufficiently charged to withstand a worst case safe mode entry and recovery, but is also not over charged, which would shorten the lifetime of the unit.
- New payload data applications: The primary mission goals of CryoSat-2 are to determine regional and basin-scale trends in perennial Arctic sea ice thickness and mass, and to determine regional and total contribution to global sea-level of the Antarctic and Greenland ice sheets. — CryoSat-2 is now providing high quality data to several user scientific communities: sea-ice, land-ice, meteorology, ocean, marine gravity, coastal zone and hydrology (Ref. 50).
• March 26, 2014: Water from melting glaciers and ice sheets, along with thermal expansion of ocean water due to rising temperatures, are causing global sea-level rise. Scientists are exploiting satellite data to understand better just how much each component contributes to this devastating consequence of climate change. The latest estimates show that global sea level is rising by about 3 mm a year, and this is one of the major threats of global warming, especially for low-lying coastal areas. 51)
Identifying the individual contributors to sea-level rise is one of the most complicated challenges in climate science. This involves tracking water as it moves in all its forms – solid, liquid or gas – around Earth. While Earth-observing satellites continuously map global and regional sea-level change, they can also be used to quantify the amount of water coming from various sources.
Under ESA's Climate Change Initiative (CCI), experts in the domains of oceans, land, atmosphere and the cryosphere are working together to quantify the various sources of sea-level change – known as balancing the sea-level budget.
Changes in the mass of ice sheets and glaciers can be mapped using satellite radar altimeters, like the one flying on ESA's CryoSat-2 that was specially designed to survey ice. By monitoring these changes, scientists can gage how much water they contribute to the ocean.
Legend to Figure 32: The rate of ice loss in Greenland and Antarctica is increasing. From 1992 to 2012, the two ice sheets contributed a total of 11.1 mm to global sea levels. This is about 20% of all sea-level rise over that 10-year period (Ref. 51).
• Feb. 2014: CryoSat-2 completed its first three years of operations on Nov. 19, 2013 when the spacecraft was declared operational. It continues to work flawlessly, acquiring and generating science data systematically, to measure the variation of sea-ice mass floating in the Arctic and trend of land-ice volume over Greenland and Antarctica. An issue affecting the onboard power system forced operations to fall back to the redundant system with little impact on the science retrieval. 52)
• Feb. 2014: Regarding collision avoidance, CryoSat-2 experienced seven conjunctions within 300 m in 2013, two required evasive maneuvers: 53)
- Oct. 11, 2013 (JSpOC alert): conjunction at ~340 m (53 m radial)
- Oct. 15, 2015 (JSpOC alert): conjunction at ~205 m (200 m radial).
• The CryoSat-2 spacecraft and its payload are operating nominally in 2014.
• January 2014: Near the center of Antarctica, measurements from CryoSat-2 show an unusual pattern in the ice sheet's elevation (Figure 33). Scientists have now found the reason for this pattern – and the discovery is leading to even more accurate measurements from ESA's ice mission. 54)
CryoSat collects data over Antarctica while passing on northbound and southbound orbits. But the data show an unusual pattern of height differences where these orbit cross, radiating from the South Pole.
Initially it was reasoned that there could be an issue with the satellite itself, such as a miscalculation of the altitude, a timing error or a problem with one of the corrections we apply to the measurements. - After eliminating the possibility of these errors through careful experimentation, scientists discovered that the pattern was caused by the way the satellite signal is scattered from the ice sheet surface.
Antarctica has some of the strongest and most persistent winds on Earth, which leave permanent erosional and depositional features on the surface and in the snow pack. The scientists found that these wind-driven features modify CryoSat-2's radar measurements in such a way as to produce the pattern that has been detected. - The pattern in Figure 33 is not an ‘error', but an artefact arising from the interaction of the polarization of CryoSat's antenna with the structure of the ice surface induced by wind.
It has long been known that wind-driven directional properties of the ice sheet surface can affect the signal received by radar altimeters, but has never been seen so clearly. The most striking feature of the pattern – the diamond ring pattern close to the pole – had not been seen by past altimeter missions because they did not fly far enough south.
Since the pattern appears to be stable over time, the data can easily be corrected, ensuring that CryoSat's past and future measurements of Antarctica are precise. The discovery also helps scientists better understand the interaction between radar waves and ice sheet surfaces.
Legend to Figure 33: There is a distinct pattern of alternating high and low elevations (shown in red and blue), which inverts closer to the South Pole. After careful analysis, it was discovered that this is an artefact caused by the interaction of the polarization of CryoSat's antenna with the structure of the ice.
• December 16, 2013: Measurements from CryoSat-2 show that the volume of Arctic sea ice has significantly increased this autumn. In October 2013, CryoSat-2 measured about 9000 km3 of sea ice – a notable increase compared to 6000 km3 in October 2012. This year's multi-year ice is now on average about 20%, or around 30 cm, thicker than last year. While this increase in ice volume is welcome news, it does not indicate a reversal in the long-term trend. - It's estimated that there was around 20 000 km3 of Arctic sea ice each October in the early 1980s, and so today's minimum still ranks among the lowest of the past 30 years. 55)
• December 11, 2013: Three years of observations by ESA's CryoSat-2 satellite show that the West Antarctic Ice Sheet is losing over 150 km3 of ice each year – considerably more than when last surveyed. The imbalance in West Antarctica continues to be dominated by ice losses from glaciers flowing into the Amundsen Sea. The ice thinning continues to be most pronounced along fast-flowing ice streams of this sector and their tributaries, with thinning rates of between 4–8 m per year near to the grounding lines – where the ice streams lift up off the land and begin to float out over the ocean – of the Pine Island, Thwaites and Smith Glaciers. 56)
The melting of ice sheets that blanket Antarctica and Greenland is a major contributor to global sea-level rise. An international team of polar scientists had recently concluded that West Antarctica caused global sea levels to rise by 0.28 mm each year between 2005 and 2010, based on observations from 10 different satellite missions. But the latest research from CryoSat-2 suggests, that the sea level contribution from this area is now 15% higher.
Figure 34: Three years of measurements from CryoSat show that the West Antarctic Ice Sheet is estimated to be losing over 150 km3 of ice each year (image credit: CPOM, ESA)
• During December 5-6, 2013, a major storm passed through northern Europe causing flooding, blackouts, grounding flights and bringing road, rail and sea travel to a halt. ESA's CryoSat-2 satellite measured the storm surge from the recent North Sea storms, as high waters passed through the Kattegat sea between Denmark and Sweden. Since the storm coincided with a period of high tides in the North Sea, there were extremely high sea levels – a ‘storm surge'. In the UK, sea levels were at their highest since the 1953 North Sea Floods, while in Germany, parts of Hamburg were flooded. 57)
- On Dec. 6, CryoSat-2 passed over the Kattegat, providing an estimate of total water levels. The observations matched predictions, helping to confirm these models. The measurements were made by CryoSat's radar altimeter that – although designed to measure sea-ice thickness – is providing outstanding results over sea and, especially, coastal areas.
- Until recently, altimeter measurements of sea-level height could only be made over open oceans, because of land interference closer to the coast. In the last few years, however, progress has been made in reducing these effects, also thanks to the new generation of radar altimeters being heralded by CryoSat-2. This has allowed scientists not only to map water levels closer to the coast, but also profile land surfaces and inland water targets such as small lakes, rivers and their intricate tributaries.
- Altimeter measurements from space can be used to validate storm surge models as well as provide near-realtime information that can be incorporated into predictions. Under ESA's Data User Element, the 'eSurge' project is helping to optimize the use of altimetry and other types of satellite data to improve storm surge forecasting.
• Sept. 2013: CryoSat-2 has been in orbit since 2010 and with the satellite still in excellent health it is now set to continue providing precision measurements until 2017.
ESA's CryoSat-2 mission has provided three consecutive years of Arctic sea-ice thickness measurements, which show that the ice continues to thin. Along with observations of ice extent, The measurements of CryoSat-2 thickness now span from October 2010 to April 2013, allowing scientists to work out the real loss of ice, monitor seasonal change and identify trends. CryoSat-2 continues to provide clear evidence of diminishing Arctic sea ice. 58)
Figure 35: Variations in spring ice thickness: Changes in ice thickness for March/April 2011, 2012 and 2013 as measured by CryoSat-2 (image credit: A. Ridout,UCL)
Figure 36: Variations in autumn ice thickness: Changes in ice thickness for October/November 2011, 2012 and 2013 as measured by CryoSat-2 (image credit: A. Ridout, UCL)
• In early July 2013, ESA is reporting that CryoSat-2 has found a vast crater in Antarctica's icy surface. Scientists believe the crater was left behind when a lake lying under about 3 km of ice suddenly drained. 59)
Far below the thick ice sheet that covers Antarctica, there are lakes of fresh water without a direct connection to the ocean. These lakes are of great interest to scientists who are trying to understand water transport and ice dynamics beneath the frozen Antarctic surface – but this information is not easy to obtain.
By combining new measurements acquired by CryoSat-2 with older data from NASA's ICESat satellite, the Cryosat team has mapped the large crater left behind by a lake, and even determined the scale of the flood that formed it.
Figure 37: Location of the crater in Antarctica (image credit: ESA)
The CryoSat science team analyzed the data acquired by the SIRAL (SAR Interferometer Radar Altimeter) instrument on CryoSat-2 and demonstrated its novel capability to track topographic features on the Antarctic Ice Sheet. The perimeter and depth of a 260 km2 surface depression was mapped above an Antarctic SGL (Subglacial Lake) and, in combination with ICESat laser altimetry data,the decadal changes were charted in SGL volume. During 2007-2008, between 4.9 and 6.4 km3 of water drained from the SGL, and the peak discharge exceeded 160 m3/s. The flood was twice as large as any previously recorded, and equivalent to ~ 10 % of the meltwater generated annually beneath the ice sheet. - The ice surface has since uplifted at a rate of 5.6 ± 2.8 m/yr. Our study demonstrates the ability of CryoSat-2 to provide detailed maps of ice sheet topography, its potential to accurately measure SGL drainage events, and the contribution it can make to understanding water flow beneath Antarctica. 60)
- In April 2013, CryoSat-2 was 3 years on orbit
- Although there are always small issues to investigate and resolve, the satellite and ground segment have been extremely reliable and robust. There has been no major anomaly with the satellite since May 2011, and CryoSat-2 has now achieved over 15,000 orbits in flight.
- As part of the nominal CS-2 support to users the FCT (Flight Control Team) aims to maintain maximum SIRAL availability and minimal loss of Science data. However in the case of support to NASA's operation IceBridge and ESA's CryoVEx (Cryosat Validation Experiment) type campaigns, the FCT aims to avoid any possible SIRAL outage over these periods. This implies that the Orbit maintenance manoeuvres and any other platform or ground segment related activities are avoided around the period of the campaigns to avoid any interruption to SIRAL availability.
• An international team of scientists using new measurements from Europe's ice mission has discovered that the volume of Arctic sea ice has declined by 36% during autumn and 9% during winter between 2003 and 2012. A team of scientists led by University College London has now generated estimates of the sea-ice volume for the 2010–11 and 2011–12 winters over the Arctic basin using data from ESA's CryoSat-2 satellite. This study has confirmed, for the first time, that the decline in sea ice coverage in the polar region has been accompanied by a substantial decline in ice volume. Since 2008, the Arctic has lost about 4300 km3 of ice during the autumn period and about 1500 km3 in winter. 63) 64) 65)
The team confirmed CryoSat-2 estimates using independent ground and airborne measurements carried out by ESA and international scientists during the last two years in the polar region, as well as by comparing measurements from NASA's Operation IceBridge.
• December 2012: ESA's ice mission is now giving scientists a closer look at oceans, coastal areas, inland water bodies and even land, reaching above and beyond its original objectives. The satellite's radar altimeter not only detects tiny variations in the height of the ice, it also measures sea level and the sea ice's height above water to derive sea-ice thickness with an unprecedented accuracy. At a higher precision than previous altimeters, CryoSat's measurements of sea level are improving the quality of the model forecasts. Small, local phenomena in the ocean surface like eddies can be detected and analyzed. 66)
Taking CryoSat a step further, scientists have now discovered that the altimetry readings have the potential to map sea level closer to the coast, and even greater capabilities to profile land surfaces and inland water targets such as small lakes, rivers and their intricate tributaries.
• The CryoSat-2 spacecraft and its payload are operating nominally in 2012.
- May 2012: While the main objective of the CryoSat-2 mission is to measure the thickness of polar sea ice and monitor changes in the ice sheets that blanket Greenland and Antarctica, the radar altimeter, SIRAL (SAR Interferometer Radar Altimeter), is not only able to detect tiny variations in the height of the ice but it can also measure sea level. 67)
Recent studies at the Scripps Institution of Oceanography in San Diego, USA, found that the range precision of CryoSat-2 is at least 1.4 times better than the US's GEOSAT or ESA's ERS-1. They estimate that this improved range precision combined with three or more years of ocean mapping will result in global seafloor topography – bathymetry – that is 2–4 times more accurate than measurements currently available.
Most satellite radar altimeters, such as the one on the joint CNES/NASA/Eumetsat/NOAA Jason-2, follow repeated ground-tracks every 10 days to monitor the changes in ocean topography associated with ocean currents and tides. - On the other hand, the 369-day repeat cycle of CryoSat-2 provides a dense mapping of the global ocean surface at a track spacing of over 4 km. Three to four years of data from CryoSat can be averaged to reduce the ‘noise' due to currents and tides and better chart the permanent topography related to marine gravity (Ref. 67).
- April 2012: After nearly a year and a half of operations, CryoSat has yielded its first seasonal variation map of Arctic sea-ice thickness (Figure 38). Results from ESA's ice mission were presented at the Royal Society in London as part of the events celebrating the 50th anniversary of the UK in space. 68)
Figure 38: Produced from CryoSat-2 data, this map shows Arctic sea-ice thickness, as well as the elevation of Greenland ice sheet, for March 2011. For sea ice, green indicates thinner ice, while yellow and orange indicate thicker ice (image credit: CPOM/UCL/Leeds/ESA/PVL) 69)
- February 2012: Ocean measurements from ESA's CryoSat-2 mission are being exploited by the French space agency, CNES, to provide global ocean observation products in near-real time. Understanding sea-surface currents is important for marine industries and protecting ocean environments. 70)
- January 2012: Although the primary objective of CryoSat-2 was to measure the thickness of ice, fast data delivery was not initially intended. The CryoSat team has changed this to demonstrate that CryoSat-2 can deliver marine information in near-real time from most of its orbits around Earth. Up to now, this new product called 'fast delivery mode' has only been provided to organizations such as the US NOAA (National Ocean and Atmospheric Organization). This is about to change: marine information is expected to be available systematically to all users from February 2012 onwards. 71)
At NOAA's LSA (Laboratory for Satellite Altimetry), the CryoSat-2 data are processed to estimate wind speed and wave height, which are then provided to forecasters at NOAA's NCEPs (National Centers for Environmental Predication). LSA combines CryoSat-2 data with information from other organizations such as CNES of France, the ECMWF (European Centre for Medium-Range Weather Forecasts) and NASA. This processing takes a matter of only three days. NOAA delivers these data to ocean modelers and forecasters worldwide. For example, Australia's Integrated Marine Observing System now uses CryoSat observations of sea level to monitor surface currents.
Figure 39: The NOAA fast delivery product displays the estimate of wind speed over oceans using data from ESA's CryoSat-2 mission from 17 Nov. to 13 Dec. 2011 (image credit: NOAA)
• Antarctic measurement campaign: In early December 2011, a team of Australian and German scientists from the University of Tasmania, the Australian Antarctic Division and the Alfred Wegner Institute (AWI, Bremerhaven) has just finished the first leg a remarkable measurement campaign. The campaign is being carried out in East Antarctica around Law Dome and the Totten Glacier. Law Dome is relatively stable but features steep surface slopes and Totten Glacier is changing rapidly – so both offer ideal locations for validating CryoSat-2 data. 72)
The campaign involves taking measurements from a Polar-6 aircraft. It carries the ASIRAS radar, which mimics CryoSat's SIRAL. Ground-truth measurements are also collected for comparison. The skidoos drag GPS to map the height of the ice, which are later compared to the aircraft and satellite measurements. The skidoo team gathered ground measurement over about 250 km transects.
Legend to Figure 40: Strong layering as a result of seasonal changes in snow accumulation is clearly visible down to about 20 m. In combination with snow pit and firn-core data, scientists can determine the spatial variability of the accumulation rate (Ref. 72).
• The first map of sea-ice thickness from ESA's CryoSat-2 mission was revealed at the Paris Air and Space Show on June 21, 2011. This new information is set to change our understanding of the complex relationship between ice and climate. CryoSat-2 has spent the last seven months delivering precise measurements to study changes in the thickness of Earth's ice. 73)
CryoSat-2's exceptionally detailed data have been used to generate this map of sea-ice thickness in the Arctic (Figure 41). Data from January and February of 2011 have been used to show the thickness of the ice as it approaches its annual maximum. Thanks to CryoSat-2's orbit, ice thickness close to the North Pole can be seen for the first time.
CryoSat measures the height of the sea ice above the water line, known as the freeboard, to calculate the thickness. The data are exceptionally detailed and considerably better than the mission's specification. They even show lineations in the central Arctic that reflect the ice's response to wind stress.
A new map of Antarctica has also been created showing the height of the ice sheet (Figure 42). This is more preliminary because more data are needed here to see what CryoSat-2 can do. Even so, the extra coverage that CryoSat-2 offers near the poles can be demonstrated: parts of Antarctica can now be seen for the first time from space.
In addition, detail of edges of the ice sheet where it meets the ocean can now closely be monitored thanks to CryoSat's sophisticated radar techniques. This is important because this is where changes are occuring.
• A month-long common ESA/NASA Arctic campaign was conducted in the spring of 2011 in support of ESA's CryoSat mission. The one-month Arctic expedition is a major undertaking, with scientific teams from numerous organizations braving temperatures of –30ºC in central Greenland, Svalbard and the Fram Strait, Devon Island and offshore from Alert, Ellesmere Island, in northern Canada. 74)
To ensure that CryoSat is delivering accurate data, the scientists are gathering a wealth of ice and snow measurements on the ground and from the air. These in situ measurements will be compared with measurements delivered by CryoSat, thereby guaranteeing that the mission is delivering the best quality data possible. Data on changes in the thickness of ice floating in the polar oceans and in the vast ice sheets on land are vital in the quest to deepen our understanding of the delicate relationship between ice, climate change and sea-level rise.
In parallel, NASA is also in the Arctic, surveying the polar ice cover from the air for their IceBridge operation (see IceBridge below).
This involved coordinated ESA–NASA flight activities and orbital ground track of CryoSat on 17 April. In Figure 43, the straight red and black thick line indicates CryoSat's ground track and the color changes where the measurement mode switches to pass over the steep ice sheet margin. The dark green straight line shows the 15 April CryoSat track, with the yellow line indicating the NASA P-3 aircraft underflight. The white line shows the paths taken by the Twin Otter carrying the ASIRAS instrument and the green lines show the AWI aircraft carrying the electromagnetic sensor, which take measurements of the area for comparison.
NASA IceBridge campaigns: IceBridge, a six-year NASA mission, is the largest airborne survey of Earth's polar ice ever flown. It will yield an unprecedented three-dimensional view of Arctic and Antarctic ice sheets, ice shelves and sea ice. These flights will provide a yearly, multi-instrument look at the behavior of the rapidly changing features of the Greenland and Antarctic ice. IceBridge will use airborne instruments to map Arctic and Antarctic areas once a year. The first IceBridge flights were conducted in March/May 2009 over Greenland and in October/November 2009 over Antarctica. Other smaller airborne surveys around the world are also part of the IceBridge campaign. 75) 76)
- IceBridge has flown more than 200 hours during science flights with the P-3 during the 2011 Arctic deployment, and only one canceled flight day due to weather. On April 29, 2011, the P-3 flew a medium-priority mission over the catchment area of Petermann Gletscher that when combined with two other surveys will result in a 10-kilometer grid over the glacier's entire catchment area. 77)
The ground measurements, multiple airborne measurements and the CryoSat-2 overpass will create a landmark dataset to shed light on fundamental issues in remotely sensing sea ice.
• On Feb. 1, 2011, ESA announced at the CryoSat Validation Workshop (Frascati, Italy, Feb. 1-3, 2011) the release of the CryoSat-2 ice data. This means that the international science community will have free and easy access to all of the measurements from CryoSat-2. This will amount to a unique dataset to determine the impact climate change is having on Earth's ice fields. 78)
• The CryoSat-2 mission was declared operational on Nov. 19, 2010. - The results of the intense commissioning phase were presented to more than 80 scientists and engineers from ESA, industry and universities at the CryoSat-2 Commissioning Results Review, held in Noordwijk, the Netherlands, on 22 October. 79) 80)
• The commissioning phase of the CryoSat-2 mission is expected to last for about 6 months.
• In mid-July 2010, the project is already releasing access data to about 150 scientists of around 40 research institutes (users outside the project team) as part of the calibration and validation procedure. The intent is to help ensure that these measurements meet the mission's exacting standards before the data are released to the wider scientific community later this year. 81)
• Taking advantage of NASA's 'Operation Ice Bridge' campaign in April 2010, measurements of Arctic sea ice have been made from an aircraft flying directly under CryoSat-2's orbital path. These measurements offer an early opportunity to check the quality of the newly launched CryoSat-2 satellite data over sea ice. The campaign uses a DC-8 aircraft carrying the ATM (Airborne Topographic Mapper) laser, which sends pulses of light in circular scans to the ground. The pulses reflected back to the aircraft are converted into elevation maps of the ice surface below.
Since the campaign is being carried out in the Arctic, ESA and NASA seized the opportunity to collaborate by timing one of the DC-8's flights to coincide with CryoSat-2 orbiting above. The NASA aircraft flew from Thule in northwestern Greenland over the Arctic Ocean on 20 April to pass directly under the satellite orbiting close to the North Pole. In parallel, ESA's European Spacecraft Operations Centre (ESOC) in Germany switched CryoSat-2's SIRAL from 'health monitoring mode' into its sea-ice measuring 'SAR mode'. 82)
- The first IceBridge underflight of CryoSat-2 took place on April 20, 2010 along a 670 km track over the Arctic Ocean. The flight was carried in using a NASA DC-8 as part of NASA's Operation IceBridge and was designated Sea Ice 07. The aircraft intercepted the CryoSat-2 groundtrack near 87º latitude, 205º longitude approximately 10 minutes after the satellite passed over (13:33:06 GMT) as shown in Figure 44. The DC-8 followed the groundtrack westward at an altitude of 7000 m for about one hour, covering 670 km. The DC-8 carried the usual complement of IceBridge instrumentation, including the DMS (Digital Mapping System) camera, and two laser altimeters: 1.) the LVIS ( Land, Vegetation and Ice Sensor), usually flown at high altitudes, and 2.) the ATM (Airborne Topographic Mapper), usually flown at lower altitudes. 83)
Figure 44: Arctic map showing flight path of Operation IceBridge Sea Ice 07 flight. Upper horizontal portion of red line follows the path of the CryoSat-2 groundtrack flown by the NASA DC-8 (image credit: NOAA, NASA)
• On April 11, 2010, the LEOP (Launch and Early Orbit Phase) was formally ended. The spacecraft is in excellent condition. Later on April 11, SIRAL (Synthetic Aperture Interferometric Radar Altimeter) was switched on for the first time and started gathering the first radar echo data. 84)
• CryoSat-2 has delivered its first data just hours after ground controllers switched on the satellite's sophisticated radar instrument for the first time. CryoSat-2 was launched on 8 April and has been performing exceptionally well during these critical first few days in orbit.
Sensor complement (SIRAL, DORIS, LRR)
SIRAL (SAR Interferometer Radar Altimeter):
SIRAL is the primary instrument of the mission, designed and developed for ESA by Thales Alenia Space (formerly Alcatel Alenia Space), France. SIRAL is of Poseidon-2 heritage flown on the Jason-1 mission. The objective is to observe ice sheet interiors, the ice sheet margins, for sea ice and other topography. 85)
The SIRAL-2 design is based on existing equipment, but with several major enhancements designed to overcome difficulties associated with measuring ice surfaces. It works by bouncing a radar pulse off the ground and studying the echoes from the Earth's surface. By knowing the position of the spacecraft - achieved with an onboard ranging instrument called DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) - the signal return time will reveal the surface altitude. Correct antenna orientation is vital for this and is maintained using a trio of star trackers.
The design of SIRAL for the CryoSat-2 mission was made completely redundant. 86)
Figure 45: A perspective view of the nose of the CryoSat-2 with the SIRAL units (image credit: ESA)
The SIRAL instrument design makes use of the DDA (Delay Doppler Altimeter) concept representing a new technology introduction into spaceborne altimetry and permitting that detailed views of irregular sloping edges of land ice, as well as non-homogenous ocean ice, can also be obtained. The new features of SIRAL have been demonstrated with the airborne D2P (Delay-Doppler Phase-monopulse Radar) of JHU/APL, first test flights were conducted in 2000 (for a DDA concept introduction see the last chapter of the description).
The SIRAL design features two receiving antennas forming an interferometer in the cross-track direction with a baseline of 1.2 m (support for SARIn mode). In addition, the return signal in along-track direction is processed to construct a synthetic aperture for enhanced ground resolution. The instrument is a Ku-band radar altimeter which uses the full-deramp range compression technique of conventional altimeters (conventional single frequency pulse-limited altimeter). However, it introduces two features that make it different from previous spaceborne altimeter implementations: 87) 88) 89) 90)
• The instrument has two parabolic antennas (including pulse-to-pulse phase coherence) and two receive chains, permitting an interferometric mode of operation (and interferometric signal processing).
• SIRAL operates at high PRF (Pulse Repetition Frequency), ensuring coherent along-track sampling for aperture synthesis (PRF>Doppler bandwidth). The distinguishing feature of SIRAL compared to conventional altimeter instruments (generally with pulse intervals of about 500 µs) is that it sends bursts of pulses separated by intervals of only 50 µs. Though the return echoes are correlated, the bursts are instead treated using "aperture synthesis" data processing techniques.
Figure 46: Illustration of the SIRAL instrument electronics (image credit: Thales Alenia Space)
The instrument consists of three major subsystems, two of these are in discrete electronic boxes:
• DPU (Digital Processing Unit), it serves all digital altimeter functions, including the digital chirp generation, the full sequencing functions of the altimeter, and the receive and processing functions of the echo
• RFU (Radio Frequency Unit). It contains all analog IF and RF electronics and a solid-state power amplifier with an RF peak power of 25 W.
• The antenna subsystem consists of two Cassegrain antennas, mounted side-by-side and forming the interferometric cross-track. Both antennas are identical; one is used to transmit and receive, whereas the other antenna is used to receive echoes (bistatic configuration). The primary super-elliptic reflectors are about 1.1 m x 1.2 m in size. They are supported by a composite sandwich plate. A high thermoelastic stability is needed to meet the interferometric instrument performance.
Table 4: SIRAL key instrument parameters
Figure 47: Block diagram of the SIRAL instrument (image credit: ESA)
The science requirements demand of CryoSat to measure variations in ice thickness of perennial sea and land ice fields to the limit allowed by natural variability, on spatial scales varying over three orders of magnitude. The natural variability of sea and land ice depends on fluctuations in the supply of mass by the atmosphere and ocean, and snow and ice density. The precisions of the measurements are expressed in terms of cm of yearly ice equivalent thickness variations. These are:
• Arctic sea-ice: 1.6 cm/year vertical measurement accuracy at 105 km2 scale (equivalent to 300 km x 300 km cells). Temporal sampling: 1 month
• Land ice (small scale): 3.3 cm/year at 104 km2 (equivalent to 100 km x 100 km cells). Temporal sampling : 1 year
• Land-ice (large scale): 0.17 cm/year over 13.8 x 106 km2 (about the area of Antarctica). Temporal sampling: 1 year.
Table 5: Overview of measurement goals
The monitoring of the interferometric behavior of the receive chains is ensured by a dedicated interferometric calibration mode that can be used as an operational mode. An additional calibration mode permits the measurement of the amplitude/phase distortions of each receive chain. By using an internal frequency synthesizer, this measurement can be done for several frequencies inside the IF bandwidth. In addition, different gain settings can be used, which makes it possible to accurately determine the gain of the receiver. Either of the receive chain (chain 1 or 2) may be selected in LRM or in SAR support modes. This in-flight capability increases the knowledge of the instrument contribution on the echo measurement.
Figure 48: SAR observation principle of SIRAL (image credit: ESA)
Figure 49: The footprint of the radar beam in the target region (image credit: ESA)
The chirp generator is composed of a digital pulse generation section, operating with a sampling rate of 160 MHz, followed by an analog multiplier section expanding the pulse bandwidth by a factor of 16, up to 350 MHz. This configuration ensures the pulse-to-pulse coherence required for the SAR modes. Parameters: chirp frequency = 4.08 GHz; bandwidth = 350 MHz; signal duration = 51 µs; SNR=30 dB.
The SSPA (Solid-State Power Amplifier) is composed of four parallel hybrid amplifiers in PHEMT technology. Parameters: frequency = 13.575 GHz; peak power >25 W; gain = 9 dB; amplitude ripple = 0.2 dB in 350 MHz; efficiency = 24%. - The FFT (Fast Fourier Transform) module, needed for the on-board tracking algorithm to estimate range and gain commands, makes use of the existing FFT module of POSEIDON 2.
SIRAL operational modes:
SIRAL provides the following operational modes for different observational support types. The complex waveform data stream from the CryoSat altimeter requires a sophisticated processing scheme in particular for exploiting the synthetic aperture and interferometry techniques over ocean and ice surfaces.
1) LRM (Low Resolution Mode) operation support: LRM uses a single receive channel and low PRF for conventional pulse-limited operation for ice sheet interiors/open oceans. The transmitted pulse length and the transmitted bandwidth are set to the same value as that for Envisat in a similar mode (51 s, 320 MHz). The PRF is kept constant over the orbit at a value around 2 kHz to ensure the decorrelation of received echoes. The averaging for tracking and ground processing is performed after the FFT (Fast Fourier Transform).
The LRM mode is useful over surfaces where the topography is homogeneous, at least as large as the antenna footprint of about 15 km. The altimeter echoes have a predictable shape and the mean surface level of this area can be derived by an appropriate model.
2) SARM (Synthetic Aperture Radar Mode) support mode (also referred to as advanced SAR mode): SARM uses a single channel and a high PRF. Closed burst timing is employed to ensure a high along-track resolution. The PRF is chosen higher than the Doppler bandwidth over the half-power beamwidth to avoid aliasing in the ground processing of the data (the PRF is about 10 times higher than that of LRM to ensure coherence between the echoes of successive pulses). Bursts of 64 pulses at a PRF of 18.5 kHz with a burst repetition frequency of 85 Hz are transmitted.
In SARM, the resolution of the radar is improved in the along-track direction. This is achieved by exploiting the Doppler properties of the echoes as they cross the antenna beamwidth. The result is equivalent to decomposing the main antenna beam into a set of 64 narrower synthetic beams in the along-track direction. The footprints of the different sub-beams over a flat surface are adjacent rectangular areas, about 250 m wide in along-track and as large as the antenna's cross-track footprint (up to 15 km). Hence, a larger number of independent measurements are available over a given area; this property is used to enhance the accuracy of the measurements over sea ice. The echoes are transmitted to the ground segment in the time domain, prior to any averaging. Hence, the data rate in SARM is significantly higher than that for LRM.
3) SARIn (SAR Interferometric) support mode. The objective is to provide improved elevation estimates over variable topography. This mode is used mainly over ice sheet margins with high surface slopes. Both receive channels are operating simultaneously at high PRF to ensure the availability of a high cross-track resolution used for ice sheet margins and coastal areas (accurate determination of the arrival direction of the echoes in along-track and in cross-track). This is needed to derive the height of the surface from the range measurement of the radar. Narrow-band tracking pulses, transmitted in-between successive wide-band measurement bursts are used in this range-tracking concept to cope with abrupt height variations.
In the SARIn mode, the addition of the interferometric feature to the SAR further improves the echo localization capabilities, as the cross-track direction angle of the echoes can be determined. This is achieved by comparing the phase of one receive channel with respect to the other.
The innovative technical features of SIRAL are:
• The capability to operate in all measurement modes
• Digital chirp generation with pulse-to-pulse coherence for Doppler processing
• Solid State Power Amplifier (SSPA) in Ku-band with high performance (25 W),
• Dual antennas forming an interferometer, mounted on an optical bench together with star-tracker heads, ensuring the accurate knowledge and stability of the interferometric baseline orientation
• Two receive chains matched together with very low distortions.
The novel feature of SIRAL, as compared with conventional altimeters, is the capability to locate a resolution cell in the 3 dimensional space. The SIRAL concept is based on a Ku-band nadir-looking radar which can be operated in the conventional mode over oceans. Over terrain (ice or land) the "advanced SAR mode" uses Doppler filtering for the enhancement of the along-track resolution. A second antenna and receiving channel provides a second take of the scene which is used for surface height retrieval as it is usually done with SAR interferometry.
Table 6: Summary of instrument parameters for operational mode support
Operations principle: Conventional radar altimeters send pulses with a long interval : about 500 µs. SIRAL sends a burst of pulses with an interval of only 50 µs between them. The returning echoes are thus correlated, and by treating the whole burst of pulses in one operation, the data processor can separate the echo into strips arranged across the track by exploiting the slight frequency shifts (caused by the Doppler effect) in the forward- and aft-looking parts of the beam. The strips laid down by successive bursts can therefore be superimposed on each other and averaged to reduce noise. This mode of operation is called the SARM (Synthetic Aperture Radar Mode). 91)
Figure 50: Schematic view of conventional LRM operations (left) and Delay-Doppler SARM operations of SIRAL (right), image credit: R. K. Raney, JHU/APL
In interferometric mode (SARIn), a second receiving antenna is activated to measure the arrival angle. It enables to receive some radar echos coming from a point not directly located beneath the satellite. The difference in the path-length time of the radar echos is tiny between radar echos on the track and radar echoes out of the track. The measure of the angle between the baseline joining the antennas and the echo direction is essential and must be very accurate. The baseline orientation is so operated by three star trackers.
The Cryosat-2 mission is the first altimeter mission to operate the SARM (SAR mode), next to the LRM (Low Resolution Mode), in the SIRAL (SAT Interferometer Radar Altimeter) instrument.
Figure 51: Artist's view of the CryoSat-2 observation concept (image credit: EADS Astrium)
DORIS (Doppler Orbitography and Radiopositioning Integration by Satellite):
DORIS measures the Doppler frequency shifts of both VHF and S-Band signals transmitted by ground beacons. Its measurement accuracy is better than 0.5 mm/s in radial velocity allowing an absolute determination of the orbit position with an accuracy of 2-6 cm. DORIS is an uplink radio frequency tracking system based on the Doppler principle. The CNES instrument provides accurate measurements for a precise orbit determination. Knowledge of the orbit is essential for exploitation of the altimeter data and the overall performance. The onboard receiver measures the Doppler shift of uplink beacons in two frequencies (2.03625 GHz for Doppler measurement and 401.25 MHz for the ionospheric correction) which are transmitted continuously by the ground stations. One measurement is used to determine the radial velocity between spacecraft and beacon, the other to eliminate errors due to ionospheric propagation delays. The 401.25 MHz frequency is also used for measurements of time-tagging and auxiliary data transmission. The DORIS instrument comprises:
• A fixed omni-directional dual-frequency antenna
• A receiver performing the Doppler measurements every ten seconds. The nominal mode of operation is an autonomously programmed mode in which the receiver tracks the beacon signals according to information provided by the navigation software (DIODE) based on an on-board table of beacon data.
• An USO (Ultra Stable Oscillator) delivering the reference frequency with a stability of 5 x 10-13 over a period of 10 to 100 s.
The mass of DORIS is 15 kg (including the antenna of 160 mm diameter). The instrument requires 20 W of power, the data rate is 4 kbit/s.
The following DORIS services are used for CryoSat operations:
- Real-time orbit determination for spacecraft attitude and orbit control (on-board)
- Provision of a precise time reference based on TAI (International Atomic Time); in addition a precise 10 MHz reference signal is used (on-board)
- Provision of on-ground POD (Precise Orbit Determination) and ionospheric modelling.
The entire DORIS system comprises a network of more than 50 ground beacons, a number of receivers on several satellites in orbit and in development, and ground segment facilities. It is part of IDS (International DORIS Service), which also offers the possibility of precise localization of user-beacons.
LRR (Laser Retroreflector):
LRR is a passive optical device. The objective is to use LRR as an additional tool and backup for precise orbit determination with the aid of the international laser tracking network. LRR is accommodated in the nadir plate of the spacecraft, its FOV of ±57.6º is suitable for range measurements above 20º elevation angles at all azimuths from the ground. For any aspect angle the predicted rms target error is below 6 mm.
Figure 52: Illustration of the LRR system (image credit: ESA)
Table 7: Performance characteristics of one LRR
Introduction of the DDA (Delay Doppler Altimetry) technology into the SIRAL design
The concept of DDA, initially proposed by Keith Raney of JHU/APL (Johns Hopkins University/Applied Physics Laboratory), represents a new technology introduction with the potential to greatly increase the value of observations from satellite radar altimetry. The DDA scheme takes advantage of the Doppler shift of the pulse frequency in the along-track direction to allow for an increase in pulse repetition frequency and a subdivision of the illuminated area along-track into discrete Doppler bins to provide a dramatic improvement in efficiency and precision. 92) 93)
A conventional pulse-limited altimeter independently averages many radar pulses as the spacecraft moves along its track during the averaging time window and its illuminated area becomes defocused with increasing significant wave height. The relatively slow repetition of pulses and the impact of the waves limit the available resolution of the instrument.
Figure 53: Comparison of a conventional pulse-limited radar altimeter's (a) illumination geometry (side view) and footprint (plan view) and (b) impulse response, with a delay/Doppler altimeter's (c) illumination geometry and footprint and (d) impulse response (image credit: JHU/APL)
The DDA (Delay-Doppler Altimeter) differs from a conventional radar altimeter concept in that it exploits coherent processing of groups of transmitted pulses and the full Doppler bandwidth is exploited to make the most efficient use of the power reflected from Earth's surface. While the conventional altimeter technique is to measure the distance between the satellite and the mean ocean surface, the DDA method differs from those instruments in two ways: 94) 95) 96) 97)
- Pulse-to-pulse coherence and full Doppler processing to allow for measurement of the along-track position of the range measurement
- Use of two antennas and two receiver channels that allow for measurement of the across-track angle of the range measurement.
This is a significant improvement over conventional Doppler beam sharpening. To exploit this full bandwidth, the range variation that exists across the Doppler bins is removed as part of the data processing. The reflected pulses from a given area of the observed surface are integrated over the entire time that the target area is within the radar beamwidth. As a result, much more of the reflected energy is captured and a smaller transmitted power is required to obtain a given level of performance.
The DDA concept (Figure 53 b) retains the inherent advantages of a pulse-limited altimeter with its spherical wavefront always providing a nadir component, thus avoiding instrument nadir-pointing errors. In addition, the DDA exploits the faster pulse repetition frequency by binning the Doppler frequency shifts in the along-track direction. These bins appear as narrow strips orthogonal to the satellite ground track. As the DDA moves along its path, the leading edge Doppler bin illuminated during the first pulse becomes the second Doppler bin during the next pulse and receives a second "look" by the instrument. This process repeats as long as the bin remains within the DDA footprint. Each pulse defines a new leading edge Doppler bin, re-samples each bin within the footprint, and integrates the retrievals as the satellite moves along its track. Since each bin is sampled many times, the samples can be coherently processed and the higher pulse repetition frequency provides for a higher resolution footprint along-track that is independent of the significant wave height. For example, a 30 Hz altimeter pulse provides a signal integration length that results in widths of the Doppler bins as narrow as 250 m.
The DDA technology provides several advantages over conventional altimetry. The sea surface height precision available from this type of instrument is approximately twice that of existing sensors. Simulations of the associated signal processing concepts have produced 0.5 cm precision in a calm sea, with precision remaining better than 1.0 cm even in significant wave heights as great as 4 m. The DDA technique is much less sensitive to errors induced by ocean waves. For a calm sea, DDA and conventional altimetry experience comparable levels of random noise; however, as the waves grow, a conventional altimeter experiences a dramatic noise level increase. With the coherent processing of the DDA, only a slight increase in random noise with wave height is experienced. This makes the DDA particularly well suited for geodetic applications where the random error due to ocean waves is the dominant error source. Wind speed and wave height retrievals from the DDA have twice the precision of current sensors.
Another advantage of DDA is the ability to sample the coastal ocean where conventional altimeters experience signal contamination from land. As the spacecraft approaches or departs a coastline where the angle of intersection with the satellite ground track is nearly orthogonal, on board processing can identify individual Doppler bins close to the coast and continue to sample it as the satellite passes over the boundary.
From a system architecture perspective, the efficiency of the DDA provides for less transmitted power by the instrument and the potential for smaller and lighter spacecraft components - and thus a less costly mission - when compared to conventional altimeters with a similar design life.
The CryoSat mission will be operated from ESA/ESOC, Darmstadt, Germany. The Kiruna ground station in Sweden functions as the prime command and data acquisition facility. The payload data segment (data processing, archiving and distribution) function is also located at the Kiruna station. The ground segment makes use of the existing infrastructure. All user interfaces are coordinated via ESA/ESRIN with dissemination of data from Kiruna.
An important aspect of the CryoSat-2 ground segment is that it had been designed for operations with a low level of manpower. Furthermore, remote operations and troubleshooting can be performed on all systems located in Kiruna.
The major elements of the ground segment are:
• RPF (Reference Planning Facility): responsible for the planning of the payload and the satellite resources verification.
• FOS (Flight Operations Segment): responsible for the telecommand scheduling, satellite command and control, and telemetry acquisition.
• PDS (Payload Data Segment): responsible of scientific data processing, archiving and distribution
• MF (Monitoring Facility): responsible for providing measures of the performance of the system, and in particular of the instruments.
• Complementary supporting ESA elements, shared with other missions:
- USF (User Services Facility)
- LTA (Long-Term Archive)
• Other elements outside ESA:
- DORIS control and processing centre (SSALTO) which provides precise orbits
- Auxiliary data providers (e.g. meteorological data)
- SLR (Satellite Laser Ranging) stations.
• User community, calibration, validation and retrieval.
Figure 54: Overview of the ground segment infrastructure (image credit: ESA)
Permanent calibration station for altimeters in Crete with microwave transponder
The Technical University of Crete (TUC) is installing a new permanent microwave transponder ground infrastructure on the Island of Crete, Greece, to serve as an alternative and independent technique for the calibration of, mainly, European altimetric missions. The facility was initially planned as a calibration site for the Sentinel-3 in the south west of Crete, Greece, using the developed transponder. However, this ground infrastructure, along with other permanent facilities in Crete, may also be used for the calibration of other Ku-band altimetric missions such Jason-2, Cryosat-2, etc. 101)
The idea for incorporating land based transponders was initially introduced in 2000. 102) A microwave transponder is an electronic equipment which receives the pulsed radar signal, transmitted by the altimeter of the over-passing satellite and actively amplifies and retransmits the signal towards the spacecraft, where it is recorded. The time delay of the signal is measured, from which the absolute range between the transponder and the satellite can be deduced. The main advantage of this technique, compared to the conventional sea-surface calibration, stands for the fact that no ocean dynamics errors are involved in satellite altimeter's calibration.
However, in the past, only few transponders have been built and implemented for this reason. The ESA premises in Svalbard, Norway host a transponder developed by RAL, UK in 1987 that has been used mainly for the Cryosat-2 calibration. The Gavdos Island Cal/Val facility in Greece hosted the Austrian Academy of Sciences transponder and that transponder has been effectively used for the calibration of Envisat and Jason-2 missions. There has been another transponder placed in Rome, Italy which was used for the Envisat sigma-0 calibration.
In 2011, the Geodesy and Geomatics Engineering Laboratory at the Technical University of Crete in Greece developed a new Ku-band microwave transponder. The TUC transponder is mobile, allowing calibration at different locations but also modular for operating in other frequencies, provided that some parts are modified. It is capable of recording the incoming and outgoing signals, while it can be controlled and operated remotely. The transponder frequency has been selected to be compatible with past, current and future European as well as international altimetry missions that operate in this microwave range (i.e., Jason series, Cryosat-2, Sentinel-3). Additionally, it is equipped with a GPS (Global Navigation Satellite System) receiver and appropriate meteorological sensors to provide precise time-tagging, as well as the atmospheric delay corrections during transponder calibration. This is of importance for the accurate determination of the altimetric range because the atmosphere affects the altimetric measurements. Furthermore, this prototype transponder is the only microwave transponder that incorporates circularly polarized antennas. The latter, allows performing calibration experiments on different satellite missions at the same location, approaching from different directions, providing that the satellite ground track is in a range of 3-5 km away from the transponder location.
Table 8: TUC's transponder radio frequency characteristics
The TUC transponder has been characterized for 4 months (March-July 2012) at the CPTR (Compact Payload Test Range) facilities in ESA/ESTEC, the Netherlands.
The transponder has already been used for the calibration of several Cryosat-2 passes (10-May, 8-June and 3-August 2013) over the SLR2 (Satellite Laser Ranging 2) site (35° 32.084' N, 24° 04.061' E) in North West Crete, Greece, and a clear response has been captured on the satellite's data (Figure 55).
A TUC transponder site has been selected on Crete Island which represents a triple cross-over point between Sentinel-3A, -3B and Jason-2&3 (and also Jason-CS, as it will most likely fly over the same Jason-series tracks). This criterion was used to finally define and freeze the ground tracks for Sentinel-3 mission.
The CDN2 (35° 20.729' N, 23° 46.577'E) site is exactly under Jason, 100 m east of Sentinel-3A and 300 m west of the Sentinel-3B ground tracks. The CNES team will verify the satellite signal observed using Jason-2 around the CDN2 candidate.
Figure 56: A triple cross-over point for Sentinel-3A (red), -3B (purple) and Jason series (yellow) exist at the CDN2 site in western Crete (image credit: TUC)
The instruments at the CDN2 site will be protected using either weather-proof boxes or a container with appropriate covers to avoid/reduce any satellite echoes by their metallic parts. Figure 57 illustrates an indicative spatial distribution of the necessary and ancillary instrumentation to be constructed at the CDN2 Sentinel-3 altimeter calibration site. Besides the instrumentation and infrastructure, the preparatory steps taken for the establishment of the CDN2 site involve also the development of appropriate software for data archival and transmission and for the determination of the transponder's precise positioning.
The Sentinel-3 altimeter calibration site is expected to be fully operational in early 2014, that is about one year prior to the Sentinel-3A launch. During this period, calibration campaigns for the Jason-2 and Cryosat-2 altimetry mission will be performed to test the transponder's operational capabilities in real-field conditions. These campaigns will aim at: a) delivering altimeter calibration values for these satellites, b) getting familiarized with the remote operation procedures to be followed, and 3) identifying potential upgrades necessary for improving the transponder's performance.
The transponder is to be upgraded, improved, and characterized before its final deployment and support for Sentinel-3A commissioning phase in 2015.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).