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- FACT SHEET: SIR-C/X-SAR
-
-
- Since the late 1970s a variety of NASA satellite missions
- have used imaging radar to study the Earth and our planetary
- neighbors. The joint U.S./German/Italian Spaceborne Imaging
- Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) is the next
- step in that program.
-
- SIR-C/X-SAR will fly aboard the space shuttle Endeavour in
- early 1994 as part of the Space Radar Laboratory (SRL). This
- will be the first of at least three flights, with the following
- two launches scheduled for 1995 and 1996.
-
- The most useful feature of imaging radar, also called
- synthetic aperture radar (SAR), is its ability to collect data
- over virtually any region at any time, regardless of weather or
- sunlight conditions. The radar waves can penetrate clouds, and
- under certain conditions the radar can also see through
- vegetation, ice and dry sand. In many cases, radar is the only
- way scientists can explore inaccessible regions of the Earth's
- surface.
-
- A synthetic aperture radar transmits pulses of microwave
- energy toward Earth and collects the energy that is scattered
- back to the antenna. The motion of the shuttle is used to
- "synthesize" an antenna (the aperture) that is much longer in
- length than the actual SIR-C/X-SAR antenna in the shuttle. A
- longer antenna produces images of finer resolution.
-
- The SIR-C/X-SAR mission is a major technical step forward in
- the evolution of spaceborne imaging radar. It is the first
- spaceborne radar system that will simultaneously acquire images
- at multiple wavelengths and polarizations.
-
- SIR-C, built by JPL and the Ball Communications Systems
- Division for NASA, is a two-frequency radar including L-band (23-
- cm wavelength) and C-band (6-cm wavelength). SIR-C will have the
- capability to transmit and receive horizontally and vertically
- polarized waves at both frequencies.
-
- X-SAR is built by Dornier and Alenia Spazio companies for
- the German space agency, Deutsche Agentur fuer
- Raumfahrtangelegenheiten (DARA), and the Italian space agency,
- Agenzia Spaziale Italiana (ASI). It is a single-polarization
- radar operating at X-band (3-cm wavelength).
-
- SIR-C/X-SAR will allow scientists to make highly detailed
- studies of the Earth's surface on a global scale, including new
- types of measurements such as biomass and soil moisture.
-
- The SIR-C/X-SAR system is a precursor mission to the Earth
- Observing System (EOS) SAR, a polar-orbiting satellite that will
- carry radars operating at the same three frequencies.
-
-
-
- SCIENCE
-
- Scientists will use SIR-C/X-SAR to measure vegetation
- structure and seasonal changes in wetlands. SIR-C/X-SAR data
- will also provide measurements of soil moisture and surface
- roughness, tropical forest flooding, seasonal changes in snow
- cover and glacier properties, and the distribution of snow over
- mountainous regions. Oceanographers will use SIR-C/X-SAR to
- image currents, eddies, frontal boundaries, ocean swells,
- internal waves and ocean bottom topography.
-
- The radar's ability to penetrate arid soil will help
- scientists understand Earth's ancient climate and water patterns.
-
- SIR-C/X-SAR will also play an important role in monitoring
- present-day geologic activity such as volcanic eruptions,
- tectonics, erosion, and desertification.
-
- Initial plans call for SIR-C/X-SAR to collect a total of 50
- hours of data, roughly corresponding to 50 million square
- kilometers (18 million square miles) of ground coverage.
- Several "supersites" of extreme scientific interest, such as the
- Galapagos Islands and the Sahara Desert, have been identified and
- will be continually monitored during the mission. SIR-C/X-SAR
- will be flown during different seasons which will allow
- scientists to make comparative measurements of these same sites.
-
- The scientists will also establish "ground truth" teams that
- will make simultaneous measurements of vegetation, soil moisture,
- sea state, snow and weather conditions during the mission. Data
- from these teams will be supplemented with information taken from
- aircraft and ships to ensure an accurate interpretation of the
- data taken from space. In addition, the astronauts will record
- their personal observations of weather and environmental
- conditions in coordination with SIR-C/X-SAR operations.
-
-
- BACKGROUND
-
- SIR-C is the latest in a series of spaceborne imaging radar
- missions that began in June 1978 with the launch of Seasat SAR
- and continued with SIR-A in November 1981 and with SIR-B in
- October 1984. Both the SIR-A and SIR-B sensors were derived from
- the Seasat SAR, and all three were capable of transmitting and
- receiving horizontally polarized radiation (commonly referred to
- as HH polarization) at a frequency of 1.28 gigahertz (L-band
- frequency).
-
- The major difference between the Seasat and SIR-A sensors
- was the orientation of the radar's antenna with respect to the
- Earth's surface. Microwave radiation transmitted by Seasat
- struck the surface at a fixed angle of approximately 23 degrees
- from the local zenith direction. SIR-A was designed to view the
- surface at a fixed 50 degree angle.
-
- SIR-B improved upon both those missions because its antenna
- could be mechanically rotated. This allowed SIR-B to obtain
- multiple radar images of a given target at different angles
- during successive shuttle orbits.
-
- The X-SAR antenna is a follow-on to Germany's Microwave
- Remote Sensing Experiment (MRSE) which was flown aboard the first
- shuttle Spacelab mission in 1983.
-
-
- SIR-C/X-SAR SENSOR CHARACTERISTICS
-
- The SIR-C antenna is the most massive piece of flight
- hardware ever built at JPL, and will nearly fill the entire
- shuttle cargo bay. Its mass is 10,500 kg (23,100 lbs) and it
- measures 12 meters by 4 meters (39.4 feet by 13.1 feet). The
- antenna consists of three leaves and each is divided into four
- subpanels.
-
- Unlike previous SIR missions, the SIR-C radar beam is formed
- from hundreds of small transmitters embedded in the surface of
- the radar antenna. By properly phasing the energy from these
- transmitters, the beam can be electronically steered without
- physically moving the large radar antenna. This feature will
- allow images to be acquired from 15 degree to 55 degree angles of
- incidence. Advancements in radar technology will allow SIR-C to
- acquire simultaneous images at L-band and C-band frequencies with
- HH, VV, HV, and VH polarizations.
-
- Polarization describes how the radar wave travels in
- space. For example, when data is acquired with HH polarization
- the wave is transmitted from the antenna in the horizontal plane
- and the antenna receives the backscattered radiation in the
- horizontal plane. With HV polarization, the wave is transmitted
- horizontally, but is received by the antenna in the vertical
- plane. It is the interaction between the transmitted waves and
- the Earth's surface that determines the polarization of the waves
- received by the antenna. Multi-polarization data contains more
- information about surface conditions than single polarization
- data.
-
- X-SAR will use a slotted-waveguide antenna which is mounted
- on a bridge structure that is tilted mechanically to align the X-
- band beam with the L-band and C-band beams. X-SAR will provide
- VV polarization images.
-
- Both SIR-C and X-SAR can be operated as either stand alone
- radars or in conjunction with each other. The width of the
- ground swath varies from 15 to 90 kilometers (9 to 56 miles),
- depending on the orientation of the antenna beams. The
- resolution of the radars can be varied from 10 to 200 meters (33
- to 656 feet.)
-
-
- DATA PROCESSING
-
- All data will be stored onboard the shuttle using new high-
- density, digital, rotary-head tape recorders with portions
- relayed to the ground via the Tracking and Data Relay Satellite
- System (TDRSS) data link. There will be 160 digital tape
- cartridges (similar to VCR tape cartridges) carried aboard the
- shuttle to record the 50 hours of data.
-
- The mission will return 32 terabits of data (32 X 1012 bits
- of data) or the equivalent of 20,000 encyclopedia volumes.
- The raw data will be digitally processed into images using JPL's
- advanced digital SAR processor and by processors developed by
- Germany and Italy for the X-SAR data.
-
- Historically, processing SAR data has required a great deal
- of computer time on special purpose computer systems, however,
- SIR-C/X-SAR scientists will benefit from rapid advances in
- computer technology that make it possible to process the images
- with a standard super mini-class computer. Yet even with these
- advances, it will still take five months to produce survey images
- from the large volume of data acquired. Detailed processing will
- take another nine months to complete. Data will be exchanged
- among Italy, Germany and the United States to meet the needs of
- the science investigators.
-
-
- SCIENCE TEAM
-
- An international team of 49 science investigators and three
- associates will conduct the SIR-C/X-SAR experiments. A dozen
- nations are represented, including: Australia, Austria, Brazil,
- Canada, China, England, France, Germany, Italy, Japan, Saudi
- Arabia and the United States. A list of investigators and their
- affiliations is attached.
-
- Dr. Diane Evans of the Jet Propulsion Laboratory is the U.S.
- project scientist. Dr. Herwig Ottl of DFVLR is the German
- project scientist and Dr. Paulo Pampaloni is the Italian project
- scientist.
-
-
- MANAGEMENT
-
- The SIR-C mission is managed by the Jet Propulsion
- Laboratory for NASA's Office of Mission to Planet Earth. Michael
- Sander is the JPL project manager.
-
- X-SAR is managed by the Joint Project Office (JPO) located
- near Bonn, Germany. Dr. Manfred Wahl of DARA is the project
- manager and Dr. Paulo Ammendola of ASI is the deputy project
- manager.
-
- #####
-
- 6-93 MAH