SeaSat (Seafaring Satellite) Mission
SeaSat (also referred to as SeaSat-A prior to launch and SeaSat-1 after launch) is a pioneering Earth observation experimental mission of NASA/JPL; the first ever civilian spaceborne imaging radar instrument (SAR) was flown on SeaSat in 1978. During its brief 110-day lifetime (end of mission due to a malfunction), SeaSat collected more information about the oceans than had been acquired in the previous 100 years of shipboard research. It established satellite oceanography and proved the viability of imaging radar for studying our planet. Most importantly, it spawned many subsequent Earth remote sensing satellites and instruments at JPL and elsewhere that track changes in Earth's oceans, land and ice. Its advances were also subsequently applied to missions to other planets.
The Seasat program had three main objectives:
1) to demonstrate techniques to monitor Earth’s oceanographic phenomena and features from space on a global scale
2) to provide timely oceanographic data to scientists studying marine phenomena, and to users of the oceans as a resource (ocean shippers, fishermen, marine geologists, etc.)
3) to determine the key features of an operational full-time ocean-monitoring system.
The SeaSat mission pioneered satellite oceanography and proved the viability of imaging radar for studying our planet. The SAR instrument provided a wealth of information on such diverse ocean phenomena as surface waves, internal waves, currents, upwelling, shoals, sea ice, wind, and rainfall (first global view of ocean circulation). Seasat provided global statistics on ocean mesoscale variability, detected strong currents, and showed the potential to detect the longer wavelengths of the circulation. - Beyond the oceans, SeaSat's synthetic aperture radar instrument provided spectacular images of Earth's land surfaces. Even though the satellite was short-lived and the SeaSat program was discontinued, it demonstrated the immense potential of the SAR observation technology, generating great interest in satellite active microwave remote sensing. SeaSat SAR observations amounted to a total area of about 126 x 106 km2 (of the northern hemisphere), including multiple coverage of many regions. 1) 2) 3) 4) 5) 6)
Background: NASA began planning for the Seasat satellite mission in 1972, the first multisensor spacecraft, dedicated specifically to ocean observations. Specific objectives were to collect data on sea-surface winds, seasurface temperatures, wave heights, ocean topography, internal waves, atmospheric water, and sea ice properties.
Requirements for Seasat were generated by a User Working Group (UWG), which included the Office of the Oceanographer of the U.S. Navy, Fleet Numerical Weather Center in Monterey, CA, Navy Surface Weapons Center in Dahlgren, VA, Naval Research Laboratory, the Johns Hopkins University Applied Physics Laboratory (APL), the Office of Naval Research, and the Navy/NOAA Joint Ice Center. NOAA was represented on the UWG by the many NOAA laboratories around the nation, including the NOAA Atlantic Oceanic Marine Laboratory (AOML) in Miami, FL, the NOAA weather center in Suitland, MD, the NOAA Pacific Marine Environmental Laboratory in Seattle, WA, and NOAA's Marine Fisheries office in Bay St Louis, MS; the Defense Mapping Agency, United States Geological Survey (USGS), the U.S. Coast Guard, and the Department of the Interior were also represented on the UWG.
The Seasat project was managed by JPL for NASA, with significant participation from NASA's Goddard Space Flight Center, Greenbelt, MD; NASA's Wallops Flight Facility, Wallops Island, VA; NASA's Langley Research Center, Hampton, VA; NASA's Glenn Research Center, Cleveland, Ohio; Johns Hopkins University Applied Physics Laboratory, Laurel, MD; Lockheed Missiles and Space Systems, Sunnyvale, CA; and the National Oceanic and Atmospheric Administration (NOAA), Washington, D.C.
Table 1: Overview of the SeaSat program history -25 years after the launch of SeaSat 7)
Figure 1: Artist's view of the deployed SeaSat-A spacecraft in orbit (image credit: NASA/JPL)
Figure 2: Alternate view of SeaSat (image credit: Lockheed Martin, NASA)
The spacecraft was designed and developed by LMSC (Lockheed Missiles and Space Company) as prime contractor and by Ball Aerospace Systems of Boulder, CO. The satellite utilized the Agena upper stage to provide satellite bus functions, including power, telemetry (S-band), attitude control, and command and control functions. A sensor package containing the mission's five experiments was attached to the Agena, as were the experiments' antenna systems. Seasat was three-axis stabilized using momentum wheels and horizon sensors. The vehicle was oriented with the SAR and other antennas remaining nadir pointing and the Agena rocket nozzle and solar panels zenith pointing. S/C size: 21 m length, 1.5 m diameter, total S/C mass=2290 kg. The spacecraft design life is 1 year with expendables, including orbit adjust capability, for three years. 8) 9) 10)
Application: Ice and ocean monitoring (sea-surface winds, sea-surface temperatures, wave heights, internal waves, sea-ice features, ocean features, ocean topography, and the marine geoid), land use, geology, forestry, and mapping.
Figure 3: Line drawing of the SeaSat spacecraft (image credit: DLR)
The Agena as the second stage of the Atlas-F/Agena launch vehicle, serves as the satellite bus providing attitude control, power, guidance, telemetry and command functions. The sensor module is tailored specifically for the SeaSat payload of five microwave instruments and their antennas. Together, the two modules are ~ 21 m long with a maximum diameter of 1.5 m without appendages deployed. Atop the Atlas booster rocket, the entire satellite is enclosed within a 3 m diameter nose fairing which matches the diameter of the Atlas. After burnout of the Agena stage and injection into the nominal orbit, SeaSat has a mass of nearly 2300 kg. 11)
In orbit, the satellite appears to “stand on end” (Figure 5) like a pencil, the sensor and communications antennas pointing toward nadir and the Agena rocket nozzle and solar panels pointing opposite toward space. The dominant feature of the SeaSat spacecraft is the SAR antenna, a 2.1 m x 10.7 m planar array deployed perpendicular to the satellite body.
ACS (Attitude Control Subsystem): The spacecraft is 3-axis stabilized using a momentum wheel/horizon sensing system to accurately point the sensors at Earth's surface. Hot gas jets provide thrust for adjusting the orbit and for attitude control during Agena burn and orbit adjustment periods.
Following orbital insertion, ACS orients the spacecraft from nose-forward to nose-down and provides stabilization during deployment of the antennas and solar arrays. These functions are performed using hydrazine reaction control thrusters for attitude control and a gyro reference unit as one attitude reference, augmented by horizon sensors for a short period prior to nose-down.
The payload pointing requirements include control to an accuracy of 0.5º in roll, pitch and yaw and telemetered data on the spacecraft orientation to an accuracy of 0.2º in all axes. Scanwheels provide pitch and roll references viewing the Earth's horizon and pitch and roll fine control. The yaw attitude is maintained by gyrocompassing. Sun sensor data is used to determine accurately the yaw orientation, but is not used for control. The scanwheels are mounted at the lower end of the sensor module near all of the critical antennas. The pitch momentum wheel and roll reaction wheel are located in a support structure above the sensor module. Excess momentum accumulated in the wheels is removed by providing adjustable torque on the satellite using electromagnets which interact with the Earth's magnetic field.
EPS (Electrical Power Subsystem): EPS was designed to provide power at 28 ±4 VDC to the spacecraft subsystems and to the payload using solar arrays and rechargeable batteries. The basic design philosophy was to provide functional redundancy. A capability was provided for component isolation (removal from circuit), cross-strapping, charge control (automatic and manual); in addition, system protection was implemented via bypass functions by commandable relays (Ref. 14).
The primary energy source for the spacecraft was the SA (Solar Array) which consisted of two wings mounted on either side of the aft rack. With the vehicle in the normal orbital attitude and the SA deployed in the X-Y plane, the wing axis lay 40º ahead (toward the direction of flight) of the +Y axis and 40º behind the -Y axis. The wings tracked the sun through 360º about this axis using error signals generated by the sun sensors located on each solar array wing. The signals generated by the sun sensors were processed in the SADE (Solar Array Drive Electronics) which provided power to control the array drive motor speed.
During periods of eclipse, the array was driven by a fixed angular rate by signals from the SADE. In addition, the rotation direction and rate could be controlled by commands. Each wing contained 11 panels. The average power output capability varied during the life of the spacecraft due to the seasonal intensity of the sun, the angle to the sun (β angle), eclipse periods, and various factors which degrade the power output capability of the solar cells. During full sun, the SA supplied power to all the loads as well as for charging the 2 type 40 NiCd batteries. The batteries supplied the total spacecraft load requirements during eclipse and supplied the surge loads when they exceeded the instantaneous capability of the SA.
Power of ~1000 W was provided at the beginning of the mission, varying throughout the mission to ~ 700 W. The average on-orbit power was about 700 W. The solar panels were rotatable on one axis; they made up an area of 14.5 m2 of solar cells.
Figure 4: Block diagram of the EPS (image credit: NASA/JPL, Ref. 14)
RF communications: The data collected by the sensors are converted from analog to digital, except for that of the SAR instrument. Data are transmitted from the satellite in three separate streams: a 25 kbit/s real-time stream containing instrument data from ALT, SASS, SMMR, and VIRR and all engineering subsystem data, an 800 kbit/s playback stream of recorded real-time data, and a 20 MHz analog SAR instrument data stream, receivable only in real-time by specially equipped tracking stations.
An onboard data storage capacity of ~ 350 Mbit is provided - the equivalent of more than two full orbits of measurements from all sensors with the exception of the SAR instrument. SAR data is not recorded.
Redundant S-band transmitters and receivers, functioning as transponders, provide the communications link for engineering and payload data. A separate S-band transmitter (5 W) with its own helical antenna provides the SAR downlink in real-time.
In addition to the primary tracking information from SeaSat's S-band communication system, two independent tracking systems aid in navigation and orbit determination. Laser tracking signals originate from ground sites and are reflected from an array of retroreflectors on the spacecraft.
A dual-frequency beacon transmits ultrastable carriers to a ground tracking network, TRANET. The TRANET, operated by DoD (Department of Defense), receives the dual-frequency Doppler beacon from SeaSat. The tracking measurements are used to supplement the STDN S-band tracking for orbit determination. Onboard equipment includes an ultrastable transmitter radiating at 162 MHz and at 324 MHz. SeaSat uses this frequency also as a source for satellite data timing.
Sensor module and payload accommodation:
The sensor module is a platform for the operation of the five sensors to achieve the mission objectives within the required resolution and accuracy. The sensors are located in positions relative to one another and to the beacon, laser retroreflector and communication antennas so that each ahs an unobstructed field of view and each achieves the required pointing and scan angle. The mounting positions were also selected to prevent electromagnetic interference between multiple radiating sources.
The sensor module's primary structure is a 25.4 cm diameter aluminum alloy tubular mast to which equipment mounts are attached.
Two scanwheel assemblies are mounted near the forward end on the tubular supports to give each unit a clear view of Earth's horizon.
The ALT (Radar Altimeter) is mounted at the end of the mast structure - nearest to Earth - the 1 m diameter reflector antenna and RF unit on the forward end and the signal processor to the side. The ring of the corner cube quartz reflectors for the laser tracking system surrounds the altimeter antenna and RF electronics module.
The SASS (Microwave Scatterometer) and Doppler beacon transmitter for the TRANET tracking system are mounted in a support structure on the side of the mast. Four slotted array stick antennas for the SASS are stowed against the structure and each is deployed separately. The TRANET antenna is attached to a deployable boom which also supports on of the two S-band communication antennas. The second is deployed on a separate boom.
The VIRR (Visible and Infrared Radiometer) consists of a scanner mounted on a deployable boom and electronics on the mast tube.
The SMMR (Scanning Multifrequency Microwave Radiometer) is mounted as a single unit on the side of the sensor module structure. The unit includes a fixed offset parabolic reflector, scan mechanism and a digital processor.
The SAR /Synthetic Aperture Radar) antenna and the electronics are installed near the base of the sensor module. The huge SAR sensor antenna is in eight segments, folded during launch and deployed to form a flat rectangular array with an area of 23 m2. The SAR downlink transmitter is mounted on the mast and its helical antenna is deployed on a short boom.
Figure 5: Illustration of the deployed SeaSat spacecraft on orbit (image credit: NASA)
Launch: The SeaSat spacecraft was launched on June 27 (UTC), 1978 on an Atlas-F/Agena launch vehicle from VAFB (Vandenberg Air Force Base), CA, USA.
Orbit: Non-sun-synchronous near circular polar orbit, inclination = 108º, apogee = 799 km, perigee =775 km, period = 101 minutes, repeat cycle of 17 days (subcycle of 3 days).
SeaSat operated successfully from late June to early October 1978 when it experienced a malfunction. The end of the SeaSat mission occurred on October 9 (UTC), 1978 - due to an abrupt power system failure in the Agena bus that was used as a part of the spacecraft. The loss of power was caused by a massive and progressive short in one of the slip ring assemblies that was used to connect the rotating solar arrays into the power subsystem. The most likely cause of this short was the initiation of an arc between adjacent slip ring brush assemblies. The triggering mechanism of this arc could have been either a wire-to-brush assembly contact, a brush-to-brush contact, or a momentary short caused by a contaminant that bridged internal components of opposite electrical polarity. 12) 13) 14)
Mission duration: 70 days (data generation) of 105 operational days (1503 orbits). During SAR operations, approximately 42 hours of SAR data were collected.
• Imagery obtained from the Seasat SAR clearly demonstrated its sensitivity to surface roughness, slope, and land-water boundaries. Seasat images have been used to determine the directional spectra of ocean waves, surface manifestations of internal waves, polar ice-cover motion, geological structural features, soil moisture boundaries, vegetation characteristics, urban land-use patterns, and other geoscientific features of interest.
• Despite its overall technological and scientific success, Seasat's relatively short lifetime precluded the acquisition of a seasonal data set. Moreover, the Seasat SAR was a single-parameter instrument using a fixed wavelength, polarization, and incidence angle. While the near-nadir incidence angle was ideal for acquiring strong ocean returns, it produced severe geometric layover distortions on terrain images of high-relief regions.
Table 2: Collection of some events/items during the SeaSat-1 mission
Figure 6: A sample SeaSat-1 SAR image of the Los Angeles metropolitan area observed in 1978 (image credit: NASA/JPL, Ref. 6)
Sensor complement: (SAR, SMMR, ALT, SASS, VIRR, LRR)
The sensor complement consisted of active and passive instruments to achieve an all-weather capability. A new era of spaceborne oceanography was ushered in with the SeaSat sensor complement. All sensors operated at the same time, over the same region of the ocean, providing a truly synoptic view of the parameters important to the understanding of the dynamics of our ocean. 15) 16)
SAR (Synthetic Aperture Radar):
The SAR instrument features: HH polarization, look angle = 20º; pixel size = 25 x 25 m (spatial resolution on the surface at 4 looks); radiometric resolution = 5 bit raw data. Sensor transmission frequency: 1.275 GHz (L-band); wavelength= 23.5 cm; swath width=100 km. Antenna: 1024-element phased array antenna of size 10.74 m x 2.16 m; PRF= 1464 to 1640 Hz; pulse duration = 33.4 µs; bandwidth (linear FM) = 19.077 MHz; transmitted peak power = 1 kW (nominal). 17) 18) 19) 20) 21)
The planar antenna array consisted of eight, 1.3 m x 2.16 m rigid and structurally identical fiberglass honeycomb panels. The panels were hinged together in series, but were individually supported by a deployable tripod substructure that governed the deployment of the truss and provided the interface of the antenna structure with the spacecraft.
The Seasat SAR sensor is regarded as the first imaging SAR system used in Earth orbit. The SAR antenna is mounted on the S/C with its boresight oriented at 20º from the vertical direction (look angle), pointing to the right of the flight path. The antenna beamwidth measures 6.2º in elevation and 1º in azimuth. A footprint of 100 km x 15 km (3 dB contour) is provided. The swath extends from 290 km to 390 km to the right of the S/C ground track (Figure 7). The received radar echoes are downlinked in S-band (analog data link at 2.265 GHz) to a total of five ground receiving stations in real-time located at: Goldstone, CA, Fairbanks, AK, Merrit Island, FL, Shoe Cove, Newfoundland, and Oakhanger, UK. No high-rate onboard recording capability of SAR data was available at the time.
Table 3: Performance characteristics of SAR instrument
Figure 7: Illustration of the SeaSat SAR viewing geometry (image credit: NASA/JPL)
Table 4: SeaSat SAR image products
The SAR instrument had a mass of 147 kg and a power consumption of 216 W (1000 W peak power). The instrument could only be operated from 10 minutes per orbit.
SMMR (Scanning Multichannel Microwave Radiometer):
The SMMR is a five-frequency instrument of Nimbus-7 mission heritage. The instrument was designed and built at JPL. Objectives: Monitoring sea surface temperatures, wind speeds, rain rate, atmospheric water content (mapping of columnar water vapor distribution over the global oceans) and ice conditions. SMMR is a multispectral, dual-polarization microwave radiometer observing at the following frequencies: 6.6 GHZ (45.4 mm), 10.7 GHz (28 mm), 18.0 GHz (16.6 mm), 21.0 GHz (14.2 mm), and 37.0 GHz (8.1 mm). Six Dicke-type radiometers were utilized. Those operating at the four longest wavelengths measured alternate polarizations during successive scans of the antenna; the others operated continuously for each polarization.
The SMMR instrument consisted of five hardware elements:
• The antenna assembly consisting of the reflector, fabricated of graphite epoxy, and the feedhorn
• The scan mechanism, including momentum compensation devices
• An RF module containing the input and reference switching networks, the mixer-IF preamplifiers, and the Gunn local oscillators
• An electronics module containing the main IF amplifiers, all the post-detection electronics, and the power supplies for the scan and data subsystems
• A power supply module which contains the dc-to-dc converters and regulators for the rest of the instrument.
The antenna was a parabolic reflector offset from the nadir by 42º. Motion of the antenna reflector provided observations from within a conical volume along the ground track of the spacecraft. SMMR had a swath width of about 600 km and the spatial resolution ranged from about 22 km at 37 GHz to about 100 km at 6.6 GHz. The absolute accuracy of sea surface temperature obtained was 2 K with a relative accuracy of 0.5 K. The accuracy of the wind speed measurements was 2 m/s for winds ranging from 7 to about 50 m/s. An identical instrument was flown on Nimbus-7 (launch Oct. 24, 1978). 22) 23) 24) 25)
The SMMR instrument had a mass of 53.9 kg and a power consumption of ~60 W.
Figure 8: Illustration of the SMMR instrument (image credit: JPL)
ALT (Radar Altimeter):
ALT is of S-193 heritage flown on Skylab and of ALT flown in the GEOS-3 mission. Objective: Determination of sea surface profiles, currents, wind speeds and wave heights (first attempt to achieve 10 cm altitude precision from orbit).
The ALT instrument was a Ku-band compressed pulse radar altimeter (first use of the full-deramp technique). With this new full-deramp technique no compression filter is required in the receiver. From SeaSat onwards, all altimeters have been using this technique, achieving a significant improvement in the resolution. The ALT instrument was designed and developed by JHU/APL. 26) 27) 28) 29) 30) 31)
Two of its unique features were a linear FM transmitter with a 320 MHz bandwidth, which yielded a 3.125 ns time-delay resolution, and microprocessor-implemented closed-loop range tracking, automatic gain control, and real-time estimation of significant wave height. This instrument flew the first microprocessor (8080-based controller/tracker) in space. The altimeter operated at 13.56 GHz (Ku-band, chirp signal at 2 kW peak power) using a 1-m parabolic antenna pointed at nadir and had a swath width which varied from 2.4 to 12 km, depending on sea state. ALT operated in chirp pulse mode with a 3.2 µs uncompressed pulse width and 3.125 ns compressed pulse width. The precision of the height measurement was 10 cm (rms). The estimate of significant wave height was accurate to 0.5 m or 10%, whichever was greater, the ocean backscatter coefficient had an accuracy of 1 dB.
In the SeaSat design the number of echo samples is increased (compared to GEOS-3). The samples are spaced 3.125 ns apart to encompass the anticipated spread in ocean return for wave heights up to 20 m. In this case waveforms sampling is implemented by a bank of filters with 312.5 kHz bandwidth and spacing. In contrast with previous designs, the samples are an integral part of the altitude tracking process and are used in such a way that the system adapts as a function of wave height to optimize tracker performances. The altitude tracking loop is closed in two parts: a coarse adjustment of the local oscillator pulse timing in 12.5 ns step, and a fine adjustment.
Calibration: The ALT instrument was calibrated for height bias using four overflight passes of Bermuda that were supported by the Bermuda laser. The estimated height bias was 0.0 ± 0.07 m. 32)
Figure 9: Illustration of the radar altimeter on SeaSat
The ALT instrument had a mass of 93.8 kg and a power consumption of 177 W.
SASS (Seasat-A Scatterometer System):
SASS (of S-193 heritage on Skylab) is a fan-beam dual-polarized Doppler scatterometer with the objective of radar backscatter measurements (sigma naught) over ocean surfaces for estimation of the wind field. Pulse transmit frequency of 14.599 GHz (Ku-band). SASS illuminated the sea surface with four fan-shaped beams (two orthogonal beams, each 500 km wide, on each side of the ground track). Doppler filters were used to discriminate resolution cells in the long dimension of the fan beam, resulting in 500 km swaths on either side of the satellite. The high wind swaths added an additional 250 km to each side. The spatial resolution was 50 km over a region of 200 to 700 km on either side of the spacecraft. The experimental SASS instrument first demonstrated the ability to accurately infer vector winds over the ocean's surface from a spaceborne platform. 33) 34) 35)
Note: The S-193 scatterometer on Skylab was also known by the name of RADSCAT.
Figure 10: Some parameters of the SASS instrument
Figure 11: Viewing geometry of the SASS instrument (image credit: NASA/JPL)
SASS was a proof-of-concept experiment for measuring ocean surface wind vectors under day/night near-all-weather conditions. The physical basis for this remote sensing technique is the generation of capillary waves on the ocean surface by the friction velocity of the wind. The amplitude of these cm-wavelength ocean waves is in equilibrium with the local wind, and the two-dimensional wave spectrum is highly anisotropic with the wind direction. The ocean radar backscatter results from Bragg scattering from these capillary waves, and the normalized radar cross section (σο) grows approximately as a power series of wind speed.
The scatterometer on SeaSat was the primary means of measuring ocean surface wind speed and direction. Nonetheless, patterns of SAR-measured normalized radar cross section clearly showed spatial structures associated with variations in wind speed and direction. In the last five years, it has become more apparent that SAR imagery can be used to make high spatial resolution estimates of wind speed. 36)
Given our experience over the last 25 years, SeaSat was clearly a dramatically visionary satellite system. It provided the precursors to many subsequent spaceborne instruments. The SeaSat SAR was designed to provide ocean surface wave images from which ocean wave spectra could be derived. However, the imagery clearly showed features associated with variations in wind speed and direction. Since that time, a new generation of calibrated SARs have been launched which makes it possible to use what we learned from SeaSat to produce, on a routine basis and in nearly real-time, high-resolution SAR wind fields (Ref. 36).
The SASS instrument had a total mass of 103 kg (electronics assembly of 59 kg, each antenna had a mass of 11 kg), power consumption of 100 W (peak).
VIRR (Visible and Infrared Radiometer):
VIRR is a supporting instrument on Seasat (of SR heritage on NOAA-1) with the objective to provide images of visual reflection and thermal infrared emission from oceanic, coastal, and atmospheric features that might aid in interpreting the data from the other Seasat sensors (also some quantitative measurements of SST and cloud top height). Scanning is accomplished by a rotating mirror mounted at 45º to the optical axis of the collecting telescope (scan angle=±51.2º). VIRR uses a 12.7 cm diameter Cassegrain-type telescope, focusing the radiation onto a field stop. A relay optical system transmits the radiation to a dichroic beamsplitter, which separates it into the visible and infrared wavelengths. The swath of the VIRR is about 2280 km wide, centered on nadir. 37) 38)
Table 5: VIRR instrument parameters
The VIRR instrument had a mass of 8.1 kg and a power consumption of 7.3 W.
LRR (Laser Retro-Reflector):
A device to support precision orbit determination for Seasat. Laser corner-cube reflectors, composed of 96 fused silica 3.75 cm hexagonal corner cube retroreflectors, and ground-based laser systems were used to obtain precise satellite tracking information. The retroreflector array was configured as a single ring of cube corners 1.27 m in diameter. Sixteen of the cube corners were tilted away from the axis of the ring by an angle of 25º and the remaining 80 cubes by an angle of 50º. Because of the great distance of the array from the center of mass of the satellite, the range correction varied from 5.28 m at zenith to 3.08 m near the horizon.
When illuminated by laser light pulses from the ground, each retroreflector cube in the array reflected the light pulses back to a telescope/receiver on the ground. A digital counter recorded the time of flight of the laser light pulses from the ground to the satellite and back to the ground. Range was determined from this time. NASA, USAF, SAO (Smithsonian Astrophysical Observatory) and foreign laser tracking stations tracked this satellite.
The Seasat mission was controlled from the real-time mission operations facility located at NASA/GSFC. Spacecraft data were received and recorded by the tracking stations of STDN (Spaceflight Tracking Data Network) and transmitted to GSFC. There, data were sorted, merged, time tagged, and recorded on magnetic tape, which were then shipped to the Instrument Data Processing System (IDPS) at JPL. SeaSat's five onboard sensor data were individually managed by the following centers:
• ALT (Radar Altimeter): Wallops Flight Center, VA
• SMMR (Scanning Multichannel Microwave Radiometer) and SAR: JPL
• SASS (SeaSat-A Scatterometer System): LaRC (Langley Research Center)
• VIRR (Visible and Infrared Radiometer): GSFC
The received SAR echoes were downlinked in S-band (analog data link at 2.265 GHz) to a total of five ground receiving stations in real-time (no onboard high-rate recording capability was possible at the time; SAR data were only acquired when the satellite was in the sight of a ground station) located at: Goldstone, CA, Fairbanks, AK, Merrit Island, FL, Shoe Cove, Newfoundland (provided by CCRS), and Oakhanger, UK (provided by ESA).
The downlinked analog SAR data were recorded at the receiving stations on film using a cathode ray tube. The data were then processed to pictures using analog Fourier Optical techniques in what is known as an “Optical SAR Processor.” SAR echo data is effectively a microwave hologram of the illuminated area, so by recording this data on film, optical processing becomes the natural approach to forming an image of the ground.
Early SAR data users were hampered by enormous amounts of data and very limited computing power to analyze the data. Until 1978, SAR images were formed using analog techniques, incorporating optical lenses and photographic film (initially, over 95% of the SeaSat data was processed in a survey mode using optical laser techniques).
Also in 1978, the first reconstruction of a SAR image was formed on a digital computer (a slow process at the time with the available computer power, but good quality imagery was generated with this technique). This SAR processor was developed by MDA (MacDonald Dettwiler) of Richmond, BC, Canada, for the purpose to manage SeaSat SAR data. These early digital SAR processors required all the processing power available of a system; they were installed on mainframe computers or on large dedicated hardware. A typical digital SAR scene of size 100 km x 100 km required 6 magnetic tapes of 1600 BPI. -- On the other hand, todays SAR images (since the late 1990s) can be formed on relatively inexpensive equipment like a workstation or a PC. 39) 40)
Figure 12: Coverage map of SeaSat SAR data (image credit: NASA/JPL)
Figure 13: SeaSat ocean data distribution plan (image credit NASA)
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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.