SIR-C/X-SAR stands for Spaceborne Imaging Radar-C/X-band Synthetic Aper ture Radar. SIR-C/X-SAR is an imaging radar system scheduled for launch aboard t he NASA Space Shuttle in 1994. It consists of a radar antenna structure and asso ciated radar system hardware that is designed to fit inside the Space Shuttle's cargo bay. On take-off, the cargo bay doors are closed as seen in the graphic on the next page. After the Space Shuttle has reached a stable Earth orbit, the ca rgo bay doors will be opened , the antenna structure will be deployed, and SIR-C /X-SAR will be switched on, to begin using its state-of-the-art radar technology to image the earth's surface. Radar images generated by SIR-C/X-SAR will be use d by scientists to help understand some of the processes which affect the earth' s environment, such as deforestation in the Amazon, desertification south of the Sahara, and soil moisture retention in the Mid-West.
¥ Deploying SIR-C
Space Shuttle doors closed
Space Shuttle doors open, showing SIR-C/X-SAR antenna
SIR-C/X-SAR is a joint project of the National Aeronautics and Space Adm inistration (NASA), the German Space Agency (DARA) and the Italian Space Agency (ASI). It is the next step in a series of spaceborne imaging radars, beginning w ith SEASAT in 1978, continuing with SIR-A (1981), Germany's Microwave Remote Sen sing Experiment (1983), and SIR-B (1984). It is a precursor to the Earth Observi ng System (EOS) imaging radar system planned for the end of the decade.
¥ Science Objectives
SIR-C/X-SAR's unique contributions to Earth observation and monitoring a re its capability to measure, from space, the radar signature of the surface at three different wavelengths, and to make measurements for different polarization s at two of those wavelengths. SIR-C image data will help scientists understand the physics behind some of the phenomena seen in radar images at just one wavele ngth/polarization, such as those produced by SEASAT. Investigators on the SIR-C/ X-SAR Science team will use the radar image data from SIR-C/X-SAR to make measur ements of the following:
¥ Vegetation type, extent and deforestation
¥ Soil moisture content
¥ Ocean dynamics, wave and surface wind speeds and directions
¥ Volcanism and tectonic activity
¥ Soil erosion and desertification
¥ SIR-C/X-SAR Instrument Description
The SIR-C/X-SAR antenna structure actually consists of three individual antennas, one operating at L-band (23.5cm wavelength), one at C-band (5.8cm wave length) and the third at X-band (3cm wavelength). The L-band and C-band antennas are constructed from separate panels that can measure both horizontal and verti cal polarizations.
The SIR-C/X-SAR antenna is the most massive piece of hardware (at a tota l of 10,500 kilograms) ever assembled at the Jet Propulsion Laboratory, and meas ures 12 meters by 4 meters. The SIR-C instrument was built by JPL and the Ball C ommunication Systems Division for NASA and provides the L-band and C-band measur ements at different polarizations. The L-band and C-band antennas employ phased array technology, which allows the antenna beam pointing to be adjusted electron ically. The X-SAR instrument is built by the Dornier and Alenia Spazio compani es for DARA and ASI and operates at a single frequency, X-band. The X-SAR antenn a is a slotted waveguide type, which uses a mechanical tilt to change the beam p ointing direction.
¥ SIR-C/X-SAR Image Data
During a week-long Shuttle flight, SIR-C/X-SAR will image an area of rou ghly 50 million square kilometers of the Earth's surface. This corresponds to a total of 50 hours of data. The peak data rate will be 225 megabits (or 225,000,0 00 bits) per second. The data collected will be processed into images with resol ution selectable from 10 to 200 meters. The width of the area mapped out by the radar will vary from 15 to 90 kilometers, depending on how the radar is operated , and the direction in which the antenna beams are pointing. Data from SIR-C/X-S AR will be used to develop automatic techniques for extracting information from radar image data, in preparation for the EOS SAR mission later in the decade.
This schematic diagram shows the SIR-C/X-SAR antennas illuminating an area on th e ground, and mapping out a swath as the Shuttle moves forward. The area shown i s a SEASAT image of Los Angeles, California. North is to the right of the image shown.
¥ More About SIR-C/X-SAR
The Shuttle Imaging Radar-C and X-Band Synthetic Aperture Radar (SIR-C/X -SAR) is a cooperative space shuttle experiment between the National Aeronautics and Space Administration (NASA), the German Space Agency (DARA), and the Italia n Space Agency (ASI). The experiment is the next step forward in NASA's Spacebo rne Imaging Radar (SIR) program that began with the Seasat Synthetic Aperture Ra dar (SAR) in 1978, and continued with SIR-A in 1981 and SIR-B in 1984. The progr am will eventually lead to TOPSAT, a mission to measure topography globally, and the Earth Observing System (EOS) SAR later in this decade. The program also be nefits from experience gained with the Magellan Mission to Venus, other internat ional spaceborne radar programs (e.g. ERS-1, JERS-1), and prototype aircraft sen sors such as the JPL Airborne SAR (AIRSAR).
SIR-C will provide increased capability over SEASAT, SIR-A, and SIR-B by acquiring digital images simultaneously at two microwave wavelengths (l): L- ba nd (l = 23.5 cm) and C-band (l = 5.8 cm). These vertically- and horizontally-po larized transmitted waves will be received on two separate channels, so that SIR -C will provide images of the magnitude of radar backscatter for four polarizati on combinations: HH (Horizontally-transmitted, Horizontally-received), VV (Verti cally-transmitted, Vertically-received), HV, and VH; and also data on the relati ve phase difference between the HH, VV, VH, and HV returns. This allows derivat ion of the complete scattering matrix of a scene on a pixel by pixel basis. Fro m this scattering matrix, every polarization configuration (linear, circular or elliptical) can be generated during ground processing. The radar polarimetric d ata will yield more detailed information about the surface geometric structure, vegetation cover, and subsurface discontinuities than image brightness alone.
Germany's imaging radar program started with the Microwave Remote Sensin g Experiment (MRSE) flown aboard the Shuttle. This X-band radar was flown on th e first SPACELAB mission in 1983. The program was continued by development of t he X-SAR, for which cooperation with Italy was initiated. X-SAR, will operate a t X-band (l = 3.1 cm) with VV polarization, resulting in a three-frequency capab ility for the total SIR-C/X-SAR system. Because radar backscatter is most stron gly influenced by objects comparable in size to the radar wavelength, this multi -frequency capability will provide information about the Earth's surface over a wide range of scales not discernible with previous single-wavelength experiments .
¥ SIR-C/X-SAR Instrumentation
SIR-C will provide multi-frequency, multi-polarization radar data. The SIR-C instrument is composed of several subsystems: the antenna array, the trans mitter, the receivers, the data-handling subsystem, and the ground SAR processor . The antenna is composed of two planar arrays, one for L-band and one for C-ba nd. Each array is composed of a uniform grid of dual-polarized microstrip anten na radiators, with each polarization port fed by a separate corporate feed netwo rk. The overall size of the SIR-C antenna is 12.0 x 3.7 meters and consists of three leaves each divided into four subpanels.
Model of the SIR-C/X-SAR antenna
Unlike previous SIR missions, the SIR-C radar beam is formed from hundre ds of small low power solid state transmitters embedded in the surface of the ra dar antenna. By properly phasing the energy from these transmitters, the beam c an be electronically steered in the range direction ±23¡ from the nominal 40¡ of f nadir position without physically moving the large radar antenna. This featur e will enable images to be acquired over a wide range of incidence angles.
X-SAR will provide VV polarization images using a passive slotted wavegu ide antenna measuring 12.0 x 0.4 meters. Other X-SAR components include a trave ling wave tube as transmitter, an exciter, receiver, and data handling subsystem . A mechanical tilt mechanism will point the X-SAR antenna to angles between 1 5 and 60¡, in the same direction as the L-band and C-band beams.
Both SIR-C and X-SAR can be operated as either stand alone radars or tog ether. Roll and yaw maneuvers of the shuttle will allow data to be acquired on either side of the shuttle nadir (ground) track. The width of the imaged swath on the ground varies from 15 to 90 kilometers (9 to 56 miles) depending on the o rientation of the antenna beams and the operational mode. Table 1 presents a sum mary of the SIR-C/X-SAR system characteristics.
Table 1: SIR-C/X-SAR System Characteristics
PARAMETER L-BAND C-BAND X-BAND
Wavelength 0.235 m 0.058 m 0.031 m
Swath Width 15 to 90 km 15 to 90 km 15 to 40 km
Pulse Length 33.8, 16.9, 8.5 µs 33.8, 16.9, 8.5 µs 40 µs
Data Rate 90 Mbits/s 90 Mbits/s 45 Mbits/s
Data Format 8,4 bits/word 8,4 bits/word 8,4 bits/word
(8,4) BFPQ (8,4) BFPQ (8,4) BFPQ
BFPQ = Block Floating Point Quantization, a form of data compression from 8 bits per sample to 4 bits per sample.
SYSTEM PARAMETERS:
Orbital Altitude 225 km
Resolution typically 30 x 30 m on the surface
Look Angle Range 17 to 63 degrees from nadir
Bandwidth 10, 20 and 40 MHz
Pulse Repetition Rate 1395 to 1736 pulses per second
Total Science Data 50 hours/channel/mission
Total Instrument Mass 11,000 kg
DC Power Consumption 3000 to 9000 W