What is SIR-C/X-SAR?


SIR-C/X-SAR stands for Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar. SIR-C/X-SAR is an imaging radar system scheduled for launch aboard the NASA Space Shuttle in 1994. It consists of a radar antenna structure and associated 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 cargo 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 used 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


The SIR-C/X-SAR Project


SIR-C/X-SAR is a joint project of the National Aeronautics and Space Administration (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 with SEASAT in 1978, continuing with SIR-A (1981), Germany's Microwave Remote Sensing Experiment (1983), and SIR-B (1984). It is a precursor to the Earth Observing 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 are its capability to measure, from space, the radar signature of the surface at three different wavelengths, and to make measurements for different polarizations 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 wavelength/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 measurements of the following:


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 wavelength) 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 vertical polarizations.

The SIR-C/X-SAR antenna is the most massive piece of hardware (at a total of 10,500 kilograms) ever assembled at the Jet Propulsion Laboratory, and measures 12 meters by 4 meters. The SIR-C instrument was built by JPL and the Ball Communication Systems Division for NASA and provides the L-band and C-band measurements at different polarizations. The L-band and C-band antennas employ phased array technology, which allows the antenna beam pointing to be adjusted electronically. The X-SAR instrument is built by the Dornier and Alenia Spazio companies for DARA and ASI and operates at a single frequency, X-band. The X-SAR antenna is a slotted waveguide type, which uses a mechanical tilt to change the beam pointing direction.


SIR-C/X-SAR Image Data


During a week-long Shuttle flight, SIR-C/X-SAR will image an area of roughly 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 225,000,000 bits) per second. The data collected will be processed into images with resolution 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-SAR 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 the ground, and mapping out a swath as the Shuttle moves forward. The area shown is 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 Italian Space Agency (ASI). The experiment is the next step forward in NASA's Spaceborne Imaging Radar (SIR) program that began with the Seasat Synthetic Aperture Radar (SAR) in 1978, and continued with SIR-A in 1981 and SIR-B in 1984. The program 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 benefits from experience gained with the Magellan Mission to Venus, other international spaceborne radar programs (e.g. ERS-1, JERS-1), and prototype aircraft sensors 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 ([[lambda]]): L- band ([[lambda]] = 23.5 cm) and C-band ([[lambda]] = 5.8 cm). These vertically- and horizontally-polarized transmitted waves will be received on two separate channels, so that SIR-C will provide images of the magnitude of radar backscatter for four polarization combinations: HH (Horizontally-transmitted, Horizontally-received), VV (Vertically-transmitted, Vertically-received), HV, and VH; and also data on the relative phase difference between the HH, VV, VH, and HV returns. This allows derivation of the complete scattering matrix of a scene on a pixel by pixel basis. From this scattering matrix, every polarization configuration (linear, circular or elliptical) can be generated during ground processing. The radar polarimetric data 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 Sensing Experiment (MRSE) flown aboard the Shuttle. This X-band radar was flown on the first SPACELAB mission in 1983. The program was continued by development of the X-SAR, for which cooperation with Italy was initiated. X-SAR, will operate at X-band ([[lambda]] = 3.1 cm) with VV polarization, resulting in a three-frequency capability for the total SIR-C/X-SAR system. Because radar backscatter is most strongly 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 transmitter, 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-band. Each array is composed of a uniform grid of dual-polarized microstrip antenna radiators, with each polarization port fed by a separate corporate feed network. 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 hundreds of small low power solid state transmitters embedded in the surface of the radar antenna. By properly phasing the energy from these transmitters, the beam can be electronically steered in the range direction +/-23deg. from the nominal 40deg. off nadir position without physically moving the large radar antenna. This feature will enable images to be acquired over a wide range of incidence angles.

X-SAR will provide VV polarization images using a passive slotted waveguide antenna measuring 12.0 x 0.4 meters. Other X-SAR components include a traveling wave tube as transmitter, an exciter, receiver, and data handling subsystem. A mechanical tilt mechanism will point the X-SAR antenna to angles between 15 and 60deg., 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 together. 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 orientation of the antenna beams and the operational mode. Table 1 presents a summary 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 us 33.8, 16.9, 8.5 us 40 us
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
Beamwidth 0.25o x 5o at C-band, 1.1o x 6o at L-band
Scan Angle Range ±23o from boresight across narrow antenna direction only
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