SWOT (Surface Water Ocean Topography) Mission
SWOT is a wide-swath altimeter mission concept - a proposal under study/definition in 2009 by NASA/JPL and CNES - for accurate future monitoring of local sea level changes at the land-sea interface. The SWOT mission has been recommended in 2007 by the NRC (National Research Council) decadal survey "Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond" for implementation by NASA. 1) 2) 3)
The overall scientific objectives and goals of SWOT, in particular by the water resources and hydrologic community, are to contribute to a fundamental understanding of the Earth system by providing global measurements of continental surface water storage changes and discharge, which are critical for water and climate cycle models. 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20)
The main mission goals of the demonstration mission are:
• The primary oceanographic objectives of the SWOT mission are to characterize the ocean mesoscale and sub-mesoscale circulation at spatial resolutions of 10 km and larger.
• The hydrologic science objectives of the SWOT mission are:
- To provide a global inventory of all terrestrial surface water bodies whose surface area exceeds 250 m2 (lakes, reservoirs, wetlands) and rivers whose width exceeds 100 m (requirement) (50 m goal)
- To measure the global storage change in terrestrial surface water bodies (for man-made reservoirs, total storage) at sub-monthly, seasonal, and annual time scales.
- To estimate the global change in river discharge at sub-monthly, seasonal, and annual time scales.
In 2006, WatER was an international (NASA, CNES, ESA) proposal and initiative (satellite mission in study phase) focused on the hydrologic processes related to the spatial and temporal variability in global terrestrial surface fresh water storage. Fresh water bodies cover at least 4% of Earth's terrestrial surface whereas tropical wetlands, particularly in the Amazon Basin, cover nearly 20% of their watershed. The vast storage capacity of wetlands, reservoirs, rivers, etc. impact the global water cycle.
The processes include the movement of water through a basin, across a continent and into the oceans; the hydrologic mass exchange that occurs between floodplains and channels; the changes in water storage on the continents; and the management of water resources. Climatic, sediment, biogeochemical, and temperature processes are inherently tied to hydrologic mass-balance and transport, thus they are included in our science goals to make for a more complete understanding of the terrestrial surface water branch of the global water cycle.
To identify the temporal and spatial variability in surface water storage requires measurements of volumetric change. Thus, the minimum requirements are the measurements of: 1) surface water area extent, 2) elevations of the water surface, h, of wetlands, rivers, lakes, reservoirs, etc., and 3) temporal changes in water levels, i.e. the derivatives in time and space (Δh/Δt, Δh/Δx). The requirements call for spatial resolutions of water surfaces in the range of 10-100 m at temporal resolutions of 1 week.
In the fall of 2012, SWOT passed two important milestones and has now entered "Phase A" in the NASA mission development life cycle. 21)
Some Background: A series of workshops and a symposium were held in Europe since 2003 [by a group of European (France, ESA, UK, etc.) and US scientists, in particular the NASA SWWG (Surface Water Working Group)] on the topic of defining a proposal for a satellite mission dedicated to terrestrial surface water.
- Workshop on Hydrology from Space, Toulouse, France, Sept. 29-Oct. 1, 2003
- Workshop at LEGOS (Laboratoire d'Études en Géophysique et Océanographie Spatiale), Toulouse, France, Nov. 15-16, 2004
- Surface Water Satellite Mission workshop at ESA/ESRIN, Frascaty, Italy March 7-8, 2005
- Symposium: 15 Years of Progress in Radar Altimetry, Venice, Italy, March 13-16, 2006
- Joint Meeting of Ocean Sciences and Surface Water Hydrology in Support of Wide-Swath Altimetry Measurements, Washington DC, Oct. 30-31, 2006
- OSTST (Ocean Surface Topography Science Team) meeting was held in Hobart, Australia on March 12-15, 2007.
The growing concern about our planet's fresh water resources, coupled with the economically driven decrease in groundbased gauge measurements, has focused attention on the possibility of using space-based data sets for remote measurements of river and lake heights. The most direct measurements of inland water heights are obtained from satellite altimeters, with long time series of such observations having been built up over the past decade.
Presently, altimeters are configured for oceanographic applications, thus lacking the spatial resolution that may be possible for rivers and wetlands. The conventional nadir-looking altimeters (of radar or lidar type) provide a very limited coverage (intersatellite distance of typically 150 km for the TOPEX/Poseidon - Jason-1 tandem), and a spatial resolution of ....... These conventional altimeter profiles are not suitable of supplying the measurements needed to address scientific and societal questions.
SWOT will repeatedly measure the spatially distributed water surface elevations (h) of wetlands, rivers, lakes, reservoirs, etc. Successive h measurements yield dh/dt, (t is time), hence a volumetric change in water stored or lost. Individual images of h yield dh/dx (x is distance), hence surface water slope, which is necessary for estimating streamflow. 22)
Figure 1: An overview of global altimeter missions, March 2012 (image credit: CNES) 23)
SWOT Development Status:
• On Sept. 29, 2015, CNES contracted with TAS to build a next-generation radar altimeter for the billion-dollar U.S.-French SWOT (Surface Water Ocean Topography) mission to launch in 2020. Under the contract, TAS will deliver SWOT's Poseidon-3C nadir altimeter. 24)
• On January 6, 2015, CNES signed the first of a planned three contracts for the French/ U.S. SWOT (Surface Water and Ocean Topography) satellite, a mission in which the two governments plan to invest some $1.1 billion for a launch in 2020. The first contract was awarded to TAS (Thales Alenia Space) of France for the development of the SWOT satellite platform and its integration. 25) 26)
• September 2014: The SWOT mission is being jointly developed by NASA and CNES, with contributions from CSA (Canadian Space Agency) and UKSA (United Kingdom Space Agency). The SWOT mission, along with the airborne conceptvalidation project, AirSWOT, would provide valuable data and information that would benefit society in two critical areas; freshwater on land, and the oceans' role in climate change. It would fulfill important observations of the amount and variability of water stored in global lakes, reservoirs, wetlands, and river channels and would support derived estimates of river discharge. SWOT would also provide critical information necessary for water management, particularly in international hydrological basins. 27) 28)
• August 18, 2014: The Canadian government announced, that Canada , via CSA (Canadian Space Agency), is investing in Canadian innovation that will play a key role in the first-ever global survey of surface water. CPI Canada ( Communications and Power Industries Canada Inc.) of Georgetown, Ontario, will develop the EIK (Extended Interaction Klystron), a satellite radar component that will generate pulses used to gather surface information. 29)
• May 2, 2014: NASA and CNES have agreed to jointly build, launch, and operate a spacecraft to conduct the first-ever global survey of Earth's surface water and to map ocean surface height with unprecedented detail. NASA Administrator Charles Bolden and CNES President Jean-Yves Le Gall signed an agreement at NASA Headquarters in Washington to move from feasibility studies to implementation of the SWOT mission. The two agencies began initial joint studies on the mission in 2009 and plan to complete preliminary design activities in 2016, with launch planned in 2020. 30) 31)
- This new agreement covers the entire life cycle of the mission, from spacecraft design and construction through launch, science operations, and eventual decommissioning. NASA will provide the SWOT payload module, the KaRIn (Ka-band Radar Interferometer) instrument, the Microwave Radiometer (MR) with its antenna, a laser retroreflector array, a GPS receiver payload, ground support, and launch services. This new agreement covers the entire life cycle of the mission, from spacecraft design and construction through launch, science operations, and eventual decommissioning.
- NASA will provide the SWOT payload module, the KaRIn (Ka-band Radar Interferometer) instrument, the MR (Microwave Radiometer) with its antenna, a laser retroreflector array, a GPS receiver payload, ground support, and launch services.
- CNES will provide the SWOT spacecraft bus, the KaRIn instrument's RFU (Radio Frequency Unit), the dual frequency Ku/C-band Nadir Altimeter, the DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) receiver package, satellite command and control, and data processing infrastructure.
SWOT will be able to measure the ocean's surface with 10 times the resolution of current technologies. This will allow scientists to study small-scale features that are key components of how heat and carbon are exchanged between the ocean and atmosphere. The higher resolution of SWOT observations also will enable researchers to compute the velocity and energy of ocean circulation. A better understanding of small-scale ocean currents and eddies is also important to impacts on coastal regions such as navigation, erosion and dispersing pollutants.
SWOT is a swath-based SAR altimetry mission designed to acquire elevations of water surfaces at spatial and temporal scales necessary for answering key water cycle and water management questions of global importance. SAR altimetry is the only method capable of producing images of high resolution water surface elevation measurements [i.e., provision of h, Δh/Δx (slope), and Δh/Δt (changes in water level over time)].
The SWOT mission architecture is pictured in Figure 2 and consists of a space segment comprising a Flight System composed of the spacecraft bus and science payload and a ground segment that includes the ground station network, spacecraft and payload operations centers, data processing systems, and data archiving systems. The Flight System collects data from its science payload and relays it via an X-band downlink of 800 Mbit/s to a global network of CNES-supplied ground stations. The large daily volume of ~7 Tbit requires ~ 20 downlinks/day. The spacecraft engineering and housekeeping telemetry, with its much more modest volume, is downlinked via S-band twice a day to the ground network. The spacecraft and science instrument commanding is done via S-band through the same ground station network. Once received, the science data are relayed to the JPL and CNES POCs (Payload Operation Centers) for processing, distribution to science and other users, and archiving. 32)
Figure 2: Overview of the mission architecture (image credit: NASA)
Key mission drivers: A number of top level mission requirements are key drivers to the mission concept design. The are the orbit, the ocean height accuracy, and the hydrology surface water spatial resolution. A non-sun-synchronous highly inclined orbit of 78º inclination was selected to minimize tidal aliasing of the ocean SSH measurements and to ensure coverage of major water bodies on land. The associated large variation in solar angle relative to the spacecraft drives the power subsystem design, resulting in large solar arrays (area ~35 m2) and increased battery sizing. The ocean height accuracy drives the interferometer baseline length, the antenna/boom stability and the roll knowledge drift requirement to < 50 marsec/2.5 minutes. The surface water spatial resolution of 50 m impacts the onboard data handling and data downlink requirements due to the large associated data volume (+ 7 TBit/day) and the raw data rate of KaRIn of 360 Mbit/s.
The SWOT measurement concept is illustrated in Figure 4. The SWOT spacecraft measures satellite and ocean range through its nadir altimeter, corrected for the effects of columnar water water through its microwave radiometer. These measurements are coupled with those made in two swaths by the KaRIn instrument. Ground based laser ranging systems and DORIS beacons, plus the spaceborne GPS (Global Positioning System) provide POD (Precision Orbit Determination) for calculating SSH (Sea Surface Height). For hydrology-related height and slope measurements for water bodies and rivers, a similar approach is taken, although the nadir altimeter measurements are not required due to less stringent height accuracy requirements. SWOT will provide measurements with a spatial resolution of 1 km for the ocean (after on-board processing), and 50 m for land water, both with centimetric height accuracy , ~ 3 cm for ocean and 10 cm for land (Ref. 32).
Figure 3: Illustration of the SWOT satellite in deployed configuration (image credit: CNES) 33)
Figure 4: The SWOT measurement concept (image credit: NASA)
While conventional altimetry relies on the power and the specific shape of the leading edge of the return waveform, which is only available for the nadir point, the interferometric technique relies on the measurement of the relative delay between the signals measured by two antennas separated by a known distance (hereafter termed "baseline", Figure 5), together with the system ranging information, to derive the height for every imaged pixel in the scene. 34)
Figure 5: KaRIn interferometer baseline (image credit: NASA/JPL, CNES)
For a given point on the ground, a triangle is thus formed by the baseline B, and the range distance to the two antennas, which can be used to geolocate in the plane of the observation. The range difference between the two antennas is determined by the relative phase difference between the two signals (Figure 6).
Figure 6: Interferometric measurement concept (image credit: NASA/JPL, CNES)
The importance of the cross-track pointing angle, including the roll knowledge (Θ), in the measurement is evident. For that reason, the SWOT flight system includes a precision gyro in the KaRIn payload module. In addition, its antenna/boom must be very stable and disturbances from spacecraft mechanisms must be minimized. The KaRIn antenna baseline (B) is specified to be 10 m.
The difference between two sides (Δ r) is obtained from the phase difference (Θ) between the two radar channels. One can determine the height measurement from the two equations stated in Figure 6.
KaRIn operates in the so- called "Non Ping Pong mode": nominally, the radar transmits out of one antenna and receives on both, thus creating an interferometric pair of each swath (Figure 7) The isolation between the two swaths is accomplished by means of offset feed reflectarray antennas which produce beams of orthogonal polarizations for each swath.
The intrinsic range resolution is 75 cm, and the ground cross-track range resolution varies from about 70 m in the near swath to about 10 m in the far swath.
Several sources of errors limit the accuracy of the final height:
• Random errors, most notably the measurement noise of the interferometric phase difference. The random error contribution depends on several factors, most remarkably the system SNR (Signal-to-Noise Ratio), the length of the interferometric baseline, and the processing algorithm.
• Spacecraft bus and instrument systematic errors, such as unknown roll, baseline, range and phase drift errors. Lack of knowledge in the spacecraft roll angle, changes in the baseline due to thermal contraction or expansion, system timing errors, and phase errors introduced by the antennas or the electronics will induce height errors.
• Orbit and electromagnetic propagation (or media) errors, such as tropospheric and ionospheric propagation delays. While KaRIn will not directly measure tropospheric and ionospheric corrections, the SWOT altimeter/radiometer suite will be used to perform each range correction at nadir. The POD suite of instruments will be used to correct orbit errors.
• Wave-related errors, such and sea-state (also termed "electromagnetic", EM) bias and SWH (Significant Wave Height) errors. The spatial variability of the wave and wind fields will introduce height biases.
Figure 7: Illustration of the Non Ping Pong mode (image credit: NASA/JPL, CNES)
Sensor complement: (KaRIn)
KaRIn (Ka-band Radar Interferometer)
SWOT's main instrument is KaRIn, a bistatic SAR system in Ka-band. The interferometric altimeter is a near-nadir viewing, 100 km wide-swath interferometric SAR (consisting of two separated 50 km swaths). The instrument concept uses two SAR antennas at opposite ends of a 10 m boom coupled to a nadir SAR to measure the highly reflective water surface. Interferometric processing of the returned pulses yield single-look 5 m azimuth and 10 m to 70 m range resolution, with an elevation accuracy of about ±50 cm. Polynomial based averaging increases the height accuracy to about ±3 cm. The entire globe is covered twice every 16 days. 35) 36) 37) 38) 39) 40) 41) 42) 43) 44)
The KaRIn measurement concept has a rich heritage based on:
1) The many highly successful ocean observing radar altimeters (ERS-1, TOPEX/Poseidon, ERS-2, Jason-1, Envisat, Jason-2, etc.)
2) The SRTM (Shuttle Radar Topography Mission) - the 11-day mission took place in the period Feb. 11-22, 2000. Obviously, this short Shuttle mission could not provide multi-temporal observations over long periods of time. The mission objective was to obtain single-pass interferometric SAR imagery in C-band and X-band for DEM generation. SRTM used incidence angles of 30º - 60º. Although C-band yields some returns from the water surface, X-band produces returns everywhere. SRTM is the only mission to produce image-based elevation measurements of water surfaces. - Changing to Ka-band and 1º - 3º incidence for SWOT, should provide returns over all water surfaces.
3) The development effort of WSOA (Wide Swath Ocean Altimeter) by NASA/JPL (however, the WSOA option for ocean monitoring was dropped from the Jason-2/OSTM spacecraft in the spring of 2005 because of concerns about programmatic risk to the core Jason-2/OSTM mission and due to cost overruns). WSOA would have provided high-resolution measurements of SSH (Sea Surface Height) on approximately a 10 km grid across a 200 km swath centered on the satellite ground track.
The KaRIn instrument covers near-nadir swaths (0.6-4.1º incidence angle) on both sides of the satellite track. There are relatively few reports on backscattering from natural surfaces in Ka-band, especially in near-nadir, and they are generally limited with respect to the surface types taken into account, and the number and range of associated parameters (local incidence angle, soil humidity and roughness, water salinity, wave height, etc.).
Interferometric near-nadir Ka-band acquisitions were realized during an airborne campaign in the Camargue area in Southern France in 2011. To the knowledge of the project, these are the very first interferometric acquisitions realized in this particular configuration, including the incidence range of KaRIn on SWOT (0.6-4.1º). Several scene types were covered, including ocean (Mediterranean), a coastal area (Piemanson), a lagoon (Vaccarès) and a hydrology site (the Rhône river south of Arles). Extensive ground truth were collected. 45) 46) 47) 48) 49)
Legend to Figure 8: SSH error spectrum (red curve) as a function of spatial frequency. Shown, for reference is the SSH spectrum for a reference Jason pass (pass 132) (jagged line). Also shown as the solid black line is the expected spectral continuation. The intersection of the spectral signal with the noise floor at 15 km determines the resolving capabilities for the SWOT instrument. As can be seen, instrument noise dominates the Jason signal for wavelengths smaller than ~100 km.
Integrated measurement approach:
SWOT utilizes an integrated measurement approach, which includes the KaRIn system to observe in two off-nadir swaths, each of 50 km. KaRIn produces ocean and surface water heights and -co-registered weather imagery and elevations of 50 m and 1 km resolutions for hydrology and ocean communities, respectively. KaRIn includes an experimental nadir channel, either as a NNI (Near-Nadir Interferometer), or as a KNA (Ka-band Nadir Altimeter). The NNI is baselined. Both provide complimentary measurements to KaRIn and to support calibration and validation. The CNES-supplied Jason-heritage nadir altimeter specifically addresses SSH for long spatial wavelengths. The Jason-heritage microwave radiometer corrects the nadir altimeter range measurements for wet tropospheric delay and the GPS and DORIS receivers, with the LRA(Laser Retroreflector Assembly), provide precision orbit determination. The integrated measurement approach is illustrated in Figure 9 (Ref. 32).
Figure 9: SWOT integrated measurement approach (NASA, CNES)
KaRIn instrument: KaRIn is the primary instrument used to measure the swath of ocean and fresh water elevations. The instrument concept and core technologies for KaRIn were originally developed from NASA's ESD (Exploration Systems Development) efforts in the 2000's to develop the WSOA (Wide Swath Ocean Altimeter), a Ku-band centimeter-level precision concept for ocean observing that was developed to a substantial level of maturity, but deferred due to budgetary constraints. It demonstrated through hardware prototypes the ability to build very stable mechanical structures, phase stability, and ocean onboard processing directly relevant to the SWOT KaRIn instrument (Ref. 32).
The KaRIN instrument is expected to have a mass of ~294 kg, an orbital average power of ~ 810 W, and an average data rate of ~ 271 Mbit/s over land and ~ 3 Mbit/s over the ocean with onboard data processing. The overall size is: 3.0 m x 2.8 m x 2.8 m in stowed configuration, and 5.0 m x 10.6 m x 1.5 m in deployed configuration. KaRIn will use two graphine composite deployment masts attached to a central metering structure which also incorporates the antenna feeds. At the tip of each mast is a reflectarray support truss structure with two deployable wings. The configuration also takes advantage of the large transmit power (~1.5 kW) from the swath antennas to add receive channels pointing slightly off-nadir to eliminate the interference nadir gap to ±4 km and implement a synthetic aperture nadir altimeter, the NNI (Near Nadir Altimeter). The instrument detail, deployable reflectarray antenna, and the science mode of operations are illustrated in Figures 10, 11, and 7, respectively.
Figure 10: KaRIn instrument detail (image credit: NASA/JPL, CNES)
Figure 11: KaRIn deployable reflectarray (image credit: NASA/JPL, CNES)
Table 1: KaRIn instrument science mode operations parameters
A precision deployable prototype Mast (10 m in length) for the KaRIn instrument is being developed and tested at NASA/JPL. 51)
Figure 12: Kinematic Mast testing: The deployable Mast is tested through its full range of motion while supported by its gravity offloading fixture to verify the kinematic working of the mechanism (image credit: NASA/JPL)
The KaRIn instrument is complemented with the following suite of instruments (Ref. 44):
1) A dual-frequency (C- and Ku-band) Nadir Altimeter, similar to the Poseidon altimeter flown on the Jason series
2) A three-frequency microwave radiometer, similar to the AMR (Advanced Microwave Radiometer) flown on the Jason series
3) A DORIS receiver, a GPS receiver, and a LRA (Laser Retroreflector Array) for POD (Precise Orbit Determination).
Instrument technology readiness: The SWOT instruments, aside from KaRIn, are based on mature technologies derived from the Jason series altimetry missions and other projects. The dual-frequency nadir altimeter is based on an evolution of the Jason Poseidon-3 instrument and/or a design based on the SIRAL (SAR/Interferometric Radar Altimeter) flown on the CryoSat-2 mission of ESA (European Space Agency). Similarly, the three frequency microwave radiometer is of Jason series heritage. The GPSP and DORIS POD receivers are from existing product lines at Broad Reach Engineering and TAS (Thales Alenia Space). The LRA is a build to point copy of those used on the Jason series of spacecraft. The KaRIn instrument contains several technology development items. These are the RFU, the HPA (High Power Amplifier), the KaRIn digital electronics subsystem, and the reflectarray antenna. Their TRLs (Technology Readiness Levels) currently (2012) range from 3 to 5. All of these instrument areas will reach TRL 6 by the KaRIn PDR (Preliminary Design Review), scheduled for mid 2015, following a detailed and rigorous technology development plan. Fall-backs, using existing mature technologies, have been identified. Otherwise, 80% of the components within the instrument assembly require no new technology (Ref. 32).
The SWOT flight system is the spaceborne element of the mission. It is composed of the spacecraft bus, supplied by CNES, and two payload modules, the nadir payload module and the KaRIn payload module. The spacecraft bus provides basic services to the payload modules and includes C&DH (Command and Data Handling) subsystem, the EPS (Electrical Power Subsystem), ADCS (Attitude Determination and Control) subsystem, TCS (Thermal Control Subsystem), and the RF communication subsystem. The 35 m2 area twin solar arrays are oversized to minimize disturbances from array articulation. The arrays are fixed along the velocity vector also to minimize dynamic disturbances.
Other features of the spacecraft bus are batteries with 320 Ah capacity to support 2 kW observatory average power requirements, a SSR (Solid State Recorder) of 2.3 Tbit capacity, and a propulsion subsystem with a propellant tank capacity to meet the French Law on orbital debris mitigation.
The spacecraft bus uses high heritage technologies and generic standards developed through the CNES ISIS program. The nadir payload module accommodates all the science instruments aside from KaRIn [microwave radiometer, nadir altimeter, DORIS,GPSP(GPS Payload receiver), and LRA], and the X-band communication subsystem for the high rate downlink of the science data, including KaRIn's. The nadir payload module faces nadir and is mounted to the KaRIn payload module, which in turn is mounted to the spacecraft bus. The KaRIn payload module accommodates the KaRIn instrument, including the reflector antennas, the RF feeds and electronics, antenna booms, and a precision gyro to measure the roll parameters.
The nadir and KaRIn payload modules will be integrated and tested in parallel prior to environmental testing at JPL and subsequent shipment to France for integration with the spacecraft bus. This approach maximizes schedule efficiency and results in a fully qualified and integrated Nadir/KaRIn payload module assembly. The flight system configuration is illustrated in Figure 13 (Ref. 32).
Figure 13: The flight system configurations (image credit: CNES, NASA/JPL)
Table 2: SWOT spacecraft characteristics
RF communication system: The SWOT spacecraft will survey at least 90% of the globe, studying Earth's lakes, rivers, reservoirs, and oceans at least twice every 21 days. The data collected by these surveys could improve ocean circulation models and weather and climate predictions, while aiding in freshwater management around the world.
Taking such frequent and extensive surveys of Earth's bodies of water requires SWOT to communicate a large amount of data to ground stations. SWOT will be capable of collecting and transferring more than 7.2 Tb of science data per day. This requires a downlink data rate of 620 Mbit/s to deliver this volume in the brief time intervals when SWOT is in view of a ground station. 52)
The 620 Mbit/s downlink data rate could be supported if the satellite had a steerable high-gain antenna with a gimbal capable of continuously pointing toward the ground station. However, using a gimbal generates vibrations that would degrade the accuracy of one of SWOT's instruments,KARIN (Ka-band Radar Interferometer). Instead, a LGA (Low-Gain Antenna) with a broad-beam radiation pattern is necessary to provide maximum contact time with ground stations. An isoflux radiation pattern allows the antenna to cover the full visible Earth and to compensate for range attenuation.
Using an LGA at the 8.025-8.400 GHz band allocated for Earth-exploration satellite service, the data will need to be divided into two channels of 310 Mbit/s to satisfy the 620 Mbit/s downlink data rate and the allocated microwave band. The two channels are transmitted using different polarizations: RHCP (Right-Hand Circular Polarization) and LHCP (Left-Hand Circular Polarization).
Figure 14: Top view of a simplified model of SWOT's NADIR deck including a 1.0 m diameter radiometer and a 1.2m diameter altimeter (image credit: NASA/JPL)
In order to use the LGA as a viable option, self-interference issues needed to be addressed. The first type of inference that needs to be accounted for is referred to as polarization interference. If SWOT uses two antennas with different polarization (RHCP and LHCP), undesired polarization from one antenna will generate interference to the other one. An antenna with a high cross-polarization discrimination (XPD) will receive an insignificant amount of undesired polarization. The SWOT telecom antenna needed to demonstrate high XPD (~25dB) to mitigate this interference, which is very difficult across the large frequency band (8.025-8.400 GHz) and especially at an angle of ± 60º from antenna boresight. No commercially available antennas met such stringent requirements.
The nadir deck of the SWOT satellite is a rich environment with a 1 m aperture radiometer reflector antenna, a 1.2 m altimeter reflector antenna, a DORIS (Doppler Orbitography and Radiopositionning Integrated by Satellite) antenna, and a LRA (Laser Reflector Array). These antennas generate the second type of interference — multipath interference. This occurs when one antenna is radiating RHCP, for instance, power will be reflected by the large reflectors and becomes LHCP.
Launch: A launch of the SWOT spacecraft is planned for the 2020 timeframe using a NASA medium-class launch vehicle.
Orbit: Near-circular (non sun-synchronous) 21-day orbit, altitude of 890 km, inclination = 77.6º.
Table 3: Orbit characteristis 53)
Figure 15: Example of the SWOT fast-sampling orbit ground track (image credit: AVISO)
Figure 16: Example of the SWOT science ground track (image credit: AVISO)
Figure 17: SWOT spacecraft concept: current design (image credit: NASA/JPL, CNES)
Figure 18: Artist's rendition of the deployed SWOT spacecraft (mage credit: NASA/JPL)
The ground segment architecture integrates all of the functions required to completely support mission operations. These elements provide various services: mission planning, sequence generation, station scheduling, spacecraft analysis, orbit determination, trajectory analysis and planning, maneuver design, instruments analysis, time correlation, data capture and distribution, science data gap reporting, generation of products for science support, as illustrated in Figure 20.
Spacecraft communication services will be provided by CNES Network Operation Center. SWOT will benefit from the CNES new multimission capabilities of ISIS (Initiative for Space Innovative Standards) for Spacecraft Command and Control, currently under development for future CNES missions.
JPL and CNES will provide engineering monitoring and commands for operating their respective instruments. Science data will be transmitted via X-band, while engineering telemetry and uplink data will be communicated via S-band, to the CNES provided Ground Station Network. There will be one pass per orbit of Inuvik/ Kiruna & Aussaguel Stations including Kiruna / Aussaguel overlapping plus one pass per day on HBK (Hartebeesthoek), South Africa. Each pass is relatively short in duration, less than 10 minutes. The spacecraft will downlink raw science data at a rate of 620 Mbit/s . Once the science data is downlinked to the ground, it is delivered to the SDS (Science Data System) at JPL and CNES.
Amongst the many key challenges to the SDS is the large data volume with an average production volume of 7 Tbit/day acquired by the Payload, and the consequent huge volume of data products produced (on the order of 23 PB for 3 years of operation). JPL and CNES have reached an initial agreement to jointly develop and operate the science data system using common data processing flow and interfaces, but each institution will develop their own unique data production and distribution system.
Figure 19: Overall Ground System architecture, including links to spacecraft (image credit: CNES, NASA/JPL)
Table 4: Proposed SWOT Ground System Elements
Figure 20: SWOT ground data system architecture (image credit: CNES, NASA/JPL)
The SWOT science data products will include ocean, hydrology, and enhanced hydrology higher level products in addition to level 1 products. These data products will be processed in a coordinated fashion by the NASA and CNES SDSs (Science Data processing Centers) and distributed to the NASA PO.DACC (Physical Oceanography Data Active Archive Center) and the CNES AVISO (Archivage, Validation et Interprétation des données des Satellites Océanographiques) systems for distribution to scientific and other users.
Table 5: Overview of SWOT data products
The SWOT NASA mission will study the global oceans at unprecedented temporal and spatial scales and provide unique insight into global terrestrial surface waters, including lake level height and river discharge volume. It is an international partnership involving the French space agency, CNES, and CSA (Canadian Space Agency). CSA is supporting the development of a sophisticated EIK (Enhanced Interaction Klystron) that will be at the heart of the interferometric radar instrument, KaRIn, on the SWOT mission.
An airborne version of SWOT, called AirSWOT, has been developed at NASA/JPL to provide calibration and validation for the mission when on orbit as well as to support science and technology during mission development. The first AirSWOT science flights will start in 2013. 54)
BUSARD campaign of CNES/ONERA:
To support SWOT and future altimetry space missions at Ka-band, an airborne measurement campaign has been realized under CNES funding, involving the ONERA BUSARD motorglider platform equipped with the DRIVE radar. The particularity of this experiment is the line of sight geometry, with very steep incident angles, along with a SAR/XTI (SAR/Cross Track Interferometry) configuration.
Figure 21: ONERA BUSARD platform with two pods (image credit: ONERA)
The objective of this airborne campaign was to acquire XTI radar data representative of future InSAR data from KaRIn, in order to validate simulation results over a large selection of surfaces (from inland, coastal and sea areas). Particular attention has been paid on contrast evaluation between water and coastal backscattering levels. 55)
Table 6: Main parameters of the radar
One validation and five measurement flights (2011) have been realized in the frame of this campaign, with an altitude around 3000 m. The project focused on the 0 to 5° incident angles range flying over different types of surfaces. The areas of interest for those campaigns, mainly dedicated to water surface measurements, were located in the Camargue region in South of France. This region is well-known for including a large number of ponds and marshlands.
During this campaign, a Ka-band radar dataset has been recorded on dry, wet soils and sea surfaces with different wind conditions: null wind, low wind (about 10 km/h) and medium wind (about 30 km/h). The main areas of interest were the Vaccarès lake (Figure 22), the Rhône river (Figure 23), the Piemanson coastal area (Figure 24), and an area over the open sea.
Figure 22: Vaccarès area of interest. (image credit: ONERA)
Figure 23: Rhône area of interest. Area dedicated to River measurements (image credit: ONERA)
Figure 24: Piemanson area of interest. Area dedicated to coastal measurements (image credit: ONERA)
Ground truth collection and data calibration:
In parallel to the radar data airborne recording an exhaustive ground truth data collection has been performed. More than three thousand pictures have been taken from an onboard camera whose line of sight is similar to the radar one, and more than one hundred pictures have been taken at ground level, with geometrical and temporal references. The locations of the trihedral reflectors, used for the calibration purposes, have been accurately measured by a partner (this information is of primary interest for SAR data processing). These pictures have been used to document the sea states during the flights, and to identify vegetation on ground.
In addition to the ground truth data collection, a particular attention has been paid to data calibration. Applying a fine calibration was a key point as these experiments aimed at getting accurate estimation of water surface backscatter coefficient. The dedicated calibration scheme used was based on three main steps:
1) An accurate control of our radar system: The radar sensor was fully characterized at laboratory level to identify precisely transmit and receive channel paths (differential gains). Additional ground tests with trihedral calibrators were done for accurate power budget estimation.
2) In flight calibration process with steep incident angles geometry: Incidence angle was centered at 15°. During the flights, the project used a dedicated calibration area where corner reflectors were installed. The objective of this second step was to determine accurately the calibration coefficients to be applied to the radar data.
3) Data calibration: The project flew over regions of interest and applied calibration coefficients estimated at previous steps.
The entire radar dataset has been acquired in interferometric condition. This is illustrated in Figure 25 on the Rhône river area of interest. One of the interferometric pair of SAR images is presented in figure (top), the coherence figure is presented in figure (middle), and the interferometric phase is presented in figure (bottom).
Figure 25: Top: The SAR image forms one of the interferometric channels; Middle: The coherence figure between the two interferometric channels; Bottom: The corresponding phase figure. Nadir is bottom of the images. The black part in the top bottom image is in the air. Far range is top of the images (image credit: ONERA)
In Figure 25 (top), one can notice there is some signal folding phenomenon around the nadir area. This is more particularly visible with signals from the river on the left and right parts of the SAR image. The radar look angle is at the left hand side from the aircraft, and water surface backscattering from the right hand side is mixing with land surface backscattering on those areas. This phenomenon extends from nadir up to about 4° incidence angle.
Figure 25 (middle) is presenting the coherence figure; those areas with folded signals are well observable with some characteristic fringes. In addition, one can observe that coherence level on water is very high, around 0.9, and the forestry area is well observed in the center part of the image even though it was not easy to find it out from the SAR image. The coherence on land and crops is ranging from 0.5 to 0.9.
Finally, Figure 25 (bottom) is presenting the interferometric phase. One can notice that on left / right ambiguities areas, fringes are rotating in opposite directions compared to others and the phase rotation velocity is much higher than the one on the water, for example.
Once calibrated following the previous calibration scheme, the project extracted from the SAR image dataset several backscattering profiles, in terms of sigma 0 (σo) values. This has been done over different surfaces: Forest area, water surfaces from river, lake, marshes and ponds, bare soils and agricultural areas with different kinds of crops. An example of sigma 0 profile is shown in Figure 26.
Figure 26: Sigma 0 profile (dBm2/m2 as a function of incidence angle) over flat water surface (image credit: ONERA)
The backscattering effect close to nadir area is quite high, with a sigma 0 value close to 17dBm2/m2. Then the backscattering is decreasing as a function of the incident angle down to 0dBm2/m2 at 15° incidence angle.
Finally, the power return due to folding mechanism (Figure 25) has been estimated using the strong hypothesis that left/right scatterers are identical. In this case, the precise computation of the illumination pattern on each side can be used to compensate for folding. This hypothesis is the most likely to be valid over water surfaces, which are the areas of main interest in the frame of the measurement series.
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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).