Space Shuttle doors closed
Space Shuttle doors open, showing SIR-C/X-SAR antenna
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.
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.
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.
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
|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.
|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|