SIR-C/X-SAR Payload on STS-59 and STS-68 Missions
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. The experiment is the next evolutionary step in NASA's SIR (Spaceborne Imaging Radar) program that began with the Seasat Synthetic Aperture Radar (SAR) in l978, and continued with SIR-A in l98l and SIR-B in l984. It also represents a continuation of Germany's imaging radar program which started with the MRSE (Microwave Remote Sensing Experiment) flown aboard the Shuttle on the first SPACELAB mission in l983.
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. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)
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.
Figure 1: Photo of the Space Shuttle Endeavour on the launch pad at KSC (image credit: NASA)
• 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 (distributed amplification) 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 2: Line drawing 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 3: The SIR-C/X-SAR antenna system (image credit: JPL)
Figure 4: Illustration of the SIR-C/X-SAR payload structure (image credit: JPL)
Figure 5: Photo of the SIR-C/X-SAR payload in the Shuttle bay (image credit: NASA)
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. 16)
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 15º to 55º angles of incidence.
Table 1: SIR-C/X-SAR system requirements
Figure 6: Block diagram of the SIR-C/X-SAR system (image credit: NASA, DLR)
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 2. 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 X-SAR antenna used metalized CFRP waveguide technology). 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.
Table 2: 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. 17) 18) 19)
Table 3: Evolution of capabilities in NASA spaceborne SAR system technology 20)
Figure 7: SIR-C/X-SAR false color composite of Central Africa, obtained on Oct. 3, 1994 (image credit: NASA)
Figure 7 shows the Virunga volcano chain along the borders of Rwanda, Zaire and Uganda. In this image red is the L-band (horizontally transmitted, vertically received) polarization; green is the C-band (horizontally transmitted and received) polarization; and blue is the C-band (horizontally transmitted and received) polarization. The area is centered at about 2.4º south latitude and 30.8º east longitude. The image covers an area 56 km x 70 km. 21) 22)
The dark area at the top of the image is Lake Kivu, which forms the border between Zaire (to the right) and Rwanda (to the left). In the center of the image is the steep cone of Nyiragongo volcano, rising 3,465 m high, with its central crater now occupied by a lava lake. To the left are three volcanoes, Mount Karisimbi, rising 4,500 m high; Mount Sabinyo, rising 3,600 m high; and Mount Muhavura, rising 4,100 m high. To their right is Nyamuragira volcano, which is 3,053 m tall, with radiating lava flows dating from the 1950s to the late 1980s. These active volcanoes constitute a hazard to the towns of Goma, Zaire and the nearby Rwandan refugee camps, located on the shore of Lake Kivu at the top left.
Figure 8: Comparison of optical (left) and SAR images of the Kamchatka region Russia (image credit: DLR)
Figure 8 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.
Figure 9: A digital elevation model that was geometrically coded directly onto an X-band seasonal change image of the Oetztal supersite in Austria (image credit: University of Colorado)
The image of Figure 9 is centered at 46.82º north latitude and 10.79º east longitude. This image is located in the Central Alps at the border between Switzerland, Italy and Austria, about 50 km southwest of Innsbruck. It was acquired on April 14, 1994 and on October 5, 1994. It was produced by combining data from these two different data sets. Data obtained in April is green; data obtained in October appears in red and blue, and was used as an enhancement based on the ratio of the two data sets. Areas with a decrease in backscatter from April to October appear in light blue (cyan), such as the large Gepatschferner glacier seen at the left of the image center, and most of the other glaciers in this view. A light blue hue is also visible at the east border of the dark blue Lake Reschensee at the upper left side. This shows a significant rise in the water level. Magenta represents areas with an increase of backscatter from April 10 to October 5. Yellow indicates areas with high radar signal response during both passes, such as the mountain slopes facing the radar. 23)
MAPS (Measurement of Air Pollution from Satellites) of NASA/LaRC, 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 4) 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: 24)
• 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. 25)
• 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. 26) 27)
• 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. 28)
• Supersite Raco (Michigan, at 46.5º N and 84º 30' W).29) 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. 30)
• 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. 31) 32)
Table 4: Survey of SRL-1 and SRL-2 Supersites
1) J. Way, D. Evans, C. Elachi, “The SIR-C/X-SAR mission,” Proceedings of IGARSS'93 (International Geoscience and Remote Sensing Symposium), Tokyo, Japan, Aug. 18-21, 1993, Vol. 2
2) “X-band Synthetic Aperture Radar (X-SAR) and its Shuttle-Borne Application for Experiments,” paper by Herwig Öttl and Francesco Valdoni
3) 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
4) Special Issue on SIR-C/X-SAR, IEEE Transactions on Geoscience and Remote Sensing, Vol. 33, No. 4, July 1995
5) 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
9) 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
10) 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
11) 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
12) 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
13) “SIR-C/X-SAR Fact Sheet, Jan. 28, 1994, URL: http://fogarty.org/tim/sir-c_fact_sheet_jan94.html
15) H. Öttl, “ The SIR-C/X-SAR missions-overview and some results,” Acta Astronautica, Vol. 41, No 3, Aug. 1997, pp. 155-163
16) 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
17) 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
18) M. Coltelli, G. Fornaro, G. Franceschetti, R. Lanari, M. Migliaccio, J. R. Moreira, K. P. Papathanassiou, G. Puglisi, D. Riccio, M. Schwäbisch, “SIR C/X SAR multifrequency multipass interferometry: A new tool for geological interpretation,” Journal of Geophysical Research, Vol. 101, Issue E10, 1996, pp. 23127-23148
19) W. Alpers, C. Melsheimer, “Chapter 17: Rainfall,” URL: http://sarusersmanual.com/ManualPDF/NOAASARManual_CH17_pg355-372.pdf
22) M. Keil, E. Akgoz, S. Carl, B. Forster, T. Hausler, A. Johlige, M. Lautner, K. Martin, “Use of SIR-C/X-SAR and Landsat TM data for vegetation mapping inthe Bavarian Forest national park and in the Ore Mountains,” Proceedings of IGARSS'99, Vol. 1, 1999, Hamburg, Germany, June 28-July 2, 1999, pp. 293-295
24) Special Issue on SIR-C/X-SAR, IEEE Transactions on Geoscience and Remote Sensing, Vol. 33, No. 4, July 1995, pp. 817-950
25) 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
26) Information provided by J. Plaut, JPL, Pasadena, CA
27) K. J. Ranson,S. Guoqing, “ An Evaluation of AIRSAR and SIR-C/X-SAR Images for Mapping Northern Forest Attributes in Maine, USA,” Remote Sensing of Environment, Vol. 59, No 2, February 1997 , pp. 203-222
28) W. Chao, G. Huadong, L. Lin, “SRL-1 CASSAR Ground Campaign and its Results,” Proceedings IGARSS '95, Vol. II, pp. 970-972
29) K. Sarabandi, et al., “Polarimetric Calibration of SIR-C using Point and Distributed Targets,” IGARSS '95, Vol. I, pp. 593-595
30) Information provided by J. Nithack and by Ch. Schmullius of DLR, Oberpfaffenhofen
31) 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
32) “SIR-C/X-SAR Mission Overview,” JPL Publication 93-29, Dec. 15, 1993
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.