SIR (Shuttle Imaging Radar) Missions
In the time frame of the early 1980s and early 1990s, NASA/JPL developed and launched a total of four SAR technology demonstration missions in the aftermath of SEASAT, designated as SIR (Shuttle Imaging Radar) missions. These were: SIR-A (1981), SIR-B (1984), and two SIR-C/X-SAR missions in 1994.
The success of the SEASAT L-band SAR mission (1978) prompted the first flight of a side-looking SAR on the Shuttle. The Shuttle was seen as an opportunity to progressively develop and fly increasingly more complex radar systems for short missions, allowing the hardware to be returned to Earth for reuse on follow-on missions.
SIR-A / OSTA-1 Payload on STS-2 Mission
SIR-A (Shuttle Imaging Radar) with payload A, an L-band SAR system of SEASAT spare parts, was developed by JPL and flown on the 2nd Shuttle mission. However, at the time of flight, the SIR-A designation wasn't used yet, the L-band SAR assembly was simply part of the so-called OSTA-1 (Office of Space and Terrestrial Applications) payload with a total mass of 2460 kg. The objectives of OSTA-1 were to: 1) 2) 3) 4)
• Demonstrate the Shuttle for scientific and applications research in the attached mode
• Operate the OSTA-1 payload to facilitate the acquisition of Earth's resources, environmental, technology, and life science data
• Provide data products to principal investigators within the constraints of the STS-2 mission (to use the SAR data for such applications as land use, geology, hydrology, forestry, and mapping to obtain a better understanding of the radar signatures).
• A secondary goal was to assess the Shuttle performance as a scientific platform for Earth observations.
Other payloads (outside of OSTA-1) on STS-2 were: DFI (Development Flight Instrumentation) with a mass of 4750 kg, ACIP (Aerodynamic Coefficient Identification Package), and IECM (Induced Environment Contamination Monitor).
Figure 1: Shuttle Columbia with its payload bay open and its payloads (image credit: NASA)
Launch of SIR-A/OSTA-1 payload on Nov. 12, 1981 (duration: 3 days, this was actually the second orbital test flight of the Shuttle Columbia, STS-2 from KSC; landing at Edwards AFB, CA on Nov. 14, 1981). Orbiter liftoff mass of 104,647 kg, total payload mass of 8,517 kg.
Orbit: Shuttle orbit, altitude of 222 km x 231 km, inclination of 38º, period of 89 min, eccentricity of 0.000757 (a total of 37 orbits were flown).
RMS (Remote Manipulator System), also referred to as Canadarm. The Canadian-built RMS arm was successfully operated in all its various operating modes for the first time. The arm, built of a light-weight carbon composite tubing 38 cm in diameter, is 15.3 m long, with a mass of 408 kg.
OSTA-1 sensor complement: (SIR-A, SMIRR, OCE, MAPS, FILE)
The OSTA-1 payload, sponsored by NASA's Office of Space and Terrestrial Applications, represented the first science and applications payload flown on Space Transportation System (STS). Five of the experiments were located on a so-called "Spacelab pallet" in the payload bay, a reusable modular structure (Al frame) of size 4 m x 3 m. The pallet was provided by ESA.
Figure 2: Illustration of the OSTA-1 payload (image credit: JPL)
SIR-A L-SAR (Synthetic Aperture Radar) assembly in L-band and HH polarization; fixed antenna look angle of 47º; pixel size = 40 x 40 m; the SAR data was recorded onto an optical medium (film).
The system characteristics for SIR-A, were defined primarily by the spare parts available from the Seasat SAR. SIR-A, therefore, had essentially the same design as Seasat: L-band wavelength and HH polarization. The antenna look angle was fixed at 47º in order to optimize geologic mapping in topographically rough terrain.
From the Shuttle altitude, the antenna (spare Seasat SAR antenna panels were used) provided a 50 km swath. The antenna was mounted in a fixed open position on the starboard side of the payload bay, with the long axis parallel to the velocity vector when the Shuttle was flying nose-forward. This allowed imaging only on the northern side of the Shuttle nadir track. Because simplicity was a requirement for SIR-A, the data were recorded optically in the payload bay and were not accessed until the film was off-loaded after landing. The optical recorder on which the data for SIR-A were recorded was a spare from the Apollo 17 Lunar Sounder Experiment and was 14 years old when flown on SIR-A. 5)
Table 1: Overview of SIR-A performance parameters
Data: Image size: 50 km x length of active orbit. Optical image correlation, transparencies and photos available. Transmission: no transmission, optical recording on-board.
The SIR-A mission provided much improved SAR imagery (about 8 hours of SAR observations were recorded) for geologic analyses, since they were relatively free of the layover distortion that accompanied Seasat images of high-relief regions. SIR-A SAR data also led to the discovery of buried and previously uncharted dry river beds beneath the Sahara Desert in Sudan and Egypt. This demonstrated the ability of a L-band radar to penetrate up to several meters in hyper-arid sand sheets.
SMIRR (Shuttle Multispectral Infrared Reflectance Radiometer), a non-imaging NASA/JPL experimental instrument (spectroradiometer). Objectives of SMIRR: a) obtain radiometric measurements from orbit of land surfaces in 10 wavelength channels (rock and mineral identification). b) determine the spectral response of known rock types under different climatic conditions worldwide; c) establish the utility of orbital narrow-band radiometry (10 nm) in the 0.5 to 2.4 µm region; d) establish the requirements for future narrow-band imaging systems. 6)
The SMIRR instrument consisted of a telescope with an aperture of 17.8 cm diameter (a modified version of the telescope used on the Mariner l0 mission to Venus and Mercury in l973), a filter wheel, two detectors (MCT), two film cameras and supporting electronics. The filter wheel contained 15 evenly spaced positions to sample the spectral bands of interest (only 10 bands were used). Instrument mass of 99 kg, size of 56 cm x 94 cm x 117 cm.
SMIRR was able to identify the mineral kaolite and limestone unambiguously, proving that direct mineral identification from orbit was possible.
OCE (Ocean Color Experiment) of NASA/GSFC. The experiment consisted of a multispectral image scanner of high sensitivity, dedicated to the measurements of water color and its interpretation in terms of major water constituents (chlorophyll, sediments, pollutants, algae, etc.). The instrument employed an 8-channel optomechanical imaging scanner design. Total wavelength range from 464 - 773 nm. FOV = ±45º. IFOV = 3.5 milliradian. 7)
OCE consisted of two elements: the scanner and the electronics modules. The scanner module (34 kg mass) was a cylinder of 75 cm in length consisting of a telescope, optics, a scanner with all associated equipment. The rotating mirror featured a FOV of ±45º in the cross-track direction. The telescope imaged a scene through a field stop (1mm x 2 mm) onto a diffraction grating and was detected by 8 silicon photodiode detectors.
Prior to and during the Shuttle mission with the OSTA-1 payload, various OCE-related experiments took place. The main objective of these activities was to perform comparative measurements from aircraft, ship and ground for correlation and verification of the Shuttle OCE test data. A duplicate instrument, referred to as OCS (Ocean Color Scanner) was shipped from GSFC to DLR and installed in aircraft for pre-Shuttle test flights in various regions of Europe.
The following organizations participated in OSTA-1/OCE related activities: GSFC (USA), DLR (Germany), University of Lisbon (Portugal), Hydrologic Institute of the Navy (Portugal), University of Lille (France), GKSS (Germany), INTA Madrid (Spain), IIP Barcelona (Spain).
FILE (Feature Identification and Location Experiment). The objective was to develop techniques to make data gathering by Earth resources satellites more efficient. Using the ratio between visual red reflectance and near infrared reflectance, this experiment attempted to characterize scenes as either vegetation, water, snow or clouds, or bare ground. The FILE device consisted of a sunrise sensor, two television cameras, a decision-making electronics unit, a buffer memory, a tape recorder, and a 70 mm camera. The sunrise sensor activated the experiment whenever the sun is 60º from the Space Shuttle's zenith (30º above the horizon).
MAPS (Measurement of Air Pollution from Satellites), a NASA/LaRC instrument. The objective was to measure the amount of carbon monoxide (CO) distribution in the middle and upper troposphere (12-18 km) to evaluate the CO sources and chemistry, and to determine the seasonal variations of this key atmospheric trace gas.
The MAPS instrument was a gas filter correlation radiometer consisting of an electro-optical sensor, an electronics module, a digital tape data recorder, and an aerial camera. MAPS was mounted onto MPESS (Multi-Purpose Experiment Support Structure) in the forward end of the Shuttle cargo bay. The instrument made nadir-viewing measurements from Shuttle orbits. 8)
The electro-optical sensor consisted of two gas cells, one containing 350 hPa of CO and another containing 149 hPa of N2O; their corresponding detectors; a direct radiation detector; an external balance and gain check system, and an internal balance system.
Table 2: Specification of some MAPS parameters
MAPS measured the absorption lines of CO and N2O of incoming radiation. The spectrum was split into three beams; one beam passed through a cell containing CO and fell onto a detector - the CO gas cell acted as a filter. A second beam fell directly onto a detector. The observed signal differences were used for CO determination in the altitude range of 0-18 km. The third beam of the incident radiation passed through a cell containing N2O and was measured by another detector. Again, the N2O gas cell acted as a filter for the effects of N2O present in the atmosphere. Since the global distribution of N2O was known, the measured values of N2O were used as an indicator for the presence of clouds in the field of view and to correct the simultaneous CO measurement for systematic errors.
The aerial camera mounted next to the MAPS electro-optical head provided information on cloud cover and terrain over which the sensor data were gathered.
MAPS was first flown on STS-2 (November 12.-14, 1981) as part of the OSTS-1 payload, then on STS-41G (October 5 -13, 1984) as part of the OSTA-3 payload. These flights revealed longitudinal variations in CO distributions and identified biomass burning in the Southern Hemisphere as a major global CO source (equal in magnitude to the amount of CO generated in the Northern Hemisphere by fossil fuel combustion). MAPS flew on STS-59 (April 1994) and STS-68 (October 1994).
Figure 3: Ground coverage of the OSTA-1 payload (image credit: NASA)
SIR-B / OSTA-3 Payload on STS-41G Mission
SIR-B (Shuttle Imaging Radar) with payload B, was a NASA/JPL payload on the Shuttle mission as part of the OSTA-3 payload. The OSTA-3 payload consisted of three experiments: SIR-B, MAPS (Measurement of Air Pollution From Satellites), and FILE (Feature Identification and Landmark Experiment). Other payloads included the LFC (Large Format Camera), flown as an experiment demonstration, as well as the IMAX camera system (third flight of IMAX), APE (Auroral Photography Equipment), RME (Radiation Monitoring Equipment), and TLD (Thermoluminescent Dosimeter). 9)
Launch of the STS-41G mission (Challenger) on Oct. 5, 1984 (duration: 1 week, landing on Oct. 13, 1984). The ERBS (Earth Radiation Budget Satellite) spacecraft of NASA was part of the mission; ERBS (primary payload) was deployed by the crew on the first day (using Canadarm). The OSTA-3 payload was mounted on a pallet carrier in the payload bay. In addition, there were 8 GAS (Getaway Special) canisters, four middeck experiments and a series of experiments associated with Canada (CANEX). STS-41G was the 13th flight in the Space Shuttle program. Orbiter mass at liftoff of 110,127 kg, total payload mass of 10.643 kg (with OSTA-3 mass of 1,929 kg).
Figure 4: The OSTA-3 payload configuration with FILE, MAPS and SIR-B (image credit: NASA)
Orbit: Shuttle orbit, 350 km x 390 km initial altitude, inclination of 57º, period of 92 min. Note: The Shuttle mission STS-41G was flown at at three different orbital altitudes. Orbits 001 - 022 were flown at an average altitude of 352 km; orbits 023 - 036 were flown at an altitude of 272 km; and orbits 037 - 128 were flown at an altitude of 225 km. The objective was to test the SIR-B SAR observation performance from different altitudes and different inclination angles. 10)
Mission status: SIR-B actually acquired only seven and a half hours of digital data and eight hours of optical data. Three problems affected the amount of data collected:
1) The Ku-band antenna gimbal failed. It could transmit only prerecorded tape data through the TDRSS (Tracking and Data Relay Satellite System) with special orbiter attitudes. This resulted in acquiring only 20% of the planned science data. Therefore, only fifteen investigators received sufficient data (50 to 75%) to meet their objectives, twenty-three investigators received a limited amount of data (10 to 50%), and six investigators received only a token amount of data.
2) The TDRSS link was lost for twelve hours, forty-two minutes during the mission.
3) Anomalies in the radio frequency feed system to the SIR-B antenna reduced transmitter power and, therefore, degraded the data.
Sensor complement: (SIR-B, LFC, MAPS, FILE)
MAPS and FILE are described under SIR-A. These were re-flies of the sensors flown the OSTA-1 payload of the STS-2 mission.
SIR-B ((Shuttle Imaging Radar-B). A reflight of an upgraded L-band SAR (Synthetic Aperture Radar) instrument as flown on the STS-2 mission - with the provision of a variable look angle of the SAR antenna. SIR-B provided for the first time the ability to mechanically tilt the SAR antenna over a range of 15 - 55º so that radar imagery from multiple angles of incidence could be obtained. The azimuth resolution was 25 m at all incidence angles; radiometric resolution = 3-6 bit on raw data.
The SIR-B radar was significantly upgraded. It had the additional flexibility of imaging geometry, particularly the effect of incidence angle on radar backscatter. One additional panel was added to the SIR-B antenna, bringing its total length to 10.7 m; again the antenna was mounted on the starboard side of the payload bay. The southern side of the track was also imaged during the SIR-B flight by rolling the shuttle 75º or yawing it 180º. A final improvement to the antenna was the ability to fold for launch in order to accommodate an additional deployable payload.
A SAR image scene consisted of the swath width of 30 - 60 km (depending on look angle and orbital altitude) x the length of the active radar track. A digital data handling assembly (DDHA) converted the analog signal to a digital signal on-board the Shuttle. The SAR source data were recorded onto HDRR (High Data Rate Recorder). SAR source data were also transmitted via a TDRS link. RF data transmission: Shuttle downlink via TDRS in Ku-band at a data rate of 40 Mbit/s. The digital data were downlinked via White Sands (NM) and via DomSat to NASA/GSFC. The digital tapes were then sent to JPL to be processed to imagery. The optical data were processed by an optical correlator at JPL. 11)
Several problems severely impacted the SIR-B data collection time. These included the Ku-band antenna gimbal failure (this implied that prerecorded tape data transmissions through TDRS could only be made at special orbiter attitudes). This resulted in a TDRSS link loss for more than 12 hours. As a result, only a total of 7.5 hours of digital data and 8 hours of optically recorded data were acquired during the mission. (optical recording of the SAR data took place when problems with the Ku-band transmissions were encountered).
LFC (Large Format Camera), a NASA experimental camera based on analog film (PI: B. H. Mollberg), built by Itek Optical Systems (Div. of Litton Systems Inc.), Lexington, MA. The objective was to evaluate the utility of orbital photography for cartographic mapping and land use studies at scales of 1:50,000. 12) 13) 14)
The LFC payload consisted of a camera system mounted inside the Shuttle cargo bay. The film magazine was preloaded with 2400 frames of five different types of 23 cm x 46 cm format high definition film (color-infrared, natural color, and three types of black and white film). In addition, LFC employed ARS (Attitude Reference System), consisting of two orthogonal cameras with 152 mm focal lengths, recording the star field at the point in time of each LFC acquisition of the Earth's surface. The known star field permitted the attitude calculation of the LFC in the orbiter bay. This information was used to calculate the precise image location on Earth's surface to an accuracy of < 1 km.
The LFC assembly had a total mass of 1550 kg (the camera itself had a mass of 450 kg), a single frame covered a surface area of about 60,000 km2; it was operated from the ground by NASA/JSC (Johnson Space Center) controllers.
Snapshot frame imagery was taken along the flight path for preselected target areas. To minimize smearing effects, the camera's film plate moved horizontally along the Shuttle's line of flight when the shutter was open. The film (total length of 1200 m) was driven by a forward motion compensation mechanism when exposed on a vacuum plate. A ground resolution of 10-20 m was achieved at altitudes of 200 to 250 km with standard photographic films. The LFC was able to obtain a forward overlapping stereoscopic coverage along the Shuttle's flight path with base-to-height ratios of 0.3, 0.6, 0.9, and 1.2. A total of 2289 photographic frames were obtained, and the experiment was considered a success. However, the large and heavy LFC system did not make any reflights (because it required dedicated payload-bay space, attitude-control fuel, and scheduled time, in contrast to hand-held photography). The LFC film products are distributed by the EROS Data Center, Sioux Falls, SD.
Table 3: Some parameters of the LFC instrument
Figure 5: Front view of the LFC assembly (image credit: EROS Data Center)
SIR-C/X-SAR Payload on STS-59 and STS-68
The SIR-C/X-SAR (Shuttle Imaging Radar with Payload C / X-SAR payload was a cooperative NASA/JPL, DARA/DLR, and ASI (Agenzia Spaziale Italiana) project flown on Space Shuttle Endeavour. The SIR-C/X-SAR project was part of NASA's Mission to Planet Earth. This payload/mission is also known under the name of SRL (Space Radar Laboratory). It consisted of a radar antenna structure and associated radar system hardware designed to fit inside the Space Shuttle's cargo bay. The total payload mass was 11,000 kg with a power consumption of payload sensors of 3 - 9.0 kW. Two Shuttle missions were conducted, each of 10 days duration. 15) 16) 17) 18) 19) 20) 21) 22) 23) 24)
1) Launch of the SRL-1 (STS-59) mission on April 9, 1994. The mission lasted until April 20, 1994.
2) Launch of the SRL-2 (STS-68) mission on Sept. 30, 1994. The mission lasted until Oct. 11, 1994. During the second flight, the orbiter was able to fly nearly the same orbit as in the first flight. This permitted to collect a significant amount of data at all frequencies for use in repeat-pass interferometric SAR processing - resulting in elevation and change detection studies.
• Conduct geoscience investigations that require the observational capabilities of orbiting radar sensors, alone or in conjunction with other sensors, that will lead to a better understanding of the surface conditions and processes on the Earth.
• Explore regions of the Earth's surface that are not well characterized because of vegetation, cloud, or sediment cover in order to better understand land and ocean surface conditions and processes on a global scale.
• Incorporate this new knowledge into global models of surface and subsurface processes.
Application: Land use, geology, hydrology, oceanography, snow and ice, vegetation, calibration, and technological experiments.
Orbit: Shuttle circular Earth orbit, mean altitude of 225 km, inclination of 57º.
The SIR-C design built upon the heritage of the SIR-A and SIR-B missions in the use of SAR technology to study Earth science. A multipolarization, distributed C-band system was added to the SIR-B payload, along with a more powerful multipolarization distributed L-band system. Instead of using a single, high-power transmitter, the distributed SIR-C radars consisted of numerous low-power solid-state transmitters distributed across the antenna aperture (phased array). Large power losses were avoided and as much as an eight-fold improvement in efficiency was possible. The distributed C- and L-band SIR-C radars allowed electronic beam steering in the range direction (123º) from a fixed antenna position of 38º (look angle), thus making it possible to acquire multi-incidence angle data without tilting the entire antenna.
The SIR-C payload implementation provided a two-frequency observation capability, a greatly increased improvement over the SIR-A and SIR-B missions. This was supplemented with a third payload in X-band (DLR/ASI instrument). Hence, the SIR-C/X-SAR payload was the first spaceborne radar system capable of obtaining simultaneously multifrequency (3) and multipolarization radar imagery.
The first introduction/demonstration of the following new technologies in the SIR-C/X-SAR missions represented major milestones in the field of spaceborne SAR observations.
• Fully polarimetric spaceborne SAR (for SIR-C payload)
• Multi-frequency (3), first use of spaceborne X-band
• Active phased array, electronically steered antenna (for SIR-C payload)
• Demonstration of ScanSAR operating mode for wide swath data acquisition
In addition, SIR-C/X-SAR provided the opportunity to identify the optimum wavelengths, polarizations, and illumination geometries for SAR imagery.
Figure 6: Configuration of the SIR-C/X-SAR antenna payload in the Shuttle bay
The overall size of the SIR-C antenna was 12.0 m x 3.7 m. The C-band antenna consisted of 18 panels, 28 T/R (Transmit/Receive) modules per panel (total: 504 T/Rs). The L-band antenna consisted of 18 panels, 14 T/R modules per panel (total: 252 T/Rs).
Figure 7: The SIR-C/X-SAR antenna system (image credit: JPL)
Figure 8: Illustration of the SIR-C/X-SAR payload structure (image credit: JPL)
SLR sensor complement: (L-band SAR, C-band SAR, X-band SAR, MAPS)
All three SAR instruments were flown on each mission. Active microwave sensor observations are independent of the day/night cycle and mostly independent of the weather. - The SRL-2 orbit was nearly identical to that of SRL-1 permitting repeat cross-track interferometric SAR processing for measuring elevation as well as to detect change in the radar direction (a significant amount of data was collected at all three frequencies).
The dedicated payload filled the entire Shuttle cargo bay. The antenna was mounted in a tilted position and the Shuttle rolled to a nominal 38º look angle. In order to acquire data at look angles other than 38º with the conventional X-band system, the X-band antenna, which was mounted along the upper portion of the array, was mechanically tilted. 25)
Unlike the previous SIR-A and SIR-B missions, the SIR-C radar beam was formed from hundreds of small transmitters embedded in the surface of the radar antenna (phased array). By properly phasing the energy from these transmitters, the beam could be steered electronically without physically moving the large radar C-band antenna. This feature allowed imagery to be acquired from 15 to 55º angles of incidence.
Table 4: SIR-C/X-SAR system requirements
Figure 9: Block diagram of the SIR-C/X-SAR system
• L-band SAR (1.250 GHz, wavelength = 23.5 cm). The L-band antenna is a planar array (12 m x 2.95 m in size) composed of a uniform grid of dual-polarized microstrip antenna radiators, active phased arrays. Further details are given in Table 5. The SIR-C instrumentation was developed and built by JPL and the Ball Communication Systems Division for NASA and provided the L-band and C-band systems/measurements at different polarizations.
• C-band SAR (5.3 GHz, wavelength = 5.8 cm). The SIR-C payload comprised the L-band and C-band SAR antenna plus instrumentation. The SIR-C antenna boresights were steered electronically to provide coverage at varying distances from the Shuttle ground track. The SIR-C phased array also provided for broadening of the beam in the elevation direction from its minimum value of 5º to 16º (selection of seven values).
The SIR-C phased array enabled the operational modes of ScanSAR and Spotlight (first implementation of ScanSAR anywhere, developed by JPL). In ScanSAR, the antenna pattern coverage on the ground was stepped in the cross-track direction during the synthetic aperture period to allow coverage over a wider swath; however, at the expense of azimuth resolution. The swath width ranged from 15 to 65 km for calibrated images and 40 to 90 km for mapping mode (ScanSAR) images. For the Spotlight mode, the boresight was positioned in azimuth to dwell on a particular area as the Shuttle flew by. This permitted an increase in azimuth resolution to a value of 7 m for the selected area, at the expense of the along-track swath. The typical image size of the SIR-C payload was 100 km (azimuth, flight direction) x 50 km (swath).
For the SIR-C payload, the digital data handling subsystem had a mass of 145 kg and consumed about 800 W of power.
• X-SAR (SAR for X-band Measurement (9.6 GHz, wavelength = 3.1 cm), provided by DARA/DLR and ASI, built by Dornier and Alenia Spazio). The X-SAR payload was of Germany's MRSE (Microwave Remote Sensing Experiment) heritage, flown aboard the Shuttle on the first Spacelab mission in l983. The program evolved eventually into the development of the X-SAR system at DLR with cooperation provided by ASI of Italy.
The X-SAR design used only vertical polarization (VV). The X-SAR instrument used a passive slotted-waveguide antenna (12 m x 0.4 m) which was tilted mechanically to align the X-band beam with the SIR-C C-band and L-band beams. The X-SAR antenna had a fixed beamwidth of 5.5º in elevation and 0.14º in azimuth as opposed to the phased array and multi-polarization antenna capabilities of SIR-C. The instantaneous area illuminated by the X-band antenna on the ground (footprint) was an ellipse of size 60 km x 0.8 km from an orbital altitude of 225 km. The electronics part of X-SAR was mounted on a cold plate structure and positioned underneath the antenna structure. A TWT (Travelling Wave Tube) amplifier was transmitting up to 1736 pulses/s at a peak transmit power of 3.35 kW. The pulses were frequency-modulated with a pulse length of 40 µs and a programmable bandwidth of 10 or 20 MHz. 26) 27)
Table 5: SIR-C/X-SAR instrument parameters
SAR data collection:
The science source data were digitally coded and formatted in DDHA - using BFPQ (Block Floating Point Quantization) a form of data compression from 8 bits/sample to 4 bits/sample - and recorded onboard by the PHRR (Payload High Rate Recorder), a system of several recorders which generated High Density Digital Tapes (HDDT). There were 180 HDDTs onboard to record the data (total volume of 32 Tbit). Portions of the science data were downlinked via TDRS (Ku-band, 50 Mbit/s) to permit quicklooks for the investigators (only one SAR data stream at a time could be transmitted). After the return of each mission, the HDDTs were taken and sent to JPL, DLR and ASI for processing and analysis.
Nominally, 50 hours of SIR-C data (on each of the four channels) and 50 hours of X-SAR data were recorded by onboard tape recorders. Data "takes" were largely over experiment sites selected prior to launch, with some in-flight "targets of opportunity." The orbital altitude was trimmed for the last days of the second flight (SRL-2) to provide a repeat-track interferometric observation geometry. - The intent was to provide data calibrated in such a way as to allow comparisons with other spaceborne SAR data (eg., ERS-1, JERS-1, Radarsat, etc.) so that a time-series view of key geophysical parameters may be realized.
The SIR-C/X-SAR mission provided for the first time spaceborne polarimetric SAR data of the SIR-C payload. This provided the derivation of the complete scattering matrix of a scene on a pixel by pixel basis.
The SIR-C/X-SAR Science Team had selected nineteen "supersites" for intensive coverage during the mission. In addition, fifteen backup supersites had been selected for added redundancy should operating parameters change during the mission. This arrangement permitted interdisciplinary studies for each supersite. In all, the two SIR-C/X-SAR missions observed more than 400 sites.
Both payloads, SIR-C and X-SAR, could be operated as either stand-alone radars or together. Roll and yaw maneuvers of the Shuttle permitted to acquire data from either side of the Shuttle nadir track.
During each SLR mission, a SIR-C/X-SAR POCC (Payload Operations Control Center) was operated at NASA/JSC (Houston, TX). POCC personnel were responsible for operating the radar antenna and ensuring that radar data were recorded onboard the Shuttle. The POCC received also the mission science data that were downlinked via TDRS for processing and analysis.
During both missions, the SIR-C/X-SAT system operation exceeded all its performance requirements (in spite of some anomalies in C-band panel performance). The SIR-C/X-SAR science team, consisting of 52 investigator teams from more than a dozen countries, were using the SIR-C/X-SAR data in studies of ecology, hydrology, geology, and oceanography. Interferometric data were used for topographic mapping and surface change monitoring. In addition, observations of rainstorms demonstrated for the first time the capability of a multifrequency, multipolarization spaceborne radar system to quantify precipitation rates and to classify rain type. 28)
Table 6: Evolution of capabilities in NASA spaceborne SAR system technology 29)
Figure 10: SIR-C/X-SAR image (100 km x 60 km) of Houston, TX, obtained Oct. 10, 1994 (image credit: NASA)
Figure 11: Comparison of optical (left) and SAR images of the Kamchatka region Russia (image credit: DLR)
Figure 11 shows two pictures of the Kliuchevshoi volcano on Kamchatka island, Russia, using optical and SAR imaging technologies. The optical photo at left was taken by Shuttle astronauts on the STS-68 mission during the early hours of the eruption on September 30, 1994. The radar image at right was acquired by SIR-C/X-SAR aboard the space shuttle Endeavour on its 88th orbit on October 5, 1994. The radar image shows an area of about 75 km x 100 km centered at 58.16º N latitude and 160.78º E longitude. 30)
MAPS (Measurement of Air Pollution from Satellites), see description under the sensor complement of mission SIR-A.
Some underflight campaigns of SLR-1/-2 missions
A total of 19 supersites were selected (Table 7) for specific interdisciplinary research projects. All of these supersite investigations were (at the minimum) accompanied by field measurements in parallel to the Shuttle observations. In addition several `underflight campaigns' were conducted (in parallel with the SIR-C/X-SAR overflights) at various supersites and with spaceborne, airborne, and ground-based instruments by a worldwide research community. Some campaign activity is reported here: 31)
• Gulfstream supersite. A major multi-organizational series of experiments was conducted off the US East Coast (located within: 42º N, 75º W; 36º N, 65º W; 30º N, 73º W) with the objective to investigate oceanographic phenomena (emphasis on current-wave and air-sea interactions) in the Gulf Stream. Participant organizations: JPL, U. of Hamburg, NOAA, NAWC, NRL, U. of Miami, Navy ONR, USGS, NASA, etc. Extensive ground/sea/air truthing data were collected and eventually compared with the SRL data. NRL-P-3 aircraft with RAR, P-3/SAR (ERIM/NAWC), ROWS; DC-8 with AIRSAR/TOPSAR, ERS-1 SAR imagery, AVHRR, NOAA/NDBC offshore buoys, RV Cape Hatteras, and stations. Specific objectives were: 1) to understand the dependence of SAR signatures of the supersite boundary on radar and environmental parameters; 2) to investigate the relationship between subsurface thermohaline circulation and near-surface atmospheric structure on the wave field responsible for the radar imagery signatures; 3) to optimize SAR sensor performance (polarimetric and interferometric radar collection modes) for detecting currents; 4) to understand imagery of the perturbation of Kelvin wakes by current and thermal fronts; and 5) to investigate the role of the hydrodynamic structure in the origin of "slick-like" features observed in near coastal regions by SAR imagery. 32)
• The NASA/JPL AIRSAR/TOPSAR system (DC-8) was used in extensive underflight campaigns during both SRL missions. Observation sites during SRL-1 were: Stovepipe Wells (Death Valley), CA; Mammoth, CA; Chickasha, OK; Gulf of Mexico; Gulf stream; Duke Forest, NC; Mahantango, PA; Howland, ME; Raco, MI; Altona (Manitoba), Canada; Prince Albert, Canada; Bighorn Basin, WY. - Observation sites during SRL-2: Chickasha, OK; Gulf of Mexico; Duck Pier, NC; Duke Forest, NC; Howland, ME; Mahantango, PA; Raco, MI; Altona, Canada; Prince Albert, Canada; Yellowstone, MT; Bighorn Basin, WY; Davis, CA. 33)
• Beijing test site (G85). During SRL-1 a concurrent airborne underflight campaign was conducted by the Chinese Academy of Sciences (CAS) with its CASSAR instrument, operated by IRSA-CAS (Institute of Remote Sensing Applications of CAS). Objective: comparison with SRL-1 imagery. 34)
• Supersite Raco (Michigan, at 46.5º N and 84º 30' W).35) The University of Michigan was involved in the development of calibration procedures and precision calibration devices to quantify the complex radar images with an accuracy of 0.5 dB in magnitude and 5º in phase. A calibration campaign took place at Raco during the SRL-1, and -2 Shuttle overflights utilizing the following equipment: an array of point calibration targets including trihedral corner reflectors and PARCs (Polarimetric Active Radar Calibrators); distributed uniform target (for characterizing radiometric calibration) consisting of a field of grass, sometimes covered with snow; parallel measurements with ground-based polarimetric scatterometers.
• Supersite Oberpfaffenhofen. The DLR E-SAR instrument was flown on a DO-228 during each of the SRL-1 and -2 Shuttle missions (five times for each SRL flight) addressing such topics as calibration, agriculture, forestry and hydrology. Researchers from seven German institutes collected in parallel ground truth data during the two missions. In between the two SRL missions E-SAR participated in the EMAC (see EMAC) campaign, establishing a multitemporal and multifrequency SAR-dataset from the beginning of April to the end of October 1994 for the Oberpfaffenhofen supersite. 36)
• Supersite Altona, Manitoba, Canada (CRSS site at 49º 4.9' N, 97º 39.6' W). Underflights were conducted with C/X-SAR in a Convair-580 aircraft (CRSS) and AIRSAR/TOPSAR on a DC-8. In addition acquisition of SIR-C/X-SAR data. Collection of ground truth data. The objectives were to evaluate the multitemporal and multifrequency SAR data and to estimate soil moisture for a variety of soil types. 37) 38)
Table 7: Survey of SRL-1 and SRL-2 Supersites
1) J. W. Moore, "OSTA-1: The Space Shuttle's first scientific payload," Proceedings of 33rd IAF/IAC Congress, Paris, France, Sept. 27-Oct. 2, 1982, 9 p.
3) "STS-2, Second Space Shuttle Mission," NASA Press Kit, Sept. 1981, http://www.jsc.nasa.gov/history/shuttle_pk/pk/Flight_002_STS-002_Press_Kit.pdf
5) J. Way, "Spaceborne Imaging Radar - From Remote Sensing Science to Earth Science Questions," Launchspace Magazine, Volume 3.04, Aug/Sep 1998
6) Manual of Remote Sensing, Second Edition, American Society of Photogrammetry, 1983, pp. 1707-1710
7) H. v.d. Piepen, V. Amann, H. Helbig, H. H. Kim, W. Hart, et al. "The Promise of Remote Sensing," IEEE paper presented at IGARSS `82, June 1-4, Munich, Germany
8) Information provided by V. Connors and D. O. Neil of NASA/LaRC
9) Space Applications, Vol. 6, Chapter 2, "NASA Historical Data Book," URL: http://history.nasa.gov/SP-4012/vol6/vol_vi_ch_2.pdf
10) J. Cimino, C. Elachi, M. Settle, " SIR-B-The Second Shuttle Imaging Radar Experiment," IEEE Transactions on Geoscience and Remote Sensing, Vol. GE-24, Issue 4, July 1986, pp. 445-452
11) Shuttle Imaging Radar-B (SIR-B), http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1984-108A&ex=1
12) B. H. Mollberg, B. B. Schardt, "Mission Report on the Orbiter Camera Payload System (OCPS) Large Format Camera (LFC) and Attitude Reference System (ARS)," 1988, Johnson Space Center, JSC-23184
14) F. El-Baz, "New Mapping-Quality Photographs of the Earth and Their Applications to Planetary Comparisons," Lunar and Planetary Science XVI, 1985, pp. 207-208; http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?1985LPI....16..207E&data_type=PDF_HIGH&type=PRINTER&filetype=.pdf
15) "X-band Synthetic Aperture Radar (X-SAR) and its Shuttle-Borne Application for Experiments," paper by Herwig Öttl and Francesco Valdoni
16) R.L. Jordan, B. L. Huneycutt, M. Werner, "The SIR-C/X-SAR Synthetic Aperture Radar System," Proceedings of the IEEE, Vol. 33, No. 4, July 1995, pp. 829-839
17) Special Issue on SIR-C/X-SAR, IEEE Transactions on Geoscience and Remote Sensing, Vol. 33, No. 4, July 1995
18) R.L. Jordan, B. L. Huneycutt, M. Werner, "The SIR-C/X-SAR Synthetic Aperture Radar System," Proceedings of the IEEE, Vol. 79, No. 6, June 1991, pp. 827-838
20) E. R. Stofan, D. L. Evans, C. Schmullius, B. Holt, J. J. Plaut, J. von Zyl, S. D. Wall, J. Way, "Overview of Results of Spaceborne Imaging Radar-C, X-Band Synthetic Aperture Radar (SIR-C/X-SAR)," IEEE Transactions on Geoscience and Remote Sensing, Vol. 33, No. 4, July 1995, pp. 817-828
21) R. L. Jordan, B. L. Huneycutt, M. Werner, "The SIR-C/X-SAR Synthetic Aperture Radar System," IEEE Transactions on Geoscience and Remote Sensing, Vol. 33, No 4, July 1995, pp. 829-839
22) M. Zink, R. Bamler, "X-SAR Radiometric Calibration and Data Quality," IEEE Transactions on Geoscience and Remote Sensing, Vol. 33, No. 4, July 1995, pp. 840-847
23) D. L. Evans, J. J. Plaut, E. R. Stofan, "Overview of the Spaceborne Imaging Radar-C/X-Band Synthetic-Aperture Radar (SIR-C/X-SAR) Missions, Remote Sensing of Environment, Vol. 59, No 2, 1997, pp. 135-140
25) F. V. Stuhr, R. L. Jordan, M. U. Werner, "SIR-C/X-SAR A Multifaceted Radar," IEEE Aerospace and Electronic Systems Magazine, Vol. 10, No. 10, Oct. 1995, pp. 15-25
28) A. Jameson, F. Li, S. Durden, et al., "SIR-C/XSAR Observations of rain storms," Remote Sensing of Environment, Vol. 59, No 2, 1996, pp. 267-279
31) Special Issue on SIR-C/X-SAR, IEEE Transactions on Geoscience and Remote Sensing, Vol. 33, No. 4, July 1995, pp. 817-950
32) S. A. Mango, et al., "Remote Sensing of Current-Wave Interactions with SIR-C/X-SAR during SRL-1 and SRL-2 at the Gulfstream Supersite," Proceedings of IGARSS'95, Volume II, pp. 1325-1327
33) Information provided by J. Plaut, JPL, Pasadena, CA
34) W. Chao, G. Huadong, L. Lin, "SRL-1 CASSAR Ground Campaign and its Results," Proceedings IGARSS '95, Vol. II, pp. 970-972
35) K. Sarabandi, et al., "Polarimetric Calibration of SIR-C using Point and Distributed Targets," IGARSS '95, Vol. I, pp. 593-595
36) Information provided by J. Nithack and by Ch. Schmullius of DLR, Oberpfaffenhofen
37) T. J. Pultz, et al., "SIR-C/X-SAR Observations of Soil Moisture over the CCRS Altona, Manitoba Test Site," IGARSS '95, Vol. II, pp. 990-993
38) "SIR-C/X-SAR Mission Overview," JPL Publication 93-29, Dec. 15, 1993
This description was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" - comments and corrections to this article are welcomed by the author.
The SIR series: