SARAL (Satellite with ARgos and ALtiKa)
SARAL is a cooperative altimetry technology mission of ISRO (Indian Space Research Organization) and CNES (Space Agency of France). In this setup, CNES is providing the payload module consisting of the AltiKa altimeter, DORIS, LRA, and Argos-3 DCS (Data Collection System), and the payload data reception and processing functions, while ISRO is responsible for the platform, launch, and operations of the spacecraft. A CNES/ISRO MOU (Memorandum of Understanding) on the SARAL mission was signed on Feb. 23, 2007.
• The development of operational oceanography (study of mesoscale ocean viability, coastal region observations, inland waters, marine ecosystems, etc. )
• Understanding of climate and developing forecasting capabilities
• Operational meteorology.
The SARAL mission is considered to be complementary to the Jason-2 mission of NASA/NOAA and CNES/EUMETSAT (it is also regarded a gap filler mission between Envisat and the Sentinel-3 mission of the European GMES program). The combination of two altimetry missions in orbit has a considerable impact on the reconstruction of SSH (Sea Surface Height), reducing the mean mapping error by a factor of 4. 4) 5) 6)
AltiKa, the altimeter and prime payload of the SARAL mission, will be the first spaceborne altimeter to operate at Ka-band.
Background: The AltiKa concept, based on a wideband Ka-band altimeter (35.75 GHz, ~500 MHz) concept, was initially proposed in 2002 as a CNES altimetry minisatellite mission (150 kg) on the Myriade platform. Feasibility studies were also made to accommodate AltiKa on the TopSat platform of SSTL (Surrey Satellite Technology Ltd.). In Dec. 2005, CNES approved the development of the AltiKa payload since an opportunity arose to embark the instrument on a cooperative mission of ISRO and CNES, namely OceanSat-3. However, early in 2006, the OceanSat-3 launch was postponed to the period 2011/12, beyond the schedule objective of AltiKa. As a consequence, CNES and ISRO established an alternative option, based on a dedicated minisatellite using a new SSB (Small Satellite Bus) platform to be developed as ISRO. 7) 8) 9) 10)
Table 1: Overview of ISRO/CNES responsibilities (Ref. 10)
Figure 1: Overview of current (mid 2009) and future altimetry missions (image credit: CNES) 11)
Spacecraft bus and payload module development at ISRO:
The minisatellite, as provided by ISRO, involves nothing less but the development of a new spacecraft bus. The general spacecraft architecture consists of two modules: namely the introduction of a new modular bus (under development at ISRO as of 2007), referred to as SSB (Small Satellite Bus), and PIM (Payload Instrument Module). Two SSB designs are under development: 12) 13)
1) The first one is being developed for a minisatellite series with a total launch mass of about 450 kg, including a payload mass of ~ 200 kg.
2) The second one is considered for a microsatellite series with a total launch mass of around 100 kg, including a payload mass of 20-30 kg
In 2012, ISRO refers to the former SSB as the IMS-2 (Indian MiniSatellite-2) bus. The IMS-2 bus evolved as an operational minisatellite class platform with complete redundancy in the mainframe subsystems. The IMS-2 development is an important milestone, it is envisaged at ISRO/ISAC to be a work horse for different type of applications / operational future missions. The first mission of the IMS-2 implementation is SARAL. 14)
The design layout is such that the bus module and the payload module of each series may be integrated and tested separately (thus reducing the interdependency during the realization between both modules). IMS-2 is also designed to accommodate different types of payloads with minor modification from mission to mission. IMS-2 is developed with the intention to permit a multiple payload launch by PSLV using the DLA (Dual Launch Adaptor), which caters to minisatellites in the class of 450-600 kg.
The structure of the bus is built with aluminum honeycomb panels. It consists of 3 horizontal decks, namely the bottom deck, top deck and payload deck, and four vertical equipment panels. The bottom deck has the interface ring (with Merman band interface 937VB) bolted to it. The design employs a number of stiffness measures to avoid any resonant frequencies of the launcher in both lateral and longitudinal directions.
The IMS-2 system design includes all general platform services functions such as the EPS (Electric Power Subsystem) with battery and solar arrays, the AOCS (Attitude and Orbit Control Subsystem), TT&C (Telemetry, Tracking & Command) subsystem, RCS (Reaction Control Subsystem), the thermal subsystem for bus (a passive system is used), and mechanisms (for solar panel deployment), etc. Most of the systems have redundancy/large margins or space capacity.
Figure 2: Overview of the SARAL spacecraft configuration (image credit: ISRO/CNES)
Table 2: Overview of IMS-2 platform capability
The minisatellite bus module has a standard simple interface for the payload module. The new bus derives its shape from previous IRS configurations with cuboid structure. The bottom deck of the minisatellite bus is measuring 900 mm x 900 mm providing an interface with launch vehicle. The launcher interface uses a Merman clamp band, 937VB. The top deck is of the same size as the bottom deck featuring four corner points extending as pillars to provide a four-point interface for the payload module.
The PIM (Payload Instrument Module) consists of a set of CFRP (Carbon Fiber Reinforced Plastic) based panels with appropriate interfaces for mounting onto the main platform and for mounting of the payloads and associated elements. The payload-related systems like data handling, SSR (Solid State Recorder), etc., are mission specific functions.
Figure 3: General configuration of the PIM (image credit: ISRO)
The PIM is also of square shape and of the same cross-section size as that of the bus with four corners (payload interface pods) interfacing with main bus. The PIM, unlike the bus module, has the freedom to grow vertically; the only limitation is the available volume within the heat shield enclosure of the launch vehicle and allowable frequency constraints. 15)
Figure 4: Functional block diagram of the IMS-2 bus (image credit: ISRO, Ref. 14)
Structure: The structural configuration of the IMS-2 bus is a cuboid of size: ~1m x 1 m x 0.6 m. Use of an aluminum honeycomb sandwich construction. The face sheet is aluminum and the core is consists of an aluminum honeycomb structure. The structure consists of two horizontal decks, namely a bottom deck and a top deck, and four vertical decks, EP01 to EP04.
Figure 5: Schematic view of the IMS-2 bus structure (image credit: ISRO)
AOCS: The spacecraft is 3-axis stabilized. Attitude is sensed by a miniature star sensor providing an attitude knowledge of < 30 µarcsec about all axes. This attitude information is also used to update the inertial angle information of the gyros. The gyros are also of a miniaturized version. Further attitude sensors are 4 sun sensors (4π FOV) and a miniature 3-axis magnetometer. - Actuation is provided by 4 reaction wheels (5 Nms) arranged in tetrahedral configuration, and 2 magnetorquers. The latter one are used for the momentum dumping of the wheels. The monopropellant RCS (Reaction Control Subsystem) uses a single tank (21 kg of propellant) with two blocks of 1N thrusters. It is mainly used for orbit maintenance.
The star sensor is a single integrated sensor package consisting of a detector, preamplifier and an FPGA, which acts as intelligent video processor. An IRU (Inertial Reference Unit) is used for attitude referencing in all phases of spacecraft operations. The IRU consists of cluster assembly where three numbers of two-degree-of freedom MDTG (Multiplexed Digital Trunk Group) mounted on a cluster and suspended with 4 space qualified vibration isolator separately.
- Pointing accuracy = 0.1º (3σ)
- Drift rate: ± 10-4 º/s (3σ)
Figure 6: Photo of the reaction wheel assembly (image credit: ISRO)
EPS: The EPS employs a single bus, a DET (Direct Energy Transfer) system, with an unregulated bus voltage of 28-33 V. An average power of ~500 W is provided with 2 articulated solar panels (total area of 3.88 m2) with UTJ solar cells. Maximum power of 850 W. The power output of the solar array is regulated by the battery charge conditions. A Li-ion battery with 46 Ah capacity is used for eclipse phase operations. The power conditioning and the power distributing converters have full redundancy.
Figure 7: Power consumption chart for SARAL (image credit: ISRO)
The BMU (Bus Management Unit) with the OBC (OnBoard Computer) take care of all data handling functions using the MIL‐STD‐1553B standard interface with all onboard subsystems. The payload data handling subsystem employs serial interfaces to the baseband data handling formatter. The formatter receives the payload data packets, annotates these with the housekeeping data, and deposits them onto the SSR. All the hardware required for BMU is optimized and implemented using four cards of size 31 cm x 31 cm. The CPU is built around the MAR31750 processor, with the MMU MAR31751 device for memory expansion. Figure 8 shows the BMU/OBC hardware.
RF communications: There is no real-time data transmission requirement for SARAL. The SSR has a capacity of 32 Gbit. The TT&C data are transmitted via S-band. The TC uplink modulation scheme is FM/PSK/PCM and the uplink data rate is 4 kbit/s. The TM data is BPSK modulated, the transmit power is 1 W. A near omnidirectional TT&C antenna is used.
The payload data handling system consists of BDH (Baseband Data Handling) system and the SSR (Solid State Recorder). The data handling system collects, formats, and stores all data for later downlink. All links, both up and down, are encrypted to prevent unauthorized access to the data. It interfaces with the payloads and acquires the data from payloads through LVDS interface. The read payload data will be formatted as per CCSDS Transfer Frame format. The playback data is RS (Reed Solomon) coded, randomized, convolution coded and given to the X-band system for transmission to the ground. The SSR memory capacity is variable and is based on mission requirement. The maximum capacity of SSR is 64 Gbit.
The SARAL payload data are transmitted in X-band at data rates of 32 Mbit/s. The transmitter uses a SSPA (Solid State Power Amplifier) with an EIRP output of ~4 W. A shaped beam antenna with a gain of +4.5 dBi at ±65° along with 4W SSPA provides the necessary link margin.
In addition, the UHF link is used to collect the data from the ground segment with the Argos-3 onboard instrument while the L-band is used to transmit the collected data to the data centers.
Figure 9: The SARAL spacecraft configuration (image credit: ISRO, CNES)
The SARAL spacecraft has a mass budget of about 407 kg. The IMS-2 has a total mass of 199 kg, while the total payload mass is 147 kg (with 5% margin). The design life is 5 years.
Launch: The SARAL minisatellite was launched on Feb. 25, 2013 from SDSC-SHAR (Sriharikota, India) on the PSLV-C20 launcher of ISRO. 16)
Orbit: Sun-synchronous near-circular dawn-dusk orbit, altitude of ~800 km, inclination of 98.538º, orbital period of 100.6 minutes, LTAN (Local Time on Ascending Node) = 6:00 hours, repeat cycle = 35 days (No of orbits within a cycle = 501).
The six secondary payloads manifested on this flight were:
• BRITE-Austria (CanX-3b) and UniBRITE (CanX-3a), both of Austria. UniBRITE and BRITE-Austria are part of the BRITE Constellation, short for "BRIght-star Target Explorer Constellation", a group of 6.5 kg, 20 cm x 20 cm x 20 cm nanosatellites who purpose is to photometrically measure low-level oscillations and temperature variations in the sky's 286 stars brighter than visual magnitude 3.5.
• Sapphire (Space Surveillance Mission of Canada), a minisatellite with a mass of 148 kg.
• NEOSSat (Near-Earth Object Surveillance Satellite), a microsatellite of Canada with a mass of ~74 kg.
• AAUSat3 (Aalborg University CubeSat-3), a student-developed nanosatellite (1U CubeSat) of AAU, Aalborg, Denmark. The project is sponsored by DaMSA (Danish Maritime Safety Organization).
• STRaND-1 (Surrey Training, Research and Nanosatellite Demonstrator), a 3U CubeSat (nanosatellite) of SSTL (Surrey Satellite Technology Limited) and the USSC (University of Surrey Space Centre), Guildford, UK. STRaND-1 has a mass of ~ 4.3 kg.
The University of Toronto arranged for the launch to carry three small satellites for universities as part of its Nanosatellite Launch Services program, designated NLS-8: BRITE-Austria, UniBRITE and AAUSat3. The three NLS satellites used the XPOD (Experimental Push Out Deployer) separation mechanism of UTIAS/SFL for deployment.
The STRaND-1 nanosatellite was deployed with the ISIPOD CubeSat dispenser of ISIS (Innovative Solutions In Space).
SARAL-AltiKa is intended as a gap filler mission between the RA-2 onboard Envisat (ESA, launched in 2002, and lost in 2012) and the SRAL onboard Copernicus/Sentinel-3 (to be launched in 2014). As such, SARAL-AltiKa will fly on the same orbit as Envisat of ESA (allowing to re-use the mean sea surface fields in the data processing, and to have sea level anomaly monitoring with high quality from the start of the mission), to ensure a continuity of altimetry observations in the long term. On the other hand, the local time of passage over the equator will be different due to specific cover requirements for the instruments constellation of the Argos system.
Figure 10: Artist's rendition of the SARAL spacecraft in orbit (image credit: CNES)
• February 3, 2020: The CNES/ISRO altimetric satellite SARAL/AltiKa was launched in February 2013 and since then has provided useful data for various scientific and operational applications in oceanography, hydrology, cryospheric sciences and geodesy. However, a Reaction Wheel problem forced relaxation of the repeatability constraint on the satellite’s orbit, which has been drifting slowly since July 2016. Beyond the expected contributions of this mission and its very good integration into the objectives of the constellation of altimetric satellites, it has become more and more apparent that specific contributions and innovations related to the main specification of SARAL/AltiKa, that is to say the use of the Ka-band, have clearly emerged. The advantages of the Ka-band are in short the reduction of ionosphere effects, the smaller footprint, the better horizontal resolution and the higher vertical resolution. A drawback of the Ka-band is the attenuation due to water/water vapor in case of rain and the resulting loss of data. The main objective of this paper is to highlight the specific advances of the Ka-band in different scientific and technical fields and to show why they are promising for the future and open the way to several missions or mission projects. Although unplanned initially, the fine coverage of the Drifting Phase brings some interesting openings especially for geodesy and hydrology applications. 17)
- The SARAL/AltiKa findings, justify the consideration of a new mission project, CRISTAL (Copernicus Polar Ice and Snow Topography Altimeter) mission, using Ku- and Ka- band together. CRISTAL is one of the 6 High Priority Candidate Missions (HPCM) of ESA's Copernicus Sentinel Expansion program. The first priorities of this polar mission is to survey the sea ice, its snow cover and the land ice, and its main objectives are to support the maritime operations in the polar oceans and to better understand the climate processes. This mission will be equipped with the first Ka/Ku SAR/SARin altimeter, and with an increased band-width (500 MHz) to improve the vertical resolution. The Ku-band insures the continuity of the long series of Ku altimetric survey of the solid and liquid water over the planet, and the Ka-band brings a clear innovation to measure the snow depth.
- Although it is already known, through SARAL/CryoSat-2 comparisons, that the Ka/Ku difference provides a good proxy for snow depth, further studies are needed to validate and determine possible corrections to this approach based on the characteristics of the snow (depth, density, grain sizes, salinity, number of layers, etc.) and its support (ice, sea ice, land). For that purpose, SARAL/AltiKa remains an essential and unique tool to accomplish the preparation of CRISTAL. Its extension should allow further analyses of the penetration of Ka- and Ku- bands in snow, in particular through comparisons with the new Ka/Ku radars on board the CryoVex aircraft, with the ICESat-2 lidar and with in situ measurements.
• In February 2018, the SARAL/AltiKa mission is completing five years on orbit. Since March 2015, technical issues have been encountered on the reaction wheels and this has led to the decision of relaxing the orbit. Then, after 4 July 2016, SARAL/AltiKa left its repetitive orbit by beginning a new phase named “SARAL Drifting Phase” (SARAL-DP). The data processing as well as data latency have not been changed. From this date, SARAL satellite does fly free of station keeping maneuvers which means there is a drift between successive ground tracks which are no more repetitive. The altimetry payload is operated nominally on the drifting orbit. The cycle duration of 35 days is no longer kept but the orbit has been chosen to preserve anyway subcycles within the range of 15 to 17 days quite relevant for mesoscale sampling. 18)
- Since the beginning of the SARAL/AltiKa mission, performances proved to be compliant with nominal specifications with an overall observed performance for the SSH (Sea Surface Height) RMS of 3.4 cm, lower than the mission requirement of 4 cm. In general, SARAL/AltiKa performances appear to be quite similar, and often better, than the Ku-band reference altimetric satellites such as Jason-2. Before launch, concerns were raised about the sensitivity of the Ka-band to rain events, leading to missing and invalid measurements. However, in practice, the SARAL/AltiKa data return is remarkably high with few missing data. Over the ocean, the data coverage is greater than 99.5%, which, once again, exceeds the mission requirements. SSH differences at crossovers is the main metric to assess the overall performance of satellite altimetry missions. In this regard, the results obtained from the SARAL/AltiKa mission are of the same order, or even better, as those obtained for Jason-2 mission.
- The same result is obtained for the SLA (Sea Level Anomalies): SARAL/AltiKa data quality is as good (or even slightly better) as Jason-2. Another key factor for altimetry applications is the spectral content of the data. AltiKa altimeter provides a signal-to-noise ratio which has never been obtained before. Compared to Jason-2 and CryoSat-2 (SAR mode) data, the spectral content of SARAL/AltiKa observations is largely improved for all wavelengths below 70 km. This is explained by the much lower white noise, due to the excellent Ka-band retracking performances and also to the 40 Hz measurement rate (higher than the standard 20 Hz on Jason-class and CryoSat-2 missions). In conclusion, the mission performances widely confirm the nominal expectations in terms of accuracy, data quality and data availability.
- SARAL/AltiKa’s main scientific objective is to provide data products to the oceanographic community with the aim to improve our knowledge of the ocean mesoscale variability, mainly associated to eddies, meandering currents, fronts, filaments and squirts. The mesoscale variability refers to ocean signals with space scales of 50 km to 500 km and time scales from a few days to a few months. The associated kinetic energy is at least an order of magnitude larger than that of the mean circulation. It is now well recognized that the mesoscale processes are a key component of the ocean circulation, shaping the mean currents, controlling the exchanges with the atmosphere and the marine biogeochemistry, and clearly playing a major role in the way the ocean participate to the climatic system.
- In summary, the SARAL/AltiKa altimeter the first oceanographic altimeter to use a high single frequency in Ka-band (35.75 GHz). A little less than five years after launch, the satellite and the instruments onboard are performing well although the control of the reaction wheels appeared to be impossible at some stage, end of 2015 and beginning of 2016, leading the leave of the exact 35 repeat orbit and a shift to the Drifting Orbit Phase. Note this change had no impact of the quality and the availability of the SARAL/AltiKa data but only on the time/space sampling. As of today, all components of the altimetric system (i.e. AltiKa itself) are working properly.
- Calibration and validation investigations have shown that the quality of the data meets the expectations and initial mission requirements and this is still correct. 19) Since the launch, the quality of all products appears to be continuously in line with the mission requirements. A large number of scientific investigations have been undertaken and SARAL/AltiKa has become a fully-fledged member of the altimetric constellation.
Table 3: A summary of the advantages/drawbacks of the Ka-band
- Regarding the oceans, SARAL/AltiKa provides an improved resolution of the SSH signal in particular of the mesoscales. Regarding the along-track SSH spatial resolution SARAL/AltiKa provides resolution down to 40–50 km wavelengths, whereas Jason and Envisat altimeters resolve down to 70–80 km wavelengths. Sentinel-3 SAR is still being analyzed, but it is suggested to also be resolved to 30–50 km wavelengths. The coastal oceans, directly benefit of the better discrimination in transition zones as well as this improved resolution of the measurements. Progress has been made in reducing the altimeter footprints and noise to approach closer to the coast, but also in processing algorithms, corrections, and products for coastal applications. The improved coastal altimeter data and their derived sea level and wind–wave data are being integrated into coastal observing systems. These data provide essential monitoring for both research and operational applications, in these coastal regions where in situ measurements are sparse.
- Regarding inland waters, SARAL/AltiKa definitely improves the quality of hydrology products derived from satellite altimetry. Especially for water level time series over lakes and rivers it was shown that SARAL/AltiKa dramatically improved the quality due to its smaller footprint and its higher pulse repetition frequency. In particular, SARAL/AltiKa allows the access to smaller rivers and lakes that were not “visible” with standard altimeters. The impact of clouds or precipitation on the resulting water levels exists, but is not so strong that the data cannot be used for reliable and accurate estimation of water level time series. With regard to Envisat for example, the waveform disruption for SARAL/AltiKa for height estimation is better with SARAL/AltiKa. SARAL/AltiKa has the potential to provide better water level heights until 6 km near to the lake shore than Envisat. In addition, SARAL/AltiKa provides more homogenous water level heights over rivers than Envisat.
- Regarding ice, the lesser radar penetration of snow is beneficial for various types of measurements. The difference between Ka and Ku-band height provides a good proxy of snow depth above sea ice that is a limitation to estimate ice thickness and is a climate indicator. Above ice sheet it ensures a mean to better constrain the waveform models and invert snowpack characteristics. Combined with a better spatial resolution, Ka-band in SAR mode for instance, would therefore allow closer monitoring of sea and continental ice. As for inland waters applications, ice applications gave evidence of the key importance of continuous observations along the same repeat orbit, as was possible with ERS-2, Envisat and SARAL/AltiKa (during the nominal phase).
- Regarding geodesy, it appears that AltiKa is an excellent altimeter for resolving short-wavelength geoid anomalies. SARAL/AltiKa is now in its Drifting Phase, the dataset that would result would be a boon to marine geophysics, bathymetric estimation, and seamount mapping. Seamount size-frequency distribution models suggest that there may be as many as 105 seamounts between 1 and 2 km in height that are uncharted and were not detected by previous Ku-band altimeters. During its Drifting Phase, SARAL/AltiKa will surely find some of these. — A general observation is that the implementation of SARAL/AltiKa data into operational systems has been extremely easy both into operational oceanography centers for the SSH signal and into meteorological centers for the SWH in particular. It has also been the case for building databases of various levels of sophistication, for example for AVISO that integrates SARAL/AltiKa data as well as other altimeter data.
- SARAL/AltiKa is in many respects a prototype of the altimetry of the future. SARAL/AltiKa represents the beginning of a new class of altimeters operating at Ka-band frequency with a small footprint and high pulse rate. The Ka-band is envisioned for several new altimetric satellite projects (i.e. AltiCryo from CNES, CryoSat-3 and SKIM from ESA) and the chosen band for the upcoming SWOT. It even opens more than previously the doors of interdisciplinary applications. Indeed, the extended capabilities that are offered by the Ka-band allow to open even more widely some new frontiers of altimetry such as coastal oceanography, cryosphere, hydrology, beyond the traditional scope of the open ocean investigations. Ka-band altimetry, with SARAL/AltiKa as the today’s most emblematic declination, can be seen also as a step towards improved resolution altimetry and a preparation for the NASA/CNES SWOT mission project and the KaRIN (Ka-band Radar INterferometer) instrument.
• Geodetic Orbit (From July 2016 until End Of Life): From July 4, 2016, SARAL enters a new phase called SARAL-DP (Drifting Phase). Its altitude of 800 km is increased by 1 km and no more maneuvers are performed on the satellite, except for collision avoidance. This phase has a cycle numbering beginning at 100 and the pass 1 starts at the first South Pole crossing after the altitude increase maneuver. 20)
• On Feb. 25, 2016, the joint CNES/ISRO SARAL mission was 3 years on orbit. It carries the first radar altimeter in Ka-band (AltiKa) providing a new path for investigation in oceanography as well as for ice studies. These higher frequencies in Ka-band (35.75 GHz instead of 13.6 GHz for Ku-band and 5.3 GHz for C-band) offer a higher spatial resolution, with more waveform samples, so data acquisitions can be spaced closer between each other along the satellite ground track (~ every 180 m) with a reduced footprint. 21)
- SARAL, with a high inclination orbit (at 98.55º) allows an optimal coverage in high latitudes, in the Arctic and the Antarctic. Its measurements are therefore naturally used for studying the cryosphere. By observing both in high latitudes, on different orbits, the combined use of the AltiKa’s Ka-band and the Ku-band of Cryosat-2 is ideally complementary.
• Feb. 2015: The SARAL spacecraft is fully operational and carefully controlled by the ISRO team. The ARGOS and AltiKa payloads are fully operational. The ARGOS service has an excellent availability and altimetry products are very valuable and provide excellent measurements on ice sheets, river and lakes which is new with respect to previous CNES altimetry satellites like Jason-2/3. AltiKa data are routinely distributed worldwide and contribute largely to the CMEMS (Copernicus Marine Service). 22)
• Nov. 2014: The current version of the products (NRT and off-line products) is V2 , installed in January 2014. The V3 is under discussion and should be installed in early 2015. The results shown in the following section are based on the V2. 23)
- Product quality assessment over the ocean: The validation and analysis of the user products quality is a key contributor to the success of any altimetry mission. The first point to look at is the data coverage, in particular over the ocean. To do so, missing measurements are monitored toward the reference ground track (i.e. assuming a full coverage of the satellite data whatever the conditions are). The SARAL/AltiKa data return is remarkably high with a very few missing data over ocean. Figure 11 shows that the data coverage is greater than 99.5 %, which largely meets the mission requirements.
SARAL/AltiKa is intended as a gap filler mission between the RA-2 onboard Envisat (ESA, launched in 2002, and lost in April 2012) and the SRAL (SAR Radar Altimeter) onboard the Sentinel-3 mission of ESA (to be launched in late 2015). As such, SARAL/AltiKa flies on the same orbit as Envisat, allowing to re-use the mean surfaces fields in the data processing, and to have sea level anomaly monitoring with high quality from the start of the mission.
• On October 27, 2014, the SARAL/AltiKa Workshop was held in Konstanz, Germany to review the calibration/validation studies of the first SARAL data and the scientific investigations of data from the Franco-Indian satellite 20 months after its launch on 25 February 2013. The workshop highlighted the new altimetry capabilities of the Ka-band in areas such as oceanography (improved resolution in the open ocean, coastal altimetry), hydrology and the cryosphere.
• Feb. 2014: The SARAL spacecraft and its payload are operating nominally. 24)
• On 6 December 2013, Cyclone Xaver pounded the coasts and the North Sea. All along the Wadden Sea, the barrier islands along the north of the Netherlands and the northwest of Germany experienced record storm surges. Two tide gages along the German North Sea coast confirmed that the storm drove sea level to about 3 m above the normal tide level. The altimeter in the mean time shows that the storm surge was noticeable as far as 400 km from the coast. 25)
- The altimeter measured wind speeds of 20 m/s nearly monotonically throughout the North Sea. An offshore anemometer corroborated this value. An offshore buoy measured wave heights of 8 m, matching quite well the measurements from the altimeter, ranging from 6 m near the German coast to 12 m further out into the North Sea.
- Compared are the altimeter-derived and in-situ sea level, wave height and wind speed products with outputs from the Operation Circulation and Forecast model of the BSH (Bundesamt für Seeschifffahrt und Hydrographie) and with a global storm surge forecast and inundation model of the JRC (Joint Research Centre) of the European Commission.
Figure 12: Storm surge measured by the radar altimeter AltiKa on-board the SARAL satellite and various types of in-situ data and models (image credit: EUMETSAT)
• Sept. 2013: The SARAL GDR (Geophysical Data Record) products are routinely generated since July 2013, and delivered to all users since September 2013 in version 'T'. GDR dissemination to all users on the AVISO ftp server. 26)
- OGDR (Operational Geophysical Data Record) and IGDR products have been distributed to all users from June 25th; GDR products are available to PIs since August 2, 2013.
- Some algorithms are still to be tuned: neural network used for radiometer data ground processing, Sea State Bias computation, altimeter wind speed and ICE2 retracking.
- According to users presentations, the main message of this meeting is the easiness to use the data (thanks to the ground segment operationnality and the expertise of the users). The level of quality achieved by the products in terms of accuracy, data latency and availability is such that the Saral/AltiKa data are now operationnaly integrated in several forecast systems.
Figure 13: Preview: the first waves' height map made from ALTIKA measurements (image credit: CNES) 29)
• The final orbit of the SARAL spacecraft was reached on March 13,2013. The cycle No 1 was started on March 14, 2013. 30) The AltiKa verification phase has begun with the cycle 1, that is to say since the well-functioning satellite reached its final orbit and crossed the equator at the predefine longitude of the first ascending node.
- After a quick validation performed by the mission project, the Near Real Time products (OGDR and IGDR) have been delivered in "T" version (for test) to the SARAL/AltiKa PIs from, respectively, 21st and 28th March, 2013.
• The subsystems of the SARAL platform were switched on between February 25 and 26, 2013. All the instruments are nominal. The first days of instrument commissioning indicate good performances, as expected. 31)
Sensor complement: (AltiKa, DORIS, LRA, Argos-3)
AltiKa (Altimeter in Ka-band):
The AltiKa project of CNES is based on a wideband Ka-band altimeter (35.75 GHz, 500 MHz), which will be the first oceanography altimeter to operate at such a high frequency. The high-resolution AltiKa altimeter has a dual-frequency radiometric function which allows the altimetry measurements to be corrected for the effects due to the signal passing through the wet troposphere. This is coupled with the DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) tracking system and an LRA (Laser Retroreflector Array) for the measurement of precision orbits. 32) 33) 34) 35) 36) 37) 38)
The key feature of the altimeter payload has been the selection of Ka-band (35.75 GHz) for the altimeter avoids the need for a second frequency (necessary when using the Ku-band) to correct the ionospheric delay; the configuration allows the same antenna to be shared by the altimeter and the radiometer. This single antenna solves the accommodation problem of a conventional altimetry payload on a minisatellite (150 kg class). The Ka-band concept allows also the improvement of the range measurement accuracy by a factor of 3:1 due to the use of a wider bandwidth and a better pulse-to-pulse echo decorrelation.
The AltiKa design and development is a partnership of CNES, scientific laboratories (LEGI/CNRS, IFREMER, CLS, etc.) and industry, with TAS-F (Thales Alenia Space-France) as prime instrument contractor. The AltiKa instrumentation consists of an integrated altimeter/radiometer instrument. It is composed of the following elements:
• ORA (Offset Reflector Antenna) shared by the altimeter and the radiometer functions
• AMU (Altimeter Microwave Unit). The objective is to gather all analog support tasks of the altimeter operations (bandwidth expansion, frequency translations, local oscillator generation, high power amplification, transmitter/receiver/antenna duplexing, low noise amplification, deramp processing, bandwidth filtering, IF processing).
• RMU (Radiometer Microwave Unit). The objective is to gather all analog support tasks of the radiometer operations (low noise amplification, bandwidth filtering, high gain amplification, power detection, low pass filtering).
• DPU (Digital Processing Unit). The objective is to gather all the (hardware and software) processing functions of the instrument (digital chirp generator, time FFT, altimeter echo processing, radiometer signal processing, instrument interface handling with the platform computer).
• RCU (Radiometer Calibration Unit). The objective is to assure the interconnection and switching between the antenna, the radiometer receivers, and the radiometer calibration loads or horn. The RCU is in charge to the connection of the radiometer receivers either to the nadir measurement path or to calibration paths: a sky horn pointing to deep space for cold reference and a load at ambient temperature for hot reference. The RCU was developed by COMDEV-UK.
Figure 14: Functional block diagram of the integrated AltiKa instrument (image credit: CNES, TAS-F)
The altimeter design principle is of Poseidon heritage (Ku-band instrument flown on the Jason missions) and is based on the classical deramp technique for pulse compression Four key evolutions on AltiKa have improved significantly the radar performance compared to conventional Ku-band altimeters:
1) The larger bandwidth from 320 MHz (of Ku-band instruments) to 480 MHz (Ka-band) provides a vertical resolution improvement from 0.5 m to 0.3 m, providing also the same magnitude of improvement in the range accuracy.
2) The shorter decorrelation time of the sea echoes at Ka-band permit an increase in the PRF (Pulse Reception Frequency) for averaging efficiently more echoes in the same integration time. Hence, the PRF has been doubled with respect to the Ku-band (Poseidon) instruments. AltiKa provides a PRF of about 4000 Hz adjustable along the orbit to cope with altitude variations in the surface profile.
3) The antenna has a smaller beamwidth due to the increase in signal frequency, thus providing a smaller footprint in the target area. At an orbital altitude of 800 km, the 6 dB footprint is around 8 km for AltiKa, compared to 30 km for Poseidon. Hence, more accurate measurements in the coastal regions can be expected (better discrimination in transition zones).
4) An innovative echo tracking concept is being employed based on an internally stored DEM (Digital Elevation Model) of the sub-satellite track (ocean and land surfaces) and on the use of the real-time satellite altitude information provided by the DORIS navigator software DIODE. These features help in providing altimetric measurements on surfaces where conventional closed-loop tracking solutions have difficulties to keep the echoes within the narrow range window.
The range measurement accuracy versus wave height is illustrated in Figure 15. The improvement for AltiKa compared to Poseidon-2 is in the order of 3.
Table 4: Key parameters of the AltiKa instrument
Radiometer design: The instrument is a total power radiometer with a direct detection capability. It consists of two RF receivers, centered on 37 and 23.8 GHz, and a calibration unit enabling the connection of the receivers either to a sky horn pointing to provides a cold space reference, or to a load at ambient temperature (hot reference).
In the nominal mode, the radiometer receivers measure the antenna temperatures (RM1 to RM5). In the internal mode, the receivers are either connected to a sky horn pointing to deep space or to a load at ambient temperature. This internal calibration can be performed every few seconds.
Table 5: Key radiometer parameters and performances
The critical technologies for AltiKa development concern the Ka-band functions except the radiometer receivers that are taken from the receivers of the Megha-Tropiques mission (ISRO/CNES). This includes:
• The multi-frequency antenna
• The Ka-band SSPA (Solid State Power Amplifier)
• The Ka-band LNA (Low Noise Amplifier)
• The Ka-band radiometer calibration unit
• The Ka-band altimeter duplexer equipment (ADE)
The AltiKa antenna (Figure 16) is a fixed offset reflector (1 m aperture diameter, 0.7 m focal length, 0.1 m offset) with a tri-band feed (35.5 - 36 GHz, 23.6 - 24 GHz, 36.5 - 37.5 GHz). The feed includes an OMT (Ortho-Mode Transducer) device for the separation of the altimeter and radiometer channels which use perpendicular polarizations, and a diplexer for the separation of the radiometer (K-band, Ka-band) channels.
The characteristics of the SSPA are the following: gain of 35 dB, useful bandwidth of 500 MHz, output power of 2 W (33 dBm). The power dies are being built in D01PH technology of OMMIC, France. The output combiner is in waveguide technology to minimize losses. All RF components of the SSPA are provided with a pulsed power converter to minimize power consumption and dissipation, and to decrease the junction temperatures of the components.
Figure 17: RF synoptic of the pulsed SSPA (image credit: CNES)
The real-time telemetry data rate of AltiKa (altimeter+ radiometer) is about 43 kbit/s.
RCU (Radiometer Calibration Unit). The RCU consists of two separate channels operating in the K- and Ka-bands. Two ferrite switches are provided in each channel to allow selection of one of the three different operational paths, corresponding to each of the operational modes. Each channel uses a very low VSWR waveguide load, which acts as the “hot” calibration source. Additionally, the Ka-band channel contains a bandpass filter to improve the isolation between the RCU channels and the altimeter.
The ADE (Altimeter Duplexer Equipment) consists of a single ferrite switch on the transmit path and four ferrite switches in the receive path. The transmit and receive paths are connected via a ferrite circulator which also provides a directional path to and from the antenna port. Two cross-guide couplers provide the calibration path. The RCU and the ADE are developed at COM DEV, UK.
Figure 18: View of the RFU instrumentation (image credit: COM DEV)
Figure 19: CAD view of the integrated AMU (Altimeter Microwave Unit), image credit: CNES
Table 6: AltiKa instrument budgets
Table 7: Main radiometer characteristics & performances (Ref. 35)
DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite):
DORIS is a dual-frequency tracking system (400 MHz and 2 GHz) based on network of emitting ground beacons spread all over the world. The DORIS on-board package is composed of:
• A dual-frequency antenna (omnidirectional antenna located on the nadir face of the satellite). The antenna has a mass of 2 kg and a size of 420 mm (length) and 160 mm in diameter.
• A BDR (DORIS Redundant Box) which is composed of two DORIS chains in cold redundancy accommodated inside the same electronic box. Each DORIS chain includes a MVR (Mesure de Vitesse radiale) unit achieving beacon' signal acquisition and processing, navigation, Doppler measurements storage and formatting, electrical satellite interfaces management functions, and a USO (Ultra-Stable Oscillator) delivering a very stable 10 MHz reference which is also used by the altimeter. BDR has a mass of 18 kg and a size of 390 mm x 370 mm x 165 mm.
Figure 20: View of the DORIS BDR and antenna (image credit: TAS-F)
LRA (Laser Retroreflector Array):
LRA is provided by CNES. The objective of LRA is to calibrate the precise orbit determination system and the altimeter system several times throughout the mission. The LRA is a passive system used to locate the satellite with laser shots from ground stations with an accuracy of a few millimeters. The reflective function is done by a set of 9 corner cube reflectors, with a conical arrangement providing a 150º wide field of view over the full 360º azimuth angle. 39) 40)
According to CNES optomechanical specifications, the LRA has been developed by Thales SESO (Société Européenne de Systèmes Optiques), Aix en Provence, France (SESO is a Thales group subsidiary as of 2011). The corner cube reflectors were provided with a very stringent dihedral angle error of 1.6 arcsec and an accuracy within ±0.5 arcsec.
During the development phase, SESO has performed mechanical, thermal and thermo-optical analyses. The optical gradient of each corner cube, as well as the angular deviations and PSF (Point Spread Function) in each laser range finding direction, have been computed. The mechanical and thermal tests have been successfully performed. A thermo-optical test has successfully confirmed the optical effect of the predicted in-flight thermal gradients. Each reflector was characterized to find its best location in the LRA housing and give the maximum optimization to the space telemetering mission.
Figure 21: Photo of the LRA with nine corner cubes (image credit: SESO, CNES)
Table 8: Main characteristics of LRA
Corner cubes: The mean error is 0.17 arcsec and the maximum error is 0.42 arcsec! Due to this high accuracy, the real optical pattern will be very close to the model and within the required performances. The PSF has been characterized and analyzed for each corner cube. This verification has permitted to confirm their position and orientation. The qualitative and quantitative verifications have been made by comparing the measured PSFs with the simulated analysis values. The Figure 22 illustrates the analyses made.
Table 9: Main characteristics of the corner cubes
Argos-3 (Data Collection System):
Argos-3 of CNES, manufactured by TAS (Thales Alenia Space). The objective is to collect data from remote terminals in the ground segment referred to as PPTs (Platform Transmitter Terminals).
The Argos-3 onboard package represents the newest generation of the Argos system. The major improvement of the new Argos-3 system is that it will now be able to send orders to its terminals whereas before the onboard instruments were only capable of receiving data (up to Argos-2 inclusive). The MetOp-A spacecraft of EUMETSAT (launch Oct. 19, 2006) is carrying the first Argos-3 instrument demonstrator package. In comparison to previous generations, system performance is enhanced via a unique downlink and a high data uplink (4800 bit/s versus 400 bit/s), while insuring complete compatibility with existing systems in the ground segment. Thanks to digital processing, the new instrument is lighter and more compact than its predecessors on analog basis. Argos-3 is capable of receiving messages from over 1000 PTTs (Platform Transmitter Terminal) simultaneously within the satellite's FOV (Field of View). 41)
The Argos-3 onboard instrument is composed of the following components:
• The RPU (Receiver Processor Unit) providing the following functions:
- Processing of the received uplink signals
- Downlink management
- Interfaces with the receiver, the TxU and the satellite
• The TxU (Transmitter Unit) is sending the emissions (messages) to the PTTs in the ground segment including error-free message acknowledgement signals.
• The harness for the RPU to TxU connection
The RPU (16 kg, 36 W) and TxU (8kg, 26 W) boxes have a cold internal redundancy that can be activated by TC level. In the same way, the USO (Ultra Stable Oscillator) has a cold redundancy. RPU has dimensions of 365 mm x 280 mm x 365 mm, TxU has dimensions of 100 mm x 280 mm x 310 mm.
RPU (Receiver Processor Unit). The RPU onboard a spacecraft processes received uplink signals @ 401.6 MHz, measures the incoming frequency, time-tags the message, creates and buffers mission telemetry, manages the downlink and acts as interface between the receiver, the TxU (Transmitter Unit) and the satellite. Featuring fully digital processing, the RPU stores messages and either relays them in real-time to the nearest regional antenna - or in deferred time to a global center (maintained by NOAA, Eumetsat). A backup RPU is included as part of the device.
Figure 23: Illustration of the Argos-3 onboard instrument package (image credit: TAS-F)
Figure 24: Block diagram of the Argos-3 payload on SARAL (image credit: CNES)
Figure 25: SARAL project organization (image credit: CNES)
• CNES is responsible for the production, the archiving and the distribution of near-realtime altimetry products (OGDR: Operational Geophysical Data Record) and delayed products (GDR: Geophysical Data Record, IGDR, S-IGDR and S-GDR) to users outside India.
• Production of the mission preliminary and precise orbits is realized by CNES, from DORIS system data, completed by the laser system data, for the precise orbit. The operational upkeep of DORIS components and system is ensured by CNES.
• CNES uses EUMETSAT support for the operation of ground stations in Sweden, as well as to produce, archive and distribute near-realtime products to users outside India
• CNES supplies the near-realtime data processor to EUMETSAT and ISRO, as well as a support for its integration, test and operation
• CNES supplies the processor for delayed products to ISRO, as well as a support for its integration, test and operation
• ISRO is responsible for the production, the archiving and the distribution to Indian users of near-realtime and delayed-time altimetry products
• ISRO is responsible for the command-control operations of the satellite.
• All housekeeping and scientific telemetry as well as auxiliary data are archived by CNES and ISRO
• CNES and ISRO are responsible for the product enhanced value and the support to their respective users.
• CNES is responsible for the coordination of AltiKa with the other altimetry missions. The enhanced value products of DUACS (Data Unification and Altimeter Combination System) generated by CNES are archived and distributed under CNES responsibility.
• CNES is responsible for the expertise and the long term CalVal.
Figure 26: SARAL system overview (image credit: CNES)
Figure 27: The SARAL ground segment (image credit: CNES, Ref. 42)
3) J. Verron, F. Ardhuin, S. Arnault, P. Bahurel, F. Birol, P. Brasseur, S. Calmant, A. Cazenave, B. Chpron, J. F. Cretaux, P. De Mey, E. Dombrowsky, J. Dorandeu, L. Eymard, J. Lambin, J. M. Lefevre, B. Legresy, P. Y. Le Traon, F. Lyard, F. Mercier, E. Obligis, P. Sengenes, F. Seyler, N. Steunou, E. Thouvenot, J. Tournadre, “ARAL/AltiKa - An Altimetry mission in Ka-band,” 2010 OSTST (Ocean Surface Topography Science Team) meeting, Lisbon, Portugal, Oct. 18-20, 2010, URL: http://www.aviso.oceanobs.com/fileadmin/documents/OSTST/2010/oral/Verron.pdf
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15) D. R. M. Samudraiah, D. V. A. Raghava Murthy, “Small Satellites planned by ISRO for Earth Observation,” 58th IAC (International Astronautical Congress), International Space Expo, Hyderabad, India, Sept. 24-28, 2007, IAC-07- B4.4.8
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17) Jacques Verron, Pascal Bonnefond, Ole Andersen, Fabrice Ardhuin, Muriel Bergé-Nguyen, Suchandra Bhowmick, Denis Blumstein, Francois Boy, Laurent Brodeauh, Jean-Franccois Crétaux, Mei Ling Dabat, Gérald Dibarboure, Sara Fleury, Florent Garnier, Lionel Gourdeau, Karen Marks, Nade`ge Queruel, David Sandwell, Walter H. F. Smith, E. D. Zaron, ”The SARAL/AltiKa mission: A step forward to the future of altimetry,” Advances in Space Research, Available online 3 February 2020, https://doi.org/10.1016/j.asr.2020.01.030
18) Jacques Verron, Pascal Bonnefond, Lofti Aouf, Florence Birol, Suchandra A. Bhowmick, Stéphane Calmant, Taina Conchy, Jean-François Crétaux, Gérald Dibarboure, A. K. Dubey, Yannice Faugère, Kevin Guerreiro, P. K. Gupta, Mathieu Hamon, Fatma Jebri, Raj Kumar, Rosemary Morrow, Ananda Pascual, Marie-Isabelle Pujol, Elisabeth Rémy, Frédérique Rémy, Walter H. F. Smith, Jean Tournadre, Oscar Vergara, ”The Benefits of the Ka-Band as Evidenced from the SARAL/AltiKa Altimetric Mission: Scientific Applications,” Remote Sensing, Vol. 10, 163, 2018, doi:10.3390/rs10020163, URL: http://www.mdpi.com/2072-4292/10/2/163/pdf
19) Pascal Bonnefond, Jacques Verron, Jérémie Aublanc, K. N. Babu, Muriel Bergé-Nguyen, Mathilde Cancet, Aditya Chaudhary, Jean-François Crétaux, Frédéric Frappart, Bruce J. Haines, Olivier Laurain, Annabelle Ollivier, Jean-Christophe Poisson, Pierre Prandi, Rashmi Sharma, Pierre Thibaut, Christopher Watson, ”The Benefits of the Ka-Band as Evidenced from the SARAL/AltiKa Altimetric Mission: Quality Assessment and Unique Characteristics of AltiKa Data,” Remote Sensing, Vol. 10, 83, 2018, doi:10.3390/rs10010083, URL: http://www.mdpi.com/2072-4292/10/1/83/pdf
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35) Frédéric Robert, Jacques Richard, Benoît Durand, Nicolas Taveneau, Nathalie Steunou , Pierre Sengenès, “AltiKa, a Ka-band instrument for space altimetry with improved performances and ocean sampling: Instrument final test results,” Proceedings of the 2nd Workshop on Advanced RF Sensors and Remote Sensing Instruments 2009, Noordwijk, The Netherlands, Nov. 17-18, 2009
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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 (firstname.lastname@example.org).