SMOS (Soil Moisture and Ocean Salinity) Mission
SMOS is an ESA Explorer Opportunity science mission, a technology demonstration satellite project in ESA's Living Planet Program, in cooperation with CNES (France) and CDTI (Center for Technological and Industrial Development), Madrid, Spain. 1) 2) 3) 4) 5)
Known as ESA's ‘Water Mission', SMOS will improve our understanding of Earth's water cycle, providing much-needed data for modelling of the weather and climate, and increasing the skill in numerical weather and climate prediction. One of the highest priorities in Earth science and environmental policy issues today is to understand the potential consequences of modification of Earth's water cycle due to climate change. The influence of increases in atmospheric greenhouse gases and aerosols on atmospheric water vapor concentrations, clouds, precipitation patterns and water availability must be understood in order to predict the consequences for water availability for consumption and agriculture. 6)
The main science objective of the SMOS mission is to demonstrate observations of SSS (Sea Surface Salinity) over oceans and SM (Soil Moisture) over land to advance climatologic, meteorologic, hydrologic, and oceanographic applications. Soil moisture is a key variable in the hydrologic cycle. Over land, water and energy fluxes at the surface/atmosphere interface are strongly dependent upon soil moisture. SM is an important variable for numerical weather and climate models as well as in surface hydrology and in vegetation monitoring. Knowledge of the global distribution of salt in the oceans and of its annual and inter-annual variability, is crucial for understanding the role of the ocean and the climate system. Ocean circulation is mainly driven by the momentum and heat fluxes through the atmosphere/ocean interface, it is dependent on water density gradients, which in turn can be traced by the observation of SSS and SST (Sea Surface Temperature). 7) 8) 9) 10) 11) 12) 13) 14) 15)
Soil moisture can be retrieved from brightness temperature observations. Due to the large dielectric contrast between dry soil and water, the soil emissivity "epsilon" at a particular microwave frequency depends upon the moisture content. At L-band in particular, the sensitivity to soil moisture is very high, whereas sensitivity to atmospheric disturbances and surface roughness is minimal. 16) 17) 18)
Figure 1: Schematic view of Earth's water cycle (image credit: ESA, CNES)
Table 1: Scientific requirements for soil moisture retrieval 19)
For sea water, the dielectric constant is determined by the electrical conductivity and the microwave frequency. The ocean surface emissivity is a function of the dielectric constant and the state of the surface roughness. In principle it is possible to retrieve SSS from brightness temperature observations. - Mission requirements call for typical values to resolve specific phenomena:
• Barrier layer effects on the tropical Pacific heat flux: accuracy of 0.2 psu (practical salinity unit), with a spatial resolution of 100 km x 100 km, and a revisit time of 30 days.
Note: SSS is defined in practical salinity units (1 psu = 0.1%) and ranges from about 32 to 37 psu. In other words, salinity describes the concentration of dissolved salts in water; the psu value expresses the conductivity ratio. The average SSS is 35 psu, which is equivalent to 35 grams of salt in 1 liter of water. The sensitivity of brightness temperature to salinity is about 0.5 K/psu at a water temperature of 20ºC, decreasing to about 0.25 K/psu at 0ºC.
Salinity links the climatic variations of the global water cycle and ocean circulation: 20)
- Salinity is required to determine seawater density, which in turn governs ocean circulation
- Salinity variations are governed by freshwater fluxes due to precipitation, evaporation, runoff and the freezing and melting of ice.
• Halosteristic adjustment of heat storage from the sea level: 0.2 psu, a spatial resolution of 200 km x 200 km, and a repeat cycle of 7 days
• North Atlantic thermohaline circulation: 0.1 psu, a spatial resolution of 100 km x 100 km, and a repeat cycle of 30 days
• Surface freshwater flux balance: 0.1 psu, a spatial resolution of 300 km x 300 km, and a revisit time of 30 days.
Background on SMOS development: Twenty-six years after the first attempt to retrieve soil moisture from space (Skylab L-band radiometer experiment in 1973, referred to as S-194) and following seven years of technology development at ESA (since 1992), the SMOS Earth Explorer Mission was selected for implementation in November 1999 by ESA's Program Board for Earth Observation (PB/EO). Since then, a successful Phase A feasibility study (2000-2001) and a Phase B (2002) for further definition and critical breadboarding have been completed (the Phase B payload design was completed in Oct. 2003). Approval for full implementation was given in Nov. 2003. The SMOS project is now well consolidated, the payload implementation Phase C/D started in mid-2004. The CDR (Critical Design Review) of the payload took place in Nov. 2005. Delivery of the fully tested payload PFM (Proto-Flight Model) to Alcatel Cannes is scheduled for the end of 2006. 21)
In addition to SMOS, the SAC-D/Aquarius mission is currently under joint development by NASA and CONAE (Argentinian Space Agency). Aquarius will follow up the successful Skylab demonstration mission and employs a combined L-band real-aperture radiometer with an L-band scatterometer. The combined measurements will be focused on measurement of global sea-surface salinity. A launch of SAC-D/Aquarius is scheduled for 2010. Aquarius will cover the oceans in 8 days with a spatial resolution of 100 km, though its sensitivity to salinity will be better than that of SMOS due to its different design.
The SMOS satellite uses the generic Proteus bus developed by CNES and Alcatel Alenia Space (formerly Alcatel Space Industries). This standard platform has been designed to accommodate a wide field of missions, orbits, attitudes, instruments, and launch vehicles. Proteus has simple, well defined interfaces. The platform architecture is generic. Adaptations are limited to minor changes in software modules and launch vehicle interface.
The S/C bus is a box, nearly 1 m per side, with all the equipment units accommodated on four lateral panels and the lower plate. The platform TCS (Thermal Control Subsystem) relies on passive radiators and active regulation with heaters. Electrical power is generated by two symmetric wing arrays with single-axis step motors. Each wing is composed of four deployable panels (1.5 m x 0.8 m) covered with silicon cells which provide 685 W orbital average after 3 year mission (EOL). The power is distributed through a single non-regulated primary electrical bus (23/36 V) using a Li-ion battery.
Figure 2: Illustration of the deployed SMOS spacecraft (image credit: ESA)
The S/C is three-axis stabilized consisting of a PSM (Proteus Service Module) and a PLM (Payload Module). Typical pointing performance of better than 0.05º (3 σ) is provided by a control system with four reaction wheels and gyro-stellar attitude determination [attitude is provided by two STA (Star Tracker Assembly)]. Coarse sun sensors (8) and two 3-axis magnetometers provide attitude measurement and magnetic torquers generate torque. In addition, two of the four reaction wheels are used to provide gyroscopic stiffness. A GPS receiver provides S/C location data for accurate orbit determinations and onboard time delivery. 22) 23)
Due to the high variety of pointings to be handled with (earth pointing, yaw steering motion, inertial pointing, etc.) the AOCS concept has been based from the beginning on a gyro stellar hybridation with reaction wheel actuators unloaded through magnetotorquer bars, while safe hold mode only relies on kinetic momentum, coarse sun sensors and magnetic sensors and actuators, without the use of the four 1N thrusters in blow down mode limited to orbit control manoeuvres. This design has a proven robust behavior on all the LEO orbits which have been used in the realized missions.
The onboard command and data handling relies on a fully centralized architecture. The DHU (Data Handling Unit) performs most of the tasks through the central processor running the satellite software. It also supports the management of the communication links with all the satellite units either via discrete point-to-point lines or via a MIL-STD-1553B bus.
Figure 3: Schematic view of the command and control architecture (image credit: CNES)
SMOS is designed to operate mostly in an autonomous mode using the FDIR (Failure Detection Isolation and Recovery) concept (this permits to reduce drastically the working hours per day from the ground). The S/C bus is designed to operate in five distinct satellite modes: 1) normal autonomous operations mode, 2) safe hold mode, 3) star acquisition mode, 4) orbit correction mode with 2 thrusters, and 5) orbit correction mode with 4 thrusters. The SMOS spacecraft has a total mass of 658 kg (bus dry mass of 275 kg, 28 kg of hydrazine, four 1 N thrusters, payload module (PLM) of 355 kg). The design life is three years with a goal of five years.
The SMOS spacecraft features an attitude in which the boresight of the antenna is forward tilted by 32.5º with respect to nadir. This configuration enables measurements at line-of-sight angles between 0º - 50º. The satellite employs yaw steering about the local normal.
Figure 4: The stowed SMOS S/C with the bus at bottom and payload module on top (image credit: ESA)
Table 2: Overview of SMOS spacecraft parameters
TCS (Thermal Control Subsystem): The TCS is required to maintain all the payload equipment (MIRAS) within the specified temperature range with minimum heater power consumption. The most challenging requirements in operation are relevant to the stringent temperature control of the LICEF (Lightweight Cost-Effective Front-end) receivers. The TCS is based mainly on a passive design, supported by heater systems. All six LICEF receivers in each segment and the eighteen LICEF receivers on the Hub are installed on an aluminum doubler to minimize the gradients among them. 24)
Figure 5: Block diagram of the SMOS-MIRAS electrical architecture (image credit: ESA)
Legend to Figure 5: The TCS has two separate parts: (1) HM used for Measurement and Calibration Modes and controlled through CCU-CMN (Correlator and Control Unit-Coontrol and Monitoring Node) chain, and (2) HS used for SHM (Safe Hold Mode) and controlled through Proteus platform.
• Passive Thermal Control Design: The passive thermal control hardware incorporates: FSSM (Flexible Second Surface Mirror coatings) for thermal radiators, MLIs (Multi-Layer Insulation Blankets), Germanium coated black Kapton foil, black paint, aluminized tapes/low emissivity surface treatments, thermal doublers, interface fillers and thermal washers.
• Active Thermal Control Design: The two heater systems in the payload are named HM and HS (Figure 5).
- Electrical resistance heaters (HM in Figure 5) are installed on thermal doublers, on CMN Units and on segments structure and they are powered through CMN commands. This heater system is used to control the electronic equipment temperature in the instrument measurement/calibration modes and in PLM Off modes.
- The HS heaters (Figure 5) are installed on thermal doublers, on CMNs, on CCU, on X-band transmitter, on pyro unit, and on Lower Platform Optical Splitter. The HS heaters are powered from Proteus and controlled by thermostats. This heater system is used to keep equipment temperatures above the minimum non-operational limits (–20 ºC for most of the units) during satellite SHM and PLM Off modes. The system is fully redundant.
The TCS, as well as the rest of the payload design, has a distributed architecture. The central computer of the payload controls in closed loop remotely distributed units (12 in total) named CMN (Control and Monitoring Node) units. Each CMN unit acquires the telemetry of the temperature sensors (6 per heater line) for the heater control lines distributed in the Arms and in the Hub. The TCS is enabled during all payload operational modes including measurement and calibration.
Launch: The SMOS spacecraft was launched on November 2, 2009 on a Rockot launch vehicle (the 3rd stage of Rockot is Breeze-KM) of ELS (Eurockot Launch Services) from the Plesetsk Cosmodrome, Russia. The first burn of Breeze-KM is to acquire an elliptical transfer orbit. The second burn serves to circularize the orbit to its nominal parameters. A secondary payload on this flight is the PROBA-2 spacecraft of ESA. 25) 26) 27)
Some 70 minutes after launch, SMOS successfully separated from the Rockot's Breeze-KM upper stage. Shortly thereafter, the satellite's initial telemetry was acquired by the Hartebeesthoek ground station, near Johannesburg, South Africa. The upper stage then performed additional maneuvers to arrive at a slightly lower orbit and PROBA-2 was released too, some 3 hours into flight.
Note: The SMOS satellite has been in storage at Thales Alenia Space's facilities in Cannes, France since May 2008 awaiting for a third stage of the Rockot launcher to be assigned to the mission and a slot given for launch from the Russian Plesetsk Cosmodrome. SMOS is the second of ESA's Earth Explorer missions to launch after the GOCE (Gravity field and steady-state Ocean Circulation Explorer), which was launched on March 17, 2009.
Orbit: Sun-synchronous polar orbit, mean altitude = 755 km, inclination =98.44º, local equator crossing time at 6:00 AM on ascending node maintained within ±15 minutes, period of 100 minutes. The repeat cycle is 23 days with a 3 day subcycle. 28)
RF communications: An onboard solid-state recorder has a capacity of 3 Gbit for payload and TT&C data. Standard TT&C S-band communications are used (the downlink data rate is 722.116 kbit/s with QPSK modulation; the uplink has a data rate of 4 kbit/s). The CCSDS protocol is used for TT&C support. - The TT&C station is located in Kiruna (Sweden), operated by CNES (mission operations at CNES). Science data acquisition is in X-band at a data rate of 18.4 Mbit/s, the ground station is located at Villafranca, Spain.
Figure 6: Artist's view of the SMOS flight configuration (image credit: CNES, ESA)
Figure 7: Artist's view of the deployed MIRAS payload (image credit: ESA-AOES Medialab)
• On November 2, 2014, the SMOS spacecraft was 5 years on orbit to improve our understanding of Earth's water cycle. Marking its fifth birthday, all the data collected over land and ocean have been drawn together to show how moisture in the soil and salinity in the ocean change over the year. 29)
- Carrying a novel sensor, it captures images of ‘brightness temperature'. These images correspond to microwave radiation emitted from Earth's surface and can be related to soil moisture and ocean salinity – two key variables in Earth's water cycle.
- Over oceans, measurements from SMOS, the longest continuous record from space, show monthly differences in sea-surface salinity with respect to the average salinity – and show large deviations in the tropical Pacific Ocean and in the Indian Ocean. This is linked to the occurrence of La Niña, which is associated with cooler than normal sea-surface temperatures in the eastern Pacific, and the Indian Ocean Dipole, which describes sea-surface temperature differences between the eastern and western equatorial Indian Ocean.
- While these results are of interest to understanding aspects of the water cycle, information from SMOS is being used for a number of practical applications. In fact, 18 TB of SMOS data are distributed every year, of which around 13 TB are used by scientists and around 5 TB for near-realtime applications by operational users.
- For example, integrating these accurate near-realtime observations into the ECMWF (European Center for Medium-Range Weather Forecasts) system is helping to improve air temperature and humidity forecasts near the surface. In addition, the inclusion of SMOS observations are helping to improve the prediction of rain.
- This is particularly true for the southern hemisphere, where the number of conventional in situ observations is comparably low. In fact, ECMWF is processing and quality-controlling SMOS data operationally to realize some 600 images every day, totalling more than one million figures since the satellite was launched. - Applications include forecasting river runoff, monitoring drought and forecasting crop yield. Again, the novel satellite measurements are most beneficial over regions lacking dense on-ground observation networks.
• September 2014: The SMOS mission operations have been extended to 2017. Both ESA and CNES, jointly operating the mission, have confirmed the extension in their respective reviews. New challenging objectives have been set for this mission extension, focusing on the: 30)
- Synergistic scientific exploitation of SMOS data with other in-orbit and future Earth observation missions, e.g., Aquarius, SMAP, Sentinels.
- Need of enhanced process understanding on time-scales exceeding the initial mission lifetime, e.g. for geophysical phenomena like El Niño and El Niña or droughts, and merged data products of SMOS data with other L-band observations for the generation of long-term data sets and thematic data records, given that soil moisture and ocean salinity have been identified as ECV (Essential Climate Variable).
- Pre-operational need for continuous observation data sets: Operational and scientific users have expressed the need for data continuity in (semi-) operational applications following the maturation of the SMOS data products.
The SMOS instrument – MIRAS – is operating nominally with the exception of some known on-board anomalies. The cumulative data loss due to instrument unavailability since the beginning of the routine operations phase in May 2010 amounts to 0.09% and the degraded data amounts to 1.07% (Ref. 30).
• July 2014: SMOS is not only delivering key information on soil moisture and ocean salinity for science, but its data are also being used for a growing number of practical applications. Reflecting this versatility along with new synergistic opportunities, the mission will now remain operational until at least 2017. Currently, the SMOS mission is in its 5th year on orbit. 31)
- Going way beyond its original scientific brief of delivering critical information to understand Earth's water cycle, it continues to demonstrate its suitability for new uses. The most recent examples from this multi-talented mission include being able to provide information to measure thin ice floating in the polar seas accurately enough for forecasting and ship routing. Sea ice that is less than 50 cm thick is particularly important for weather and climate as it controls the exchange of heat and water between the ocean and atmosphere.
- While it wasn't designed to measure ice, radiation emitted by the ice allows SMOS to ‘see' through the surface, yielding ice-thickness measurements down to 50 cm – mainly the thinner younger ice at the edge of the Arctic Ocean. In recent years, this information has been much sought after by operational users. Taking data from SMOS, a product has been developed by the University of Hamburg and, with ESA's help, has now been set up as a service.
- By piggybacking on Germany's IRO-2 (Ice Forecast and Route Optimization) project, through which a prototype system for sea-ice forecasting and ship routing is being developed, ESA carried out the SMOS-ice field campaign earlier this year to validate the new data product.
Figure 8: Animation of the Laptev Sea ice thickness from ESA's SMOS satellite (image credit: University of Hamburg Institute of Oceanography)
- Based on achievements such as these and the fact that the satellite is still in very good health, ESA's Member States and the French space agency, CNES, which is responsible for operating the satellite platform, have decided to extend the mission's original planned life of five years. The extension gives the project more opportunities to look at new scientific and pre-operational applications which otherwise wouldn't have been done. Moreover, new synergies will now be possible. For example, SMOS data can be combined with those of NASA's SMAP (Soil Moisture Active Passive) mission, that will be launched in November of 2014. There are also opportunities to combine SMOS with the data from the Copernicus Sentinel missions. For example, the sea-surface salinity data from SMOS could be used in synergy with sea-surface temperature and sea-surface height information from Sentinel-3 (Ref. 31).
• May 2014: ESA's SMOS mission has gone beyond its original scientific brief of delivering critical information for understanding the water cycle – this versatile satellite is now being used to predict drought and improve crop yield in regions prone to famine. 32)
- The US Department of Agriculture (USDA) uses satellite images and soil moisture data to help identify abnormal weather that may affect the production and yield of crops. Using this information, they publish monthly estimates of world production, supply and distribution. — This data offers traders and commodity markets a source of unbiased information. The estimates provide decision-makers with critical information for countries that may need food aid as a result of severe droughts.
- Identifying when and where there may be a risk of famine involves measuring soil moisture in the ‘root-zone' during the growing season, and detecting the onset and severity of drought. Analysts use information linked to drought from a range of observing systems to compile these crop production forecasts.
- Through SMOS, the USDA service obtains timely information on soil moisture patterns, which help to predict how the health of plants will change and, therefore, how productive they will be. Testing the SMOS readings for this purpose, they received very positive feedback from analysts in southern Africa. This is a challenging area because there are very few working rain gages. — The new product is available on the Crop Explorer website of USDA. 33)
• May 2014: The TCS (Thermal Control Subsystem) for MIRAS, comprising 69 single antenna elements, has stringent thermal requirements. They were for the receivers of the 69 antennas to guarantee the validity of the scientific data: the target temperature of the receivers is 22ºC with a maximum spatial gradient of 6ºC among all of them and a maximum orbital excursion of 4ºC (Ref: 24).
The results are:
- Fulfilment of thermal requirement for LICEF receivers: THe LICEF receivers temperature evolution does not show any degradation of the thermal control loop. The average temperature of all LICEF receivers is centered at 22ºC showing a seasonal variation. Dispersion between receivers meets the requirement of < 6 ºC except marginally on the equinoxes. Orbital temperature excursions for LICEF receivers are < 4 ºC, in some cases as low as 1ºC.
- Thermal seasonal variations for LICEF receivers: LICEF receivers show a seasonal variation depending on the month (due to sun illumination), and cycles are identified with changes in start and end of eclipse periods and also when the sun elevation and azimuth angles are nearly constant.
- Thermal seasonal variations for NIR antennas: NIR antennas have thermal fluctuations in the range –5 ºC to 30ºC, varying much according to the external environment. Temperature seasonal variations occur due to sun illumination, as observed also for LICEF receivers.
- NIR antennas cooling effect during first 8 months of the mission: A cooling effect of the NIR antennas took place. At the beginning of the mission, the temperature range of the units was 4ºC to 27ºC, while after 8 months it was –5 ºC to 30 ºC. Stabilization of components in the space environment appears to be the reason for this effect.
- NIR_AB antenna temperature jump: A sudden temperature jump took place on the antenna NIR_AB, and it recovered by itself in a few weeks. This quite unusual event was due to a pure thermo-mechanical effect in the ground plane of the antenna (the structure around the antenna which is made of several layers of material).
- In-orbit failure of B1 segment temperature readings: One sector of the Y-shape antenna (Sector B1) had a permanent failure in January 2011, and the redundancy of Arm B was changed to continue the operations. The failure was due to a chip malfunctioning in the Thermal Acquisition Board of the CMN that controls sector B1.
- Alternative thermal control for B1 segment: It could be possible to return to the side of Arm B where a failure of Sector B1 occurred. It can be accomplished by using an Alternative Thermal Control, that uses the temperatures measured from thermistors in the instrument that have similar evolution of those in the segment that failed. It was shown that the quality of the scientific data is within acceptable ranges when this type of Alternative Thermal Control is used (Ref: 24).
• January 2014: The SMOS mission is operating nominally, continuing to provide global soil moisture and ocean salinity data.
Most interesting results were obtained by the DOME-Cair (airborne) field campaign that was carried out in January 2013 (12 day campaign) at the Concordia research station at Dome-C in Antarctica to validate data from ESA's SMOS and GOCE missions (Ref. 90). This campaign revealed a remarkable similarity between spatial patterns observed by the microwave and gravity instruments (see the chapter below: SMOS preparatory and in-flight validation campaigns). 34)
• December 2013: The SMOS instrument – MIRAS – is operating nominally with the exception of some wellknown on-board anomalies. The cumulative data loss due to instrument unavailability since the beginning of the routine operations phase in May 2010 amounts to 0.11% and the degraded data amounts to 1.42%. 35)
- No data loss has occurred during the acquisition of MIRAS raw data at the ground stations since the beginning of the routine operations phase in May 2010. This result has been achieved by implementing an on-board data recording overlap strategy.
• In May 2013, ESA is reporting that the RFI (Radio Frequency Interference) situation with regard to the SMOS mission keeps improving in particular over Europe and North America, which has greatly improved the seasurface salinity data over the northern hemisphere above 60° latitude. 36)
• April 2013: The saltiness of the oceans is being closely monitored from space by both ESA's SMOS and NASA's Aquarius missions, but in slightly different ways. By joining forces, researchers are exploiting these complementary missions to benefit climate science even further. 37)
Salinity is controlled largely by the balance between evaporation and precipitation, so it is an important component of Earth's water cycle and closely coupled to weather and climate. It is also an important driver in ocean circulation, which in turn, is crucial in moderating the climate. - In fact, ocean salinity is an 'essential climate variable' – a key parameter of climate change.
ESA's SMOS satellite and NASA's Aquarius sensor, carried on Argentina's SAC-D satellite of CONAE, both use an L-band radiometer to map ocean salinity but offer different resolutions and revisit times. - For example, Aquarius provides better ‘pixel' accuracy than SMOS, whereas SMOS provides higher revisit times and spatial resolution.
While it has been shown clearly that their datasets agree and provide similar information (Figure 9), the differences in the data can be exploited to yield even more detail about variations in the salinity of our oceans.
The benefits and complementarity of both missions were discussed by scientists of both projects at: SMOS & Aquarius Science Workshop, IFREMER, Brest, France, April 15-17, 2013. 38)
Figure 9: Global salinity maps from SMOS (top) and Aquarius (bottom), image credit: IFREMER, ESR, ESA, NASA
• Feb. 22, 2013: New results unveiled at ESA's ESAC (European Space Astronomy Centre) in Villanueva de la Cañada, near Madrid, Spain show that SMOS is now providing new insights into the movement of the Gulf Stream – one of the most intensely studied current systems. -Originating in the Caribbean and flowing towards the North Atlantic, the current plays an important role in the transfer of heat and salt, influencing the climate of North America's east coast and Europe's west coast. 39)
Salinity observations from SMOS show that warm, salty water being carried north by the Gulf Stream meets the colder, less-salty water transported southward along North America's east coast by the Labrador Current, mixing the water masses off Cape Hatteras (NC, USA). SMOS can distinguish between and follow the resulting eddies that are ‘pinched off' from the current and form little parcels of warm and salty water in the Labrador Current, and the colder, fresher water in the Gulf Stream.
• November 2012: The SMOS spacecraft and its payload are in very good shape technically at the end of their nominal mission lifetime (3 years in Nov. 2012); thus, they are able to continue to provide data beyond 2012. 40)
New data products provided by SMOS have been opening the door for novel applications. Soil moisture and sea-surface salinity are two variables in Earth's water cycle that scientists need on a global scale for a variety of applications, such as oceanographic, meteorological and hydrological forecasting, as well as research into climate change.
Moisture and salinity decrease the ‘emissivity' of soil and seawater respectively, and thereby affect microwave radiation emitted from Earth's surface. To observe soil moisture over Earth's landmasses and salinity over the oceans, SMOS effectively measures the microwave radiation emitted from Earth's surface. MIRAS picks up faint microwave emissions to map levels of moisture in the ground and the saltiness of the oceans, and these are provided as 2D images, or ‘snapshots', of ‘brightness temperature'.
SMOS data are currently being used in weather and flood forecasting. Drought monitoring systems based on SMOS observations are under development, as well as crop yield forecasting systems. Each of these areas is highly relevant for our everyday lives. Improved weather forecasts, especially of extreme events, can potentially help mitigate impacts and save billions of euros.
Recently, new data products provided by SMOS have been opening the door for novel applications. For example, it was found that the freeze-thaw cycle of soils can be determined, and this could result in the improved monitoring of the active surface layer in permafrost regions and a more accurate description of the gas exchange between land surfaces and the atmosphere, which is highly relevant for our climate system.
Soil moisture is a key variable determining the exchange of water and energy between the land surface and the atmosphere. SMOS soil moisture data have their own scientific value, because they provide an independent estimate of the current state of the land surface. Monitoring the spatial and temporal global dynamics of soil moisture is important for a variety of applications, such as water resources management, weather forecasting, agriculture, flood prediction and climate research. An additional socio-economic value of these Earth observation data comes from their use in forecasting systems and the subsequent decision-making process.
Early studies by ECMWF (European Centre for Medium-Range Weather Forecasts) indicated that the use of satellite-derived soil moisture estimates reduces the errors in the temperature and humidity forecasts and influences a variety of weather parameters.
The first soil frost depth datasets based on SMOS observations were derived by the Finnish Meteorological Institute for two consecutive winter periods starting in 2010. Key elements, such as the southward progression of soil freezing in autumn and early winter, were monitored successfully as well as the late onset of winter in northern Europe in 2011.
These results are potentially interesting for climate applications because they could enhance our understanding of the temporal behavior of the active layer in permafrost regions and the gas exchange process. - More research involving the climate modelling community will be dedicated to this topic over the next few years when multi-year time-series of SMOS observations will allow the analysis of seasonal and inter-annual variations (Ref. 40).
Figure 10: SMOS RFI distribution worldwide, with indication of strength (status: May 30, 2012), image credit: ESA
• July 2012: Over a dozen radio signals, that have hindered data collection on ESA's SMOS water mission, have been switched off. The effort also benefits satellites such as NASA's Aquarius mission, which measures ocean salinity at the same frequency. 43)
It may be recalled, that the SMOS mission immediately after launch revealed that many unlawful signals were being transmitted around the world in this frequency range, rendering some of its measurements unusable for scientific purposes. Over the years, ESA has investigated exactly where the interference is coming from. As national authorities have collaborated with ESA to pinpoint the origin and switch these unlawful emissions off, the interference has waned. - One of the largest areas of contamination in the northern hemisphere is over the North Pacific and Atlantic oceans, primarily from military radars.
At least 13 sources of interference have now been switched off in the northern latitudes. This has significantly improved SMOS observations at these high latitudes, which were previously so contaminated that accurate salinity measurements were not possible above 45 degrees latitude as the satellite headed north (Ref. 43).
Legend to Figure 11: The two images show the RFI at northern latitudes in February 2011 and February 2012. Several radars are observed (the red ‘dots', visible because they exceed the natural variability for brightness temperature measurements over land) over Northern Canada and at the southern tip of Greenland. The authorities from Canada and Greenland were informed, and requested to take actions. Canada started to refurbish their equipment in autumn 2011, while Greenland switched off their transmitters in March 2011.
• In December 2011, ESA is reporting that the data of SMOS are being analyzed by FMI (Finnish Meteorological Institute) scientists to detect and map frozen soils. Not only can the extent be mapped, but also the depth of the frozen layer can be inferred. 44)
- ESA's SMOS mission is proving to be extremely versatile. Not only does this pioneering satellite offer crucial data on soil moisture and ocean salinity, but it can also map the thickness of ice floating in the polar seas. The measurement of brightness temperature of the SMOS MIRAS instrument can also be used map the extent and thickness of sea ice. The Institute of Oceanography at the University of Hamburg has developed algorithms to interpret the MIRAS data accordingly. This new information will be very useful in monitoring ice as it grows in the winter and recedes in the spring. It will be used to investigate the exchange processes to improve our understanding of warming in the Arctic region. 45)
• First instrument performance assessment in the summer 2011: For SMOS, being a truly Earth Explorer mission carrying a new and innovative payload, the learning slope within its first 1½ years on orbit has been pretty steep. The consolidation of the calibration techniques and strategy, as well as of the baseline image reconstruction algorithms, the selection of the operational mode of operation, the assessment of the technical performance of the instrument, the initial validation of the scientific products and the ways to handle the encountered RFI (Radio-Frequency Interference) from illegal ground transmissions have all been accomplished. It can be concluded that presently SMOS is very close to fulfilling its objective of 4% volumetric soil moisture accuracy, while is achieving 0.3-0.5 psu sea surface salinity accuracy to be compared against a more stringent 0.1 psu mission goal. New applications as permafrost and thawing monitoring in the boreal forest, ice thickness determination in the Arctic Ocean and hurricane wind observations have been demonstrated. 46)
Nonetheless, there is still much work to be done, as a more precise assessment of the accuracy in the 3rd and 4th Stokes parameters, the full understanding of the cold sky images, and the further improvement of the image reconstruction at Level-1 and the scientific parameters retrieval algorithms at Level-2, just to broadly name the main tasks.
MIRAS instrument performance: The in-orbit radiometric performance of MIRAS is summarized in Table 4. It has been assessed by comparing SMOS measured brightness temperature images against:
- (a) the sky map surveyed by terrestrial radio-telescopes
- (b) the modelled brightness temperature of a uniform portion of the Pacific Ocean and
- (c) Antarctica brightness temperature, simply assuming that this is a time-space invariant target.
Of the three targets, the cold sky is the best known but it is imaged in non-nominal pointing and thermal conditions. On the other hand, the Pacific Ocean and Antarctica are observed in nominal conditions but some effects may be mismodelled, as the galactic glint, the sea surface roughness or the ice sheet reflectivity changes due to surface wind-ice interaction, to name a few.
Table 4: MIRAS in-orbit radiometric performance
Overall, MIRAS fulfils specifications except in two cases: (a) the systematic error over the ocean and (b) the long term stability. The systematic error consists of a stable spatial ripple error across the image, mainly coming from an imperfect knowledge of the antenna patterns and residual calibration errors. Reaching several Kelvin this spatial error is a major problem in the retrieval of sea surface salinity.
The long term stability of MIRAS needs still to be better assessed, and probably, improved. When evaluated using the cold sky, the instrument long term stability is within the desired limit of 0.18 K/year (this limit is to be consolidated with the oceanographers), but when measurements are compared against the ocean model or Antarctica (assuming this target is time-space invariant) then a drift of 0.49 K or 1.46 K is obtained, respectively. It is difficult to quantify how much of this drift is due to mismodelling of the ocean and Antarctica views and what fraction is truly due to instrument variation. Furthermore it will be necessary to accumulate several years of data before the seasonal variations can be separated from long term variations.
MIRAS evolution: Some clear improvements to MIRAS have been identified based on the in-orbit experience, to achieve better radiometric accuracy, precision and robustness against RFI. These improvements could be applied to a future SMOSops follow-on operational mission. The main modifications which have been identified are:
1) Enlargement of the alias-free field of view by reducing the spacing between elements
2) Hexagonal array instead of a Y-shaped array to improve significantly side lobe levels against RFI
3) Parallel H-V channels with higher sampling rate to improve sensitivity
4) Stable Local Oscillator units to minimize relative phase fluctuations
5) All LICEF operation to simplify calibration and instrument operation
6) Centralized CAS system using opto-electronics to simplify calibration
7) Improved thermal design of antennas and overall system to improve long term stability
8) Shortened integration time and increased downlink date rate to become more robust against RFI.
It is estimated that such MIRAS evolution would allow improving the sensitivity by a factor of 2, at least, from SMOS, achieving 1000 km alias-free swath, at 40 km spatial resolution, in a truly polarimetric mode and with better radiometric accuracy (Ref. 46).
• RFI (Radio Frequency Interference) problem in June 2011: A major international effort to shut down radio signals that have, at times, been blinding the instrument (MIRAS) on ESA's SMOS water satellite is producing a marked improvement in the quality of the mission's data. MIRAS operates in the L-band spectrum (1400–1427 MHz), capturing snapshots of ‘brightness temperature' that correspond to microwave radiation emitted from Earth's surface. From this information, the amount of moisture held in the surface layers of soil and salinity in the surface waters of the oceans can be derived. 47)
According to radio regulations set by the ITU (International Telecommunications Union), the 1400–1427 MHz frequency range is allocated to the Earth Exploration Satellite Service, space research and radio astronomy – other transmissions in this band are prohibited. - However, soon after SMOS was launched, the data revealed there were many signals being transmitted within this protected passive band, rendering some of the data unusable for scientific purposes.
In June 2011, SMOS is well on the way to meeting its research objectives over areas free of this radio-frequency interference. However, the mission has clearly not been reaching its full potential because significant amounts of data have had to be discarded.
• On Feb. 7, 2011, following the return to nominal operations on January 12 of the MIRAS instrument after the onboard anomaly experienced from December 31, 2010 had been resolved, the necessary recalibration activities have now been completed and nominal data quality restored from 7 February 2011 at 09:40 UTC. 48)
• January 24, 2011: Following the anomaly related to the anomalous temperature readings in one segment of antenna arm B, the SMOS instrument MIRAS is now back to nominal operations as of 12 January 2011. - To return to nominal data production re-calibration activities are necessary to ensure the quality of the SMOS data delivered to the user, which are presently performed and assessed. 49)
SMOS experienced a sequence of anomalies related to the temperature readings on one of the antenna segments of arm B since January 5, 2011. The interruption of the (correct) temperature readings has made the SMOS instrument MIRAS temporarily unavailable for providing scientifically meaningful data.
• On November 2, 2010, SMOS celebrated its first year on orbit. All data (brightness temperatures (level-1) and soil moisture and ocean salinity data (level-2)) have been released to the science community at large. 50)
• 2010: The first global map of both soil moisture and ocean salinity that was delivered by the SMOS Earth Explorer. By consistently mapping soil moisture and ocean salinity, SMOS is advancing our understanding of the exchange processes between Earth's surface and atmosphere, and also helping to improve weather and climate models. 51)
Figure 12: Global SSS (Sea Surface Salinity) and SM (Soil Moisture), image credit: ESA
Legend to Figure 12: For the first time, data on soil moisture and sea-surface salinity have been combined on one map. The map was generated combining global measurements acquired in August 2010. The SSS data have a spatial resolution of 1º x 1º. Comparison with in situ measurements in the oceans averaged over 1º x 1º in August reveal a quasi-global accuracy of 0.4 practical salinity units, which is very promising after just one year in orbit.
• RFI (Radio Frequency Interference) problem in the fall of 2010: Although the results of the SMOS mission have been impressive so far, the observations of the MIRAS instrument, operating in the L-band in the range of 1400-1427 MHz, have been bugged by patches of interference from radar, TV and radio transmissions in what should be a protected band. Painstaking efforts to reduce these unwanted signals are now paying off. 52) 53) 54)
Soon after the SMOS (Soil Moisture and Ocean Salinity) mission was launched, it was realized that, in some places, the data were being badly contaminated by RFI. At times, this interference was effectively blinding the instrument, rendering the data over certain areas unusable. Nevertheless, SMOS was still clearly meeting its scientific requirements in areas free of RFI. However, to maximize the benefits of the mission the RFI issue needed to be addressed, which could only come about through international collaboration.
The MIRAS instrument revealed that there were many incidences of signals within this ITU (International Telecommunications Union) protected band, particularly in southern Europe, Asia, the Middle East and some coastal zones - that violate the ITU standards. The main culprits appear to be TV transmitters, radio links and networks such as security systems. Also terrestrial radars appear to cause interference. ESA embarked upon the tricky and lengthy process of having the illegal transmissions shut down and the excessive out-of-band emissions reduced (Ref. 52).
Figure 13: Soil moisture 3 days synthesis, August 14-16, 2010 (image credit: CESBIO) 55)
• The SMOS mission has been delivering observations of 'brightness temperature' to the science community since mid-July 2010. As a measure of radiation emitted from Earth's surface, this information can be used to derive global maps of soil moisture every three days and maps of ocean salinity at least every 30 days. 56)
• During the week of May 21, 2010 ESA's SMOS satellite completed its six-month commissioning and formally began operational life. This milestone means the mission is now set to provide much-needed global images of soil moisture and ocean salinity to improve our understanding of the water cycle. - The SMOS Team came to this result after a three-day review meeting in Avila, Spain. 57)
• During the spring of 2010, the HUT-2D (Helsinki University of Technology 2D Radiometer), an airborne 1.41 GHz interferometric radiometer of the Aalto University (Espoo, Finland) was used to measure three SMOS validation target areas, one in Denmark and two in Germany. In addition to the HUT-2D radiometer system a novel L-band polarimetric radiometer EMIRAD (Electromagnetics Institute Radiometer) of DTU (Technical University of Denmark) was installed into the research aircraft. The radiometer system includes two fully polarized receivers. One of the EMIRAD's antennas one pointed to nadir and the other one is tilted 40º off nadir. The half power beam width of the antennas is approximately 36º. The test areas were: 58)
- The Upper Danube Catchment (UDC), a temperate agricultural area situated mostly in Southern Germany, is one of two major SMOS validation test sites in Europe covering 77.000 km2.
- The second test site in Germany, referred to as REC (Rur Erft Catchment), encompasses the catchment basins of the rivers Rur and Erft, which are located in the Belgian-Dutch-German border region near the city of Aachen.
- The third test site is situated within the Skjern river catchment in Western Denmark (ca. 2500 km2) at short distance to the coast line.
• After mid-Feb. 2010, in less than four months since launch, the first calibrated images are being delivered by ESA's SMOS mission. These images of 'brightness temperature' translate into clear information on global variations of soil moisture and ocean salinity to advance our understanding of the water cycle. 59)
Since its launch on Nov. 2, 2009, engineers and scientists from various institutes in Europe have been busy commissioning the SMOS satellite and instrument. This commissioning phase, which will continue until the end of April, initially involved testing the Proteus platform – a generic 'satellite bus' developed by the French space agency CNES and Thales Alenia Space – and the all-important MIRAS instrument developed by EADS-CASA in Spain under contract to ESA. Both platform and instrument have shown excellent performance during their first four months in orbit.
Figure 14: A calibrated image of brightness temperature over Australia in Feb. 2010 (image credit: ESA)
• In early December 2009, SMOS completed its first phase of life in orbit – the Launch and Early Orbit Phase (LEOP), which means that all the systems are working as they should and the satellite is ready for full commissioning. The MIRAS instrument will now undergo the various calibrations that are needed to ensure that the best possible data are delivered.
Instrument calibration and fine-tuning of the data processors is ongoing and it is expected to release first routine processed data products to the calibration/validation team by end of March 2010. After a quick look at the initial processed data, it is already obvious that there is a lot of radio frequency interference within the protected band, which is, normally, reserved for radio astronomy and passive remote sensing (Ref. 62).
• The MIRAS instrument has been switched on (Nov. 17, 2009) and is operating nominally (initial functional verification test). MIRAS is working beautifully well with all key subsystems, including all of the receivers, the optical fibers and the correlator unit, in perfect functioning condition. With the critical launch and early orbit phase completed, the engineers can now evaluate the quality of the downlinks and concentrate on the calibration of the MIRAS instrument. 60)
• CNES, responsible for operating the satellite, has confirmed on Nov. 2. 2009 that the MIRAS instrument's three antenna arms have deployed as planned, and that the instrument is in good health. 61)
Figure 15: One of the first processed orbits of SMOS brightness temperature data (vertical polarization, calibration not complete), image credit: ESA 62)
Sensor complement: (MIRAS)
MIRAS (Microwave Imaging Radiometer using Aperture Synthesis):
MIRAS was developed by the prime contractor EADS-CASA Espacio, Madrid, Spain. The SMOS sensor assembly (the only instrument of the PLM) is a dual polarized 2-D interferometer operating at L-band (1.41 GHz) and based on the MIRAS concept of several airborne campaigns (Note: in 2004 the name MIRAS was also adopted for the instrument on the SMOS spacecraft).
The overall objective of MIRAS is to provide records of pixel brightness temperatures over incidence angles from 0º up to 55º across a 900 km swath, with a spatial resolution in the range of 30-50 km. From these profiles and auxiliary information, like surface physical temperature, roughness and ionospheric total electron content among others, soil moisture and sea surface salinity will be retrieved.
The design of the passive spaceborne instrument uses three coplanar arms consisting of an elementary antenna regularly spaced (0.875 λ, maximum redundancy) in a Y-shaped configuration (also referred to as a sparsely populated antenna, a thinned array is substituted for a filled array, each deployable antenna arm has a length of 4 m). In this concept, an interferometric Fourier synthesis is applied to derive images from the correlations between each pair of antenna elements (small independent receivers) operating in the microwave region. The 2-D SMOS interferometer permits the brightness temperature to be measured simultaneously at different incidences, and at two polarizations. Moreover, the instrument records an entire scene instantaneously. As the S/C moves, a given point within the 2-D FOV is observed from different view angles. A series of independent measurements is obtained permitting the derivation of surface parameters with improved accuracy. The brightness temperature field of such a design is reconstructed with a resolution corresponding to the spacing between the outmost receivers. 63) 64) 65) 66) 67) 68) 69) 70) 71) 72) 73)
MIRAS, operating at L-band (1.4-1.427 GHz, or 21 cm wavelength), employs a Y-shaped antenna arrangement, consisting of three arms, separated at 120º. Every antenna arm supports a row of evenly spaced receivers along its span. Each arm consists of three deployable segments.
There are 69 antenna elements - the so-called LICEF (Lightweight Cost-Effective Front-end) receivers -, which are equally distributed over the three arms and the central structure (hub). Each LICEF is an antenna-receiver integrated unit (four-probe patch antenna with a combiner/polarization circuit) that measures the radiation emitted from the Earth at L-band (1404-1423 MHz). The LICEF units are built by MIER Comunicaciones SA, Barcelona, Spain, and include a dual-polarized antenna and a critical bandpass filter.
Each LICEF measures the thermal noise emitted by the Earth in L-band by means of a complex amplification chain and filtering in RF and IF comprised of MMIC (Monolithic Microwave Integrated Circuit) for reducing consumption and weight. The result is a 1 or a 0 quantization of the in-phase and quadrature components of the received signal, depending upon the noise uptake level. LICEF is a highly frequency-selective receiver that avoids capturing disturbing signals from adjacent bands that would corrupt the noise sought from earth for measurement. 74) 75)
Figure 16: Top view of the LICEF receiver and the RF interface box at right (image credit: MIER,ESA)
Figure 17: Illustration of an LICEF receiver assembly in the laboratory as of 2006 (image credit: ESA)
Figure 18: Illustration of the MIRAS hub with LICEF receiver arrangement (image credit: ESA)
The total deployed arm length is about 4 m. The hub interfaces with the platform and houses some common equipment, such as a total-power radiometer for absolute calibration. The instrument structure (modular design) uses CFRP (Carbon Fiber Reinforced Plastic) material for reasons of stiffness. Each receiver can be commanded to measure either H and V polarization, or an internal load. The calibration network (subsystem) enables correlative measurements for the entire array. It consists of the following elements (noise sources that are being fed into the receivers):
• A central redundant noise diode, distributed to all receivers of the hub
• Distributed noise diodes on each arm segment; this allows measurements not only of the segment receivers, but also of the adjacent ones on either side of the hub or the next element.
Furthermore, three NIR (Noise Injection Radiometer) at the hub are measuring the H and V polarization simultaneously to enable the "zeroth baseline" correlation. All individual receivers digitize the measured I and Q signals and send them, via a point-to-point optical fiber connection, to DICOS (Digital Correlator System), in which the correlations between all possible receiver pair are made and accumulated for the duration of the integration period (1.2 s). The calibration frequency depends on the thermal gradient and the receiver behavior with the temperature. The operating temperature of MIRAS is kept within a range of 0 to 50ºC. 76)
Figure 19: Block diagram of MIRAS (image credit: ESA/ESTEC) 77)
Background on L-band radiometry: Operating the radiometer in the L-band provides the maximum sensitivity of the emissivity to both SM (Soil Moisture) and OS (Ocean Salinity). However it implies antenna diameters of several meters in order to meet the spatial resolution requirement for SM mapping which, for the design of SMOS, has been set at 50 km. A large real-aperture antenna on a LEO satellite poses a number of practical difficulties. A problem also arises from the need to achieve global coverage of the Earth within three days, which implies a large instrument swath. Microwave imaging by aperture synthesis provides an effective alternative to more classical solutions like the use of mechanically or electrically steered antennas and of pushbroom instruments. This interferometric approach to microwave radiometry, inspired by the techniques developed in radio-astronomy over several decades, is based on the use of many small antenna/receiver units, geometrically arranged so as to sample the signal that would have been received by a real-aperture antenna. This sampling in the spatial domain, enabled by the use of multiple antenna elements, provides a scanning capability through a wide swath by means of (ground) signal processing only. 78) 79)
Background on main instrument characteristics of MIRAS: Each antenna/receiver unit is based on a patch antenna without dielectric substrate with about 70º half-power beamwidth, directivity of about 8 dB and provides both H and V polarizations with excellent cross-polarization characteristics (co-polarization/cross-polarization ratio > 25 dB). A single receiver chain per antenna element is available, so each unit can operate on either H or V polarization upon command from a control unit. In each receiver the antenna signal is filtered to the selected bandwidth (1404 to 1423 MHz, in a region of the spectrum reserved for passive measurements), amplified and finally sampled and converted to a 1-bit digital signal. The MIRAS output data stream, combining both I and Q components and at a rate of about 130 Mbit/s, is transmitted to the correlator unit by means of an optical fibre link. Each element also receives (via a second optical fibre link) a centrally-generated reference clock signal in order to perform the frequency down-conversion and the sampling with phase coherence among all elements. An oversampling by a factor of about 2 with respect to the Nyquist criterion is achieved in each receiver, which improves the radiometric sensitivity. In the correlator unit, after conversion from optical to electrical signals, a massive bank of 1-bit/2-level correlators implemented in dedicated integrated circuits performs the cross-correlations between all signals. Horizontal and vertical polarization images are interlaced and the cross-correlation for each polarization is performed over a 0.3 s period. Up to 5 images are then averaged, so two images (one per polarization) are available every 3 s.
Each antenna arm provides 3 x 6 receivers with a polarization capability in H+V. The antenna features four balanced feeds to obtain a co/crosspolarized ratio >25 dB over an angular extension of ±30º.; there is also an intermediate stage for E- and H-radiation pattern alignment. Each receiver outputs the I- and Q-digitized signals. Each receiver is furnished with a suitable filter to reject interference from adjacent bands and to shape the frequency response. A multiplexer converts the eight parallel I- and Q-level bit streams from each group of four receivers into a ten-bit serial data stream (quantization) which are fed into a central demultiplexer for serial/parallel conversion. The digital correlator, at the end of the processing chain, performs a complex correlation (at zero delay) between each pair of antenna elements of the interferometric array, resulting in the so-called "visibility function" at the spatial frequency defined by that particular antenna element baseline. The visibility function is ideally the Fourier transform of the brightness temperature of the scene, weighed by the element gain pattern, and recovered by the inverse Fourier transform. The output is the visibility map.
A typical MIRAS receiver includes the following elements: 1) RF device (with an input switch, a filter and an amplifier), 2) a phase/quadrature frequency down converter, 3) two two-level quantizers and samplers. The RF bandwidth is limited to 1400-1427 MHz, the input RF filter band is fixed at 1404-1423 MHz (interference avoidance). The I (In-phase) and Q (Quadrature) outputs of all receivers are processed by a matrix of 1-bit digital correlators, located in the hub. 80)
The MIRAS configuration provides a FOV (swath) of about 900 km (the FOV has a hexagon-like shape at an orbital altitude of 763 km, sufficient for a 3-day equatorial revisit time. The antenna boresight is tilted at an incidence angle of 32.5º in the forward along-track direction for nominal radiative flux observations. The instrument tilt maximizes the footprint area for a given resolution goal and antenna spacing ratio. The tilted antenna footprint is represented by the sum of the overlapping individual antenna receivers, each representing a pixel of a distorted hexagon shape with curved sides; all pixel projections are of various sizes and incidence angles.
Figure 20: Payload FOV (Field of View), image credit: CNES
A snapshot brightness temperature map of the FOV is taken every 0.3 seconds with an average resolution of about 5 K over 200 K. Due to the platform motion in orbit, each pixel is measured several times with different spatial and radiometric resolutions and incidence angles. The minimum FOV along-track dimension is about 800 km, this corresponds to 380 snapshots at 0.3 s each. Ground data processing must account for all observation variations in size, shape, angles, overlapping conditions, weighing and averaging schemes, etc., resulting eventually in a brightness temperature map for each snapshot as well as for the accumulated and averaged along-track observation incidences within FOV. The repetitive measurement scheme of radiation, from varying footprints and incidences, has a similar summation effect on the retrieval of the overall signal, as a TDI (Time Delay Integration) scheme for an optical imager.
The overall instrument mass is 369 kg, power ~ 375 W. MIRAS data processing requires the introduction of corrections due to atmospheric, ionospheric, and galactic effects over ocean surfaces. In addition, SSS retrieval requires a knowledge of sea surface temperature and sea roughness. For observations over land surfaces, knowledge of the surface temperature is needed with an accuracy of 2 K. These reference parameters have to be obtained from instruments of other missions.
Table 5: Science requirements for the MIRAS instrument
MIRAS instrument command and data handling. The ICU (Instrument Control Unit) interfaces with the MIL-STD-1553B bus of the S/C. The S/C time signal is used to generate time stamps for the science data, generated and packetized in DICOS. The ICU interfaces with the receivers and the calibration network via CMN (Command and Monitoring Nodes), located on the hub of each arm segment. The CMN provides a number of function: switching of receivers and calibration network, it acquires some housekeeping information, provides some power conversion and distribution to the receivers, and provides the receivers with a local oscillator signal derived from DICOS.
The MIRAS instrument has three main operational modes:
• Dual-polarization mode, in which all receivers are switched synchronously to either H or V polarization
• Full polarimetric mode, in which segments of the array are switched according to a predefined sequence between H and V
• Calibration modes, in which measurements of the internal load, the noise diodes, or the so-called "fringe washing function" are determined.
Note: After the commissioning phase, one of the two operational modes (dual-polarization or full-polarimetric) will be selected and from that point onwards, MIRAS will be operated in the selected mode.
MIRAS has two types of in-orbit calibration: external, in which the instrument makes a maneuver to point to the cold sky, and internal, in which noise is injected to the receivers. This one in turn has two modes, one of short duration carried out periodically interspersed with scene measurements, and other lasting a full orbit and used to obtain the more stable parameters and the sensitivity of all parameters with temperature. 81) 82)
Table 6: Overview of in-orbit calibration schemes
Figure 21: Overall SMOS In-Orbit calibration time line (Image credit: UPC)
Internal calibration is based on periodically injecting noise to all receivers via dedicated input switches. Due to the difficulty of making a large noise distribution network, a distributed noise approach is used for large baselines. Internal calibration also includes a so-called selfcalibration mode that is used for automatically correcting samplers offset and quadrature error 83)
External calibration: Part of an orbit is devoted to observe the sky to complete the in-orbit calibration, in particular those parameters that cannot be properly calibrated by internal calibration. They are the following: NIR units, Deviations in the antenna patterns and Parametric G-matrix. In one month there will be two identical external calibration events every two weeks. The calibration is carried out by observing an external target selected in the sky in inertial attitude for part of the orbit.
Internal calibration: Various onboard means are provided to measure the accuracy and stability of the payload data. These are:
• Matched loads in each of the receivers
• Centralized noise source/network on the hub
• Distributed noise sources/networks in the arms
• Fringe wash function in the CCU (Correlator and Control Unit)
In all calibration activities, the antenna proper is not involved; this represents the "unknown" factors. Therefore, provisions have been made to perform "external" calibrations, by pointing the entire spacecraft to specific targets: either to deep space, the moon, the sun, or known ground targets.
The LICEF (Lightweight Cost-Effective Front-end) receiver parameters of MIRAS are sensitive to temperature and ageing. Hence, they need to be calibrated in-flight to ensure that the accuracy requirements of the mission can be met. For this purpose MIRAS has been provided with:
9) 3 accurate NIR (Noise Injection Radiometer) subsystems as reference radiometers
10) An onboard CAS (Calibration Subsystem)
Both subsystems were developed by Ylinen Electronics (Finland) and HUT (Helsinki University of Technology) or TKK, Helsinki, Finland - as subcontractors to EADS-CASA Espacio of Spain.
The CAS device of MIRAS includes a total of ten Noise Source (NS) units, twelve two-to-six Power Divider units (PD), and interconnecting RF cables. CAS forms a noise distribution network that generates and distributes correlated noise into all the receivers of MIRAS at two different noise levels (hot and warm). There are two different configurations of NS units; each of the arm segments has an ANS (Arm Noise Source) connected to adjacent PD units from two outputs (Figure 22). The hub section (Figure 23), has one HNS (Hub Noise Source). The HNS has three outputs, which are connected to three PD units on the hub. 84) 85) 86)
Figure 22: Block diagram of CAS configuration in one of the three ams of MIRAS (image credit: TKK)
Figure 23: Block diagram of CAS configuration in the MIRAS hub segment (image credit: TKK)
The configuration of the noise distribution network enables each individual radiometer receiver to be fed by two different NS units. The overlapping scheme enables to keep track of the signal over the whole calibration network and thus, to establish a relationship between phase and amplitude among the receivers. The three NIR units are used as absolute reference. The distributed noise generation concept is adopted due to mass and volume constrains of the satellite payload; centralized noise generation (using a single noise source and sufficient division) would require a much larger cable harness.
The block diagram of one HNS is presented in Figure 24. The ANS units are identical with the exception that one of the three outputs is internally terminated. Test ports 1 and 2 are being used on-ground to characterize the relative attenuation and phase differences between the output ports of a NS unit.
Using the RF switches, three distinct noise temperature levels can be generated at the NS output; hot level (approx. 65000 K), warm level (approx. 5000 K), and ambient. Reduced to the output of the CAS subsystem (i.e., the input of individual receivers), the correlated noise levels (thermal noise subtracted) are approximately 1200, 75 and 0 K, respectively.
One ANS device is shown in Figure 25 HNS is otherwise identical to ANS, but it has three outputs instead of two. The mass of the units is approx. 135 g, 295 g and 300 g for PD, ANS, and HNS, respectively. The dimensions of PD and NS units are 95 mm x 57 mm and 85 mm x 115 mm, respectively, not including mounting feet and connectors.
Figure 24: Block diagram of the hub noise source (image credit: TKK)
Figure 25: Test setup of CAS ANS device in thermal test chamber (image credit: Ylinen Electronics, TKK)
The NIR (Noise Injection Radiometer) subsystem, which consists of three NIR units, work as part of the MIRAS instrument. The main objectives of NIR are: 87)
• To provide precise measurement of the average brightness temperature scene for absolute calibration of the MIRAS image map
• To measure the noise temperature level of the internal active calibration source for individual receiver calibration CAS (Calibration Subsystem).
• To form interferometer baselines with the regular receivers units in the MIRAS array (so called mixed baselines).
The NIR performance is of great importance to the SMOS mission. The measurement challenges are based on the following scenario:
- There is only one accurate external target (cold sky)
- The individual antennas have a wide field of view
- The temperature of the NIR is not actively stabilized varying about 1.5º C per orbit
- Certain calibration parameters cannot be measured on orbit, but on ground only.
The NIR instrument consists of:
• Two radiometer receivers, one for vertical and one for horizontal polarization. The NIR receivers, manufactured by MIER Telecomunicaciones, Spain, are almost identical to the regular LICEF receivers of MIRAS.
• A controller. The controller incorporates an antenna that receives the target noise (Figure ).
• Four phase stable low-loss RF cables that connect the controller to the receivers.
Figure 26: Schematic view of a NIR unit onboard MIRAS (image credit: Ylinen Electronics, TKK)
The functions of the controller are to: a) inject reference noise into the two receiver chains, b) regulate the amount of the injected noise to keep the system balanced with antenna temperature or with the calibration noise from CAS, and c) control the switches of NIR (Dicke-switches of the receivers and the noise switches of the controller) according to the selected operation mode.
The NIR subsystem has several operational modes for measuring the antenna and CAS noise temperature and for calibration. The antenna temperature is measured using a noise injection mode called as the NIR-A mode, and the CAS noise level is measured using a noise injection mode called as the NIR-R mode. Also, total power modes are used, for example, to solve the receiver noise temperature.
The NIR calibration approach includes two steps: 1) certain parameters are characterized on-ground, and 2) these parameters are applied during orbital calibration, which is carried out frequently. The in-orbit calibration is based on the measurement of the cold sky, which has a well-known brightness temperature level.
The following parameters were solved during on-ground characterization: 1) antenna and front-end loss, 2) the phase imbalance of antenna and front-end, 3) non-linearity correction, and 4) the temperature dependency of noise injection level. The measured performance meets the requirements set for the NIR subsystem of SMOS. The retrieved ground calibration parameters will be applied during in-orbit calibration when the level of the noise injection, receiver noise temperature and phase imbalance of the channels are solved. After this, the correction for the non-linearity is applied to the measured level of brightness temperature.
Figure 27: Photo of the NIR controller (image credit: Ylinen Electronics, TKK)
Figure 28: Block diagram of the NIR subsystem (image credit: Ylinen Electronics, TKK)
MIRAS (Microwave Imaging Radiometer with Aperture Synthesis) technology development
The MIRAS instrument technology development represents a rather complex program of ESA research and validation activities over an extended period which started in1997 with feasibility studies (in 2000 the SMOS Phase-A definition of a spaceborne mission was superimposed on this framework effecting in turn the layout of the MIRAS instrument) and is documented in the following reference. Only a few items are mentioned here. 88) 89)
In 1998, ESA started MDPP-1 (MIRAS Demonstrator Pilot Project-1) to provide a technology solution to the inherent challenges of L-band radiometry. The initial objective was to build a representative element of the MIRAS instrument (one entire segment of the arm) to test the technology of a distributed system. This activity was performed by EADS-CASA (Spain) and ended in Oct. 2002 with all critical areas of technology validated. The MDPP-1 included:
• The SMOS parametric mission design
• The STM (Structure and Mechanisms) for a complete segment, including a deployment mechanism
• Four LICEF (Lightweight Cost-Effective Front-end) antenna receivers. Each LICEF is an antenna-received integrated unit that measures the radiation emitted from Earth at L-band.
• The CAS (Onboard Calibration System) to service one entire segment and the complete hub
• The MOHA (MIRAS Optical Harness) to service one entire segment of the hub. MOHA employs the IEEE 1393-1999 SFODB (Spaceborne Fiber Optic Data Bus), a redundant, cross strapped, ring-based configuration to provide the following services/functions:
- Connection of the radiometer receivers to central correlation unit in star topology
- Synchronous, one directional data transmission at 112 Mbit/s data rate
- 1300 nm laser diode transmitter and InGaAs PIN photodiode receiver
Note: MIRAS is the first satellite payload to rely critically on fiber optics communications. There are 144 links at 110 Mbit/s (72 to and 72 from the antenna elements). Typical characteristics of the fiber optics implementation are:
- very low EM emission levels (from Tx/Rx)
- galvanic isolation
- mechanically flexible and lightweight
- better phase stability when bended.
In 2000, a parallel and follow-on activity was started at ESA, referred to as MDPP-2, with the objectives to demonstrate the following critical aspects of the program:
• A deployment demonstration test for one complete arm of MIRAS (three segments)
• An image validation test using a 12-LICEF MIRAS array.
• Proof-of-concept activities involving: 1) a NIR (Noise-Injection Radiometer), 2) an advanced DISCOS-2 (Digital Correlator System-2), and 3) an advanced BPF-2 (Band-Pass Filter-2)
• The breadboarding of MOHA-3, the optical harness from SMOS Phase-A.
The end of MDPP-2 was in the fall of 2003.
Figure 29: Four 2nd generation LICEF receivers within MDPP-2 (image credit: MIER)
MOHA: The connections between the various elements of the instrument are performed with an optical fiber digital network called MOHA (MIRAS Optical Harness). The choice for optical interconnects as opposed to the more classical solutions with copper wires was driven by the following inherent advantages of such a system:
• Low electromagnetic emissions, which is vitally important for MIRAS
• Good phase stability, by comparison with coaxial, over temperature and when bent
• Insensitivity to ground differential voltages (galvanic isolation)
• Optical fiber cables are light-weight and very flexible.
The TCXO in the MOHA/CCU module generates the master reference clock for the instrument. This clock signal is converted to an optical signal and distributed via optical fibers and optical splitters to all the units in the instrument, where the integrated MOHA modules perform the reverse conversion to an electrical clock signal. The multiplexed IQ data generated by the LICEF receivers is then transported via an optical fiber link to the MOHA Rx modules (one fiber per receiver). After conversion to electrical signals, the IQ data is de-multiplexed and presented via LVDS lines as I and Q data streams to the correlator module in the CCU. The reference clock is also distributed to the CMN units, where it is up-converted to form the LO signal for the LICEF receivers.
Overall, the MOHA subsystem contains 74 solid state lasers, 168 optical receiver diodes and approximately 800 m of optical fiber cable. These elements are distributed over 91 optoelectronic modules and 13 optical splitters. SMOS is the first ESA mission with an optical harness (manufacturer: Contraves, Switzerland).
Figure 30: Block diagram of the MOHA (MIRAS Optical Harness), image credit: ESA
SMOS preparatory and in-flight validation campaigns (technology verification)
• The DOME-Cair (airborne) field campaign was carried out in January 2013 (12 day campaign) at the Concordia research station at Dome-C in Antarctica to validate data from ESA's SMOS and GOCE missions. The results were presented to ESA in late 2013. This campaign revealed a remarkable similarity between spatial patterns observed by the microwave and gravity instruments (Figure 32). The images also show the flight tracks during the campaign. This could lead to completely new scientific discoveries beneath the surface of the Antarctic ice. 90) 91)
The DOME-Cair campaign was conducted by AWI (Alfred Wegener Institute for Polar and Marine Research) of Bremerhaven, Germany and by DTU [Danmarks Tekniske Universitet (Technical University of Denmark), Lyngby, Denmark].
- The campaign was followed by data quality checks, data processing and first scientific analyses.
- The aircraft was the AWI Polar-6
- The L-band microwave instrument was the EMIRAD-2 developed by DTU 92)
- The gravimeter (airborne) was the AWI LaCoste-Romberg-Gravimeter.
Based at the isolated Concordia research station at Dome-C, the campaign involved spending two weeks flying a Basler-67 airplane over an area of 350 km2. The plane carried two key sensors: a radiometer used to record surface microwave emissions to verify SMOS data and a gravity meter.
The results of the campaign were presented in late 2013 to ESA and included some major surprises: SMOS and GOCE use completely different types of instruments to measure completely different aspects of Earth. Nevertheless, the measurements taken from the aircraft's two sensors show the similar patterns across the Antarctic ice surveyed by the plane.
Figure 31: Illustration of research stations at Antarctica (AWI, DTU, Ref. 91)
Figure 32: Different data reveal similar patterns under the ice: Gravity anomalies at Dome-C compared to brightness temperature at Dome-C (image credit: S. Kristensen and F. Forsberg, DTU)
Fully polarimetric (i.e. 4 Stokes) brightness temperatures at L-band were measured, and are now available on request, along with the final report from DTU (Technical University of Denmark). Figure 33 shows the measured brightness temperature at nadir for vertical (left panel) and horizontal (right panel) polarization over 11 tracks that were acquired over the test area. The data set is freely available and interested users should submit a request to the ESA EO campaigns data at: https://earth.esa.int/web/guest/campaigns
• DOMEX-3 campaign (period: Dec. 2012 to Dec. 2015) at the location DOME-C, Antarctica, providing multi-year, continuous observations of the ice sheet at the Concordia research station in Antarctica. The measurements are made by a new multi-frequency, dual-polarization, passive microwave radiometer positioned on a tower. 93)
This new instrument is based on the Radomex radiometer built by IFAC-CNR's Institute of Applied Physics in Florence, Italy, which operated successfully in 2008–2010. However, Radomex was not designed for long autonomous operation in the extremely harsh Antarctic climate. The newly designed radiometer will be able to operate autonomously and transmit data to Europe remotely, despite the demanding environment. The radiometer has been calibrated, tested and prepared for shipping to the Concordia station. It is expected to start operations in late 2012, during the austral summer. This multi-year experiment is in support of ESA's SMOS (Soil Moisture Ocean Salinity) mission.
Figure 34: Photo of the tower-mounted Radomex instrument (image credit: ESA, IFAC)
The objectives are:
- Continuous measurement data from the surface of the ice sheet by simultaneous observations from a multi-frequency dual-polarization radiometer and a collocated thermal infrared radiometer over a period of time, spanning multiple annual cycles.
- Collection of a time-series brightness temperature measurements at L-band with sufficient calibration accuracy, stability, drift, and of sufficient duration to allow analyses of seasonal variances owing to changes in emission and temperature of the observed snow and ice layers.
- Mitigation and correction for interference sources in the data.
- Provision of external ice-sheet target calibration reference brightness temperature, and the basis for calibration monitoring, and/or verification of brightness temperature products in support of the SMOS mission, especially for re-processing exercises.
- Evaluation of the TEC (Total Electron Content) impact on the temporal stability of the SMOS h-, v-polarised and 1st Stokes parameter measurements in a peak period of Sun activity using Total Electron Content data collected onsite for the duration of the experiment.
DOMEX-3 has so far documented the stability of L-band brightness temperatures over long time periods and hence the suitability of DOME-C for vicarious calibration of L-band radiometers (e.g. SMOS, SMAP and AQUARIUS). 94) 95) 96)
• As ESA's SMOS mission nears the end of its commissioning phase in May 2010, a new ground network, referred to as ISMN (International Soil Moisture Network), will provide harmonized global datasets of soil moisture measurements collected from the ground, ensuring the data received from space are as accurate as possible. 97)
ISMN was set up collaboratively by the following partner institutions: GEWEX (Global Energy and Water Cycle Experiment), GEO (Group on Earth Observations), CEOS (Committee on Earth Observation Satellites), and ESA. ISMN provides an integrative platform to host quality controlled soil moisture measurements emerging from various ground validation campaigns and operational networks to share freely with the scientific community.
As an important part of the SMOS mission, a number of field campaigns are being carried out to validate the observed data. Both the observed brightness temperature data and the derived data products for soil moisture from SMOS need to be validated.
The soil moisture data from field campaigns can now be fed into ISMN where they are brought into a common volumetric soil moisture unit to allow for accurate comparison with data received from the SMOS spacecraft. This is not as trivial as it may seem because there is neither a standard measurement technique nor a standard protocol for measuring soil moisture. - Hence, the ISMN will advance the process of validation of remotely sensed soil moisture datasets from SMOS and NASA's future SMAP (Soil Moisture Active/Passive) mission, as well as modelled soil moisture data.
Funded by ESA, the Institute of Photogrammetry and Remote Sensing at the Vienna University of Technology (TU Wien, Austria) is responsible for the network (ISMN) and data distribution. ISMN bridges the gap between organizations measuring soil moisture on the ground and people interested in using these data. - For improved worldwide validation of satellite and model estimates of soil moisture, all campaign and operational soil moisture networks are welcome to share their data with the scientific community through the ISMN (International Soil Moisture Network).
ESA's SMOS campaigns: Three preparatory campaigns were conducted during the Phase-A study period to analyze signal dependence and to validate existing and improved models. The campaigns were: 98) 99) 100)
• WISE (Wind and Salinity Experiment). WISE was conducted in 2000 (Nov. 25 to Dec. 18, 2000) and 2001 (Jan. 9 to 16, 2001) from an oil rig, about 50 km off the coast of Tarragona in the northwest Mediterranean. The experiments were conducted by UPC (Technical University of Catalonia), and ICM (Institute of Marine Sciences) both of Barcelona, Spain. The LAURA (UPC L-band AUtomatic RAdiometer) was deployed in REPSOL's Casablanca oil rig, in conjunction with other oceanographic and meteorological instrumentation. Study of the impact on the brightness temperature of sea state, as well as the effect of wave development, swell, currents, rain, and oil spills; retrieval of sea surface salinity from multi-angular polarimetric L-band radiometric data. 101) 102) 103) 104)
• EuroSTARRS (Salinity Temperature and Roughness Remote Scanner in Europe). The objective of EuroSTARRS-2001 was to acquire SMOS-like observations for addressing the critical issues relevant to the soil-moisture objectives of the SMOS mission. STARRS, owned by NRL (Washington, DC), was operated during the campaign aboard a Dornier-228 by DLR, Oberpfaffenhofen.
• LOSAC (L-band Ocean Salinity Campaign). The objective of LOSAC was to address azimuthal dependence of the first two Stokes parameters (Tb,v and Tb,h) on wind speed and direction. The EMIRAD polarimetric L-band radiometer of TUD (Technical University of Denmark) was used aboard the C130 aircraft operated by the Royal Dutch Air Force over the North Sea. First flights were conducted in 2001. Overall, the campaign lasted until 2003. 105)
• DOMEX (Microwave Emission of the East Antarctica Plateau): An experimental campaign (DOMEX) with ground based radiometers was carried out at Dome-C in the summer 2004/2005 to control the stability of L- and C -band emission at a monthly scale, within the framework of the ESA-SMOS calibration and validation activities. The DOMEX campaign was organized with CNR-IFAC, Florence, Italy. The campaign (tower-based measurements) took place in Dec. 2004 at DOME-CONCORDIA in Antarctica. The campaign concluded in 2005.
• CoSMOS-2, Airborne measurements in Australia in Nov. 2005 (Aero Commander aircraft of Flinders University, Adelaide), dedicated to SM (Soil Moisture) observations. Cooperation with Australian University of Melbourne, Newcastle in the framework of the National Australian Field Experiment (NAFE). Participation of European Teams [Free University of Amsterdam, (The Netherlands), University of Valencia (Spain), CESBIO (France)]. The objective of the CoSMOS airborne campaign was to perform a "long-term" acquisition of data under different geo/bio-physical as well as meteorological and oceanographic conditions to address open issues related to the retrieval and validation of the SMOS products.
• CoSMOS-OS with the goal to map sea surface salinity: An airborne campaign (4 weeks) in Norway in April 2006 using the second generation EMIRAD (Electromagnetics Institute Radiometer) digital detection instrument of TUD (Technical University of Denmark) on the Skyvan aircraft of HUT/TKK (Helsinki University of Technology), Finland. The objective was to perform the acquisition of data under different oceanographic conditions to address open issues related to the retrieval and validation of the SMOS products. For all flights, the time at target has been chosen to coincide with an ENVISAT/ASAR overpass to obtain information about the actual surface roughness, and meteorological information was provided from oil-drilling installations in the target area. 106) 107)
• SAM (Small-scale Airborne prototype of MIRAS). ESA-CASA is developing SAM as prime contractor within the framework of the MDPP-3 project (MIRAS Demonstrator Pilot Project-3), sponsored by ESA. Several European institutions are participating in the development of the MDPP-3 instrument and test campaigns (UPC, HUT, etc.). The SAM demonstrator consists of 12 LICEF receivers (4 per arm), 1 reference radiometer (NIR) and includes the same internal calibration system scheme as foreseen for MIRAS. 108)
In the near future, SAM will be flown in a light aircraft, the Skyvan of the LST/HUT (Laboratory of Space Technology/Helsinki University of Technology), to undertake several airborne campaigns over land and sea. These airborne campaigns are devised to test the capability of SMOS calibration and retrieval algorithms to provide soil moisture and ocean salinity in the natural environment.
• TuRTLE (Topography effects on RadiomeTry at L-band Experiment). UPC (Spain) carried out a set of experiments to study the impact of different soil and vegetation properties on land emission: 109)
- The vine canopy, observation position, and soil moisture (SMOS REFerence pixel L-band Experiment, REFLEX, 2003 and 2006
- The soil type, and moisture profile (MOnitoring Underground Soil Experiment, MOUSE, 2004, and the soil roughness (Terrain-Roughness EXperiments T-REX, 2004 and 2006
The objective of TuRTLE was to acquire radiometric measurements of pixels with changing topography and to compare them to existing brightness temperature models. TuRTLE 2006 was carried out in a mountainous area about 50 km North of Barcelona (Spain). Radiometric measurements covering the mountain slope, and up to the sky were acquired. Concurrently, ground-truth and meteorological data were registered. Radiometric measurements have been compared to the emissivity obtained by simulation using a facet model which considers the high resolution digital elevation model and land cover map of the area. Polarization mixing due to surface tilting and integration over the antenna pattern have also been included in the simulator, and results agree with the radiometric measurements.
The campaign was carried out in May 12-19, 2006. The radiometric data was acquired by the UPC full-polarimetric, Dicke radiometer LAURA (L-band AUtomatic RAdiometer. The mountainous scene in front of the radiometer was observed at seven elevation (from 45º-to 105º referred to nadir, in 10º steps) and nine azimuth angles, covering the mountain slope terrain and the mountain-sky transition.
• AMIRAS (Airborne MIRAS) is a new instrument that is similar to the MIRAS radiometer on SMOS. AMIRAS, however, is much smaller than MIRAS and designed to operate from an aircraft to provide scientists with data similar to those expected from the SMOS mission. In Dec. 2006, AMIRAS has successfully delivered images from its maiden flight over Finland. AMIRAS consists of a Y-shaped array with four antennae elements per arm. It had its maiden flight onboard the Helsinki University of Technology's (HUT, also referred to as TKK) Skyvan in June 2006 covering both land and sea areas around Helsinki, Finland. EADS-CASA Espacio, Spain, is the prime contractor for AMIRAS. The AMIRAS instrument consists of 12 receivers (4 per arm), 1 reference radiometer (NIR) and includes the internal calibration system foreseen for MIRAS. 110) 111)
Figure 35: Configuration of the AMIRAS instrument (image credit: UPC)
• HUT-2D (Helsinki University of Technology 2D Radiometer). HUT-2D is an airborne L-band two-dimensional interferometric radiometer using aperture synthesis. The instrument was designed and manufactured at LST (Laboratory of Space Technology) of HUT (Helsinki University of Technology) in cooperation with Ylinen Electronics Ltd., Finland. The HUT-2D radiometer instrument consists of 36 receivers, local oscillator subsystem, calibration subsystem and one-bit digital correlator unit. Each pair of receivers (baseline) builds a correlating receiver. The cross correlation of the receiver outputs are processed in the correlator unit. The radiometer has been accommodated below the fuselage of the HUT remote sensing aircraft (Short SC-7 Skyvan) using three specifically designed attachment points.
In 2006 and 2007, the HUT-2D instrument represented a significant contribution to the SMOS mission preparatory campaigns in providing the first successful airborne demonstration of the complete system. This complex and sophisticated instrument has been used in calibration and validation campaigns to assess retrieval algorithms for soil moisture and ocean salinity. The instrument, even though the antennas are in a different configuration (U-shape vs Y-shape on MIRAS), is very suitable for various verification and validation methods for the MIRAS instrument of SMOS. 112)
• AACES (Australian Airborne Calibration/Validation Experiments for SMOS). The AACES campaign was under way between mid-January and the end of February, 2010 in the Murrumbidgee catchment in southern New South Wales, Australia. Given the recent launch of SMOS (Nov. 2, 2009), AACES represents the first extensive and probably most comprehensive of such experiments undertaken during the commissioning and later operational phase of SMOS. The project was led by Jeff Walker and Chris Rudiger from the University of Melbourne and supported by the invaluable help of their PhD students. Moreover, thanks to the generosity of various research institutes from France, Germany, Denmark, Poland, and the Netherlands in addition to the flights covered by ESA, a total of 14 people have joined this campaign to boost the manpower of the sampling teams. 113) 114) 115)
The Murrumbidgee catchment is unique as it comprises a distinct variety of topographic, climatic and land cover characteristics, and therefore represents an excellent validation site for the land component of this satellite mission. A total of 10 patches of 100 km x 50 km (aligned with the synthetic SMOS grid, and therefore including two independent SMOS footprints) are covered by an aircraft carrying an L-band radiometer and VIS/NIR/SWIR/TIR sensors. 116)
With a flight altitude, the ground pixel resolution is 1km, resulting in an almost complete coverage of the catchment transect of 500 km. The aircraft flights are aligned with SMOS morning overpasses. At the same time, two ground teams are covering an area of 5km x 2km each, collecting soil moisture (50 m spacing along the 5 km transects), soil temperature, and soil salinity. A sub-group of each team was in parallel collecting LAI, dew, destructive vegetation samples, and hyperspectral measurements of the surface conditions.
Figure 36: The AACES est site in the Murrumbidgee Catchment (outlined in red) in southeast Australia (image credit: ESA)
SMOS ground segment:
SMOS is an ESA Earth Explorer mission with significant national contributions provided by the French and Spanish space agencies, CNES and CDTI. The collaborative approach for the development of the SMOS mission will be continued in the operations phase, with the ground segment consisting of different stations covering various functions. For the operations phase, ESA will be responsible for the overall coordination of the mission and the ground segment operations, and CNES will be operating the spacecraft. 117)
The main stations for day-to-day running will be ESA's European Space Astronomy Centre (ESAC), in Spain, hosting the main part of the Data Processing Ground Segment (DPGS), and CNES at Toulouse, hosting the Satellite Operations Ground Segment (SOGS). Global soil moisture data are important variables for operational meteorological applications.
1) SOGS (Satellite Operations Ground Segment) located at CNES in Toulouse, France.
2) DPGS (Data Processing Ground Segment) of ESA/CDTI (ESA/Centro para el Desarrollo Technologico Industrial), located at ESA/ESAC (European Space Astronomy Centre) in Villafranca, Spain - and at ESA/ESRIN (user interface) in Frascati, Italy. DGPS is devoted to acquire, process, and dispatch the SMOS data product levels L1 and L2 (L1A, L1B, L1Csm, L1Cos, L2Csm, L2Cos) to the user community (Figure 39). The following elements provide main functions for DPGS: 122) 123)
- SPGF (SMOS Plan Generation Facility): Instrument and acquisition planning information
- Data are acquired with the 3.5 m XBAS (X-Band Acquisition System) receiving antenna
- PXMF (Payload X-Band Monitoring Facility) is part of PLPC development. It provides a means for the operator to monitor the X-band telemetry
- CEC (Calibration Engineering Centre). CEC provides product quality [SPQC (SMOS Product Quality Control)], calibration and validation tools, and system level monitoring.
3) CATDS (Centre Aval de Traitement des Donnees SMOS) is at CNES, dedicated to process, archive, and dispatch the SMOS data product levels L3 and L4 (Figure 40). The product levels L3 and L4 are derived from the L1 and L2 levels. Note: IFREMER will be in charge of CATDS operations to optimize costs and the maximum reuse of data. 124)
4) PLPC (Payload Operations and Programming Centre) at ESA/ESAC, Villafranca, Spain.
In 2006, ESA member states approved an add-on to the original mission configuration by introducing another X-band receiving station at Svalbard, Norway, which will guarantee this service. Above the Arctic Circle, Svalbard will provide 10 out of the 14 orbits per day that ESAC is not able to acquire in real-time due to its geographical location (Ref. 117).
Figure 37: Overview of the SMOS ground segment (image credit: ESA)
Figure 38: Overview of SMOS ground segment elements in Europe (image credit: ESA)
Figure 39: The DGPS data interface architecture (image credit: ESA)
In addition, further data services will be provided:
• IFREMER [Institut Francais de Recherche pour L'Exploration de la Mer (French Ocean Agency in Brest, France)] will provide the oceanographic community with a homogeneous time series of value-added data relevant to the sea surface state (wind fields, fluxes, waves or sea-ice)
• CESBIO [Centre d'Études Spatiales de la Biosphére (Toulouse, France)] will host the SM Expertise Centre; and will deliver the L3+L4 algorithms specifications and mock-up (with the help of the other laboratories involved in L3/L4 SM)
• IFREMER will host the OS Expertise Centre; and will deliver the L3+L4 algorithms specifications and mock-up (with the help of the other laboratories involved in L3/L4 OS)
• ECMWF [European Centre for Medium-Range Weather Forecasts (Reading, UK)] will provide key auxiliary data of ocean and land
• Spain (CDTI, UPC, INDRA Espacio). Spain will develop CP34, another ground-segment dedicated to SMOS L3+L4 products processing
Figure 40: Overview of the CATDS data interface architecture (image credit: CNES)
SMOS data product level objectives:
- L1/L2: The SMOS SM (Soil Moisture) objective is an accuracy of 4% on volumetric soil moisture, with three days revisit and a sampling step better than 50 km.
- L1/L2: The SMOS OS (Ocean Salinity) objective is an accuracy better than 0.1 PSU, with a 10 days to monthly grid scale (200 km). Knowing that single measurement will be poor (~1 PSU), spatial and temporal averages will be needed to reduce the noise.
- L3: L3SM will consist of soil moisture global maps (regular grid 0.15º x 0.15º), produced every day and based on the three previous days.
- L3 OS maps are produced every day with GODAE (Global Ocean Data Assimilation Experiment) specifications (5-10 days sliding maps with a 200 km x 200 km resolution).
1) M. Martin-Neira and the SMOS Project Team, "The Soil Moisture and Ocean Salinity (SMOS) Mission," URSI 2005 Commission F Symposium on Microwave Remote Sensing of the Earth, Oceans, Ice, and Atmosphere, Ispra, Italy, April 20-21, 2005
2) P. Silvestrin, M. Berger, Y. H. Kerr, J. Font, "ESA's Second Earth Explorer Opportunity Mission: The soil Moisture and Ocean salinity Mission - SMOS." IEEE Geoscience and Remote Sensing Newsletter (118), 2001, pp.11-14
3) J. Blouvac, B. Lazaed, J. M. Martinuzzi, " CNES Small Satellites Earth Observation Scientific Future Missions, IAA 2nd International Symposium on Small Satellites for Earth Observation, Berlin, April 12-16, 1999, pp. 11-14
4) Y. H. Kerr, P. Waldteufel, J.-P. Wigneron, J. Font, M. Berger, "The Soil Moisture And Ocean Salinity Mission," Proceedings of IEEE/IGARSS 2003, Toulouse, France, July 21-25, 2003
5) H. M. J. P. Barre, B. Duesmann, Y. H. Kerr, "SMOS: The Mission and the System," IEEE Transactions on Geoscience and Remote Sensing, Vol. 46, No 3, March 2008, pp. 587-593
6) Mark Drinkwater, Yann Kerr, Jodi Font, Michael Berger, "Exploring the Water Cycle of the 'Blue Planet'," ESA Bulletin, No 137, Feb. 2009, pp. 6-15
7) M. Martin-Neira, J. Font, M. Srokosz, I. Corbella, A. Camps, "Ocean Salinity Observations with SMOS Mission," Proceedings of the IEEE IGARSS 2000 Conference, Honolulu, HI, July 24-28, 2000
8) Y. H. Kerr, P. Waldteufel, J.-P. Wigneron, J. Font, "The Soil Moisture and Ocean Salinity Mission: The Science Objectives of an L-band 2-D Interferometer," Proceedings of the IEEE IGARSS 2000 Conference, Honolulu, HI, July 24-28, 2000
9) Y. Kerr, J. Font, et al., "Next Generation Radiometers: SMOS - A Dual Pol L-band 2-D Apertures Synthesis Radiometers," 2000 IEEE Aerospace Conference, March 2000, Montana,USA
10) Y. H. Kerr, J. Font, P. Waldteufel, M. Berger, "The Soil Moisture and Ocean Salinity Mission -SMOS," ESA Earth Observation Quarterly, No 66, July 2000, pp. 18-26
11) Y. H. Kerr, P. Waldteufel, J. P. Wigneron, J. Font, "Description of the Soil Moisture and Ocean Salinity Mission," COST 712 -WG 3 report, 2001, European Union, Brussels
13) M. Berger, A. Camps, J. Font, Y. Kerr, et al., "Measuring Ocean Salinity with ESA's SMOS Mission," ESA Bulletin 111, Aug. 2002, pp. 113-121
14) F. Bermudo, M. Venet, M. Le du, Y. Kerr , H. Barre, "SMOS: An L-band Interferometric Radiometer Mission," 5th IAA Symposium on Small Satellites for Earth Observation, April 4-8, 2005, Berlin, Germany
16) J. P. Wigneron, A. Chanzy, P. Waldteufel, J. C. Calvet, O. Marloie, J. P. Hanocq, Y. H. Kerr, "Retrieval capabilities of L-Band 2-D interferometric radiometry over land surfaces (SMOS Mission), VSP, Netherlands, 2000
17) J. P. Wigneron, P. Waldteufel, A. Chanzy, J. C. Calvet, Y. H. Kerr, "Two-D microwave interferometer retrieval capabilities of over land surfaces (SMOS Mission)," Remote Sensing Environment, Vol. 73, No 3, 2000, pp. 270-282
19) M. Berger, Y. Kerr, J. Font, J.-P. Wigneron, et al., "Measuring the Moisture in the Earth's Soil - Advancing the Science with ESA's SMOS Mission," ESA Bulletin No 115, Aug. 2003, pp. 40-45, URL: http://www.esa.int/esapub/bulletin/bullet115/chapter5_bul115.pdf
20) Y. Kerr, P. Waldteufel, F. Cabot, A. Hahne, S. Mecklenburg, Thierry Pellarin, "The SMOS Mission Current Status," Earth Observation and Water Cycle Science - Towards a Water Cycle Multi-mission Observation Strategy, ESA/ESRIN, Frascati, Italy, Nov. 18-20, 2009
21) M. Martin-Neira, "MIRAS - A Two-Dimensional Aperture-Synthesis Radiometer for Soil-Moisture and Ocean-Salinity Observations," ESA Bulletin No 92, Nov. 1997, URL: http://www.esa.int/esapub/bulletin/bullet92/b92marti.htm
22) Francois Bermudo, Michel Venet, Michel Ledu, Yann Kerr, "SMOS - An L-band Interferometric Radiometer Mission," Proceedings of the 60th IAC (International Astronautical Congress), Daejeon, Korea, Oct. 12-16, 2009, IAC-09-B1.1.2
23) Michel Le Du, Roger Jegou, "SMOS Mission, a Successful Cooperation based on the Flight Proven PROTEUS Platform," Proceedings of the Symposium on Small Satellite Systems and Services (4S), Funchal, Madeira, Portugal, May 31-June 4, 2010
24) Mariano Kornberg, Elena Checa, Silvio Dolce, Manuel Martin-Neira, Pilar Rubiales, Josep Closa, Guillermo Buenadicha, Jorge Fauste, "Thermal Control in SMOS Payload Operations: Anomalies, Seasonal Effects, Failure & Recovery Issues," SpaceOps 2014, 13th International Conference on Space Operations, Pasadena, CA, USA, May 5-9, 2014
28) Note: The revisit cycle is the frequency at which any point on the globe is seen again at any position within the swath. The repeat cycle is the frequency at which any point on the globe is seen again at the same position within the swath.
29) "Five years of soil moisture, ocean salinity and beyond," ESA, Nov. 3, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/SMOS-Five_years_of_soil_moisture_ocean_salinity_and_beyond
30) "ESA's Water Mission," SMOS Newsletter, Issue 8, September 2014, URL: https://earth.esa.int/documents/10174/234899/SMOS_Newsletter_8
31) "Versatility extends life of water mission," ESA, July 18, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/SMOS/Versatility-extends_life_of_water_mission
32) "Water Mission Boosts Food Security," ESA, May 21, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/SMOS/Water_mission-boosts_food_security
34) Information provided by Malcolm Davidson of ESA/ESTEC, Head of ESA's EO campaign section
35) "Data and Processors, Data availability," SMOS Newsletter, Issue 6, Dec. 2013, URL: https://earth.esa.int/documents/10174/234899/SMOS_Newsletter-6_December_2013
36) "SMOS status," ESA Bulletin No 154, May 2013, p. 70
37) "Taking two bites at ocean salinity," ESA, April 23, 2013, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/SMOS/Taking_two-bites_at_ocean_salinity
38) "SMOS-Aquarius Science Workshop Summary," URL: http://www.congrexprojects.com/docs/default-source/13c07_docs3/workshop-summary-new.pdf?sfvrsn=0
39) "SMOS: The Golbal Success Story continues," ESA, Feb. 22, 2013, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/SMOS/SMOS_the-global_success_story_continues
40) Matthias Drusch, Susanne Mecklenburg, Yann Kerr, "SMOS over Land - New applications for ESA's water mission," ESA Bulletin No 152, November 2012, pp: 38-49, URL: http://esamultimedia.esa.int/multimedia/publications/ESA-Bulletin-152/offline/download.pdf
41) Elena Daganzo-Eusebio, Roger Oliva, Yann H. Kerr, Sara Nieto, Philippe Richaume, Susanne Mecklenburg, "SMOS radiometer in 1400-1427 MHz: Impact of the RFI environment and approach to its mitigation and cancellation," Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Munich, Germany, July 22-27, 2012
42) Steen S. Kristensen, Jan Balling, Niels Skou, Sten S. Søbjærg, "RFI in SMOS Data Detected by Polarimetry," Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Munich, Germany, July 22-27, 2012
46) M. Martín-Neira, M.A. Plaza, I. Corbella, J. Kainulainen, R. Oliva, F. Cabot, F. Torres, J. Closa, F. Martín-Porqueras, J. Tenerelli, R. Castro, A. Gutierrez, J. Barbosa, G. Buenadicha, J. Benito, A. Zurita, E. Daganzo, S. Mecklenburg, "SMOS Results and MIRAS Evolution Studies," Proceedings of the 3rd Workshop on Advanced RF Sensors and Remote Sensing Instruments (ARSI), Noordwijk, The Netherlands, Sept. 13-15, 2011
48) "SMOS MIRAS - return to nominal data quality," ESA, Feb. 11, 2011, URL: http://earth.esa.int/object/index.cfm?fobjectid=7480
49) "SMOS MIRAS instrument back to nominal operations," ESA, January 24, 2011, URL: http://earth.esa.int/object/index.cfm?fobjectid=7435
50) "SMOS status," ESA Bulletin, No 145, February 2011, p. 82
52) "SMOS water mission winning battle with interference," ESA, Oct. 6, 2010, URL: http://www.esa.int/SPECIALS/smos/SEMB5USOREG_0.html
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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).