|Subj:||Re: Stuff from JPL|
|Date:||2/3/02 5:32:53 AM Pacific Standard Time|
On Sun, 3 Feb 2002 06:07:14 EST BARDSQUILL@aol.com writes: Have a simple link to information about the system?
by Tony Freeman
Jet Propulsion Laboratory
An imaging radar works very like a flash camera in that it provides its own light to illuminate an area on the ground and take a snapshot picture, but at radio wavelengths. A flash camera sends out a pulse of light (the flash) and records on film the light that is reflected back at it through the camera lens. Instead of a camera lens and film, a radar uses an antenna and digital computer tapes to record its images. In a radar image, one can see only the light that was reflected back towards the radar antenna.
A typical radar (RAdio Detection and Ranging) measures the strength and round-trip time of the microwave signals that are emitted by a radar antenna and reflected off a distant surface or object. The radar antenna alternately transmits and receives pulses at particular microwave wavelengths (in the range 1 cm to 1 m, which corresponds to a frequency range of about 300 MHz to 30 GHz) and polarizations (waves polarized in a single vertical or horizontal plane). For an imaging radar system, about 1500 high- power pulses per second are transmitted toward the target or imaging area, with each pulse having a pulse duration (pulse width) of typically 10-50 microseconds (us). The pulse normally covers a small band of frequencies, centered on the frequency selected for the radar. Typical bandwidths for an imaging radar are in the range 10 to 200 MHz. At the Earth's surface, the energy in the radar pulse is scattered in all directions, with some reflected back toward the antenna. This backscatter returns to the radar as a weaker radar echo and is received by the antenna in a specific polarization (horizontal or vertical, not necessarily the same as the transmitted pulse). These echoes are converted to digital data and passed to a data recorder for later processing and display as an image. Given that the radar pulse travels at the speed of light, it is relatively straightforward to use the measured time for the roundtrip of a particular pulse to calculate the distance or range to the reflecting object. The chosen pulse bandwidth determines the resolution in the range (cross-track) direction. Higher bandwidth means finer resolution in this dimension.
Radar transmits a pulse
GeoSAR: Geographic Synthetic Aperture Radar
By the end of this century, a new tool will be available for goelogists, earthquake researchers, emergency management agencies, and forestry and land use management agencies. An airborne radar system call GeoSAR will generate high-resolution, three-dimensional maps to explore and study California.
GeoSAR is being developed by a consortium consisting of the California Department of Conservation, Calgis Inc., and NASA's Jet Propulsion Laboratory, with funding provided the the Defense Advanced Research Projects Agency (DARPA). The project will develop a dual-frequency airborne radar system that will be able to collect 249 square kilometers (94 square miles) of data a minute.
A special feature of GeoSAR will be its ability to acquire three-dimensional images of the Earth's surface through a technique call interferometry. Because GeoSAR uses radar, the system will be able to operate both day and night, under almost any weather condition. GeoSAR will be the first instrument that will be able to map both above, through, and below the vegetation canopy providing important information such a data about landslides that are overgrown with vegetation. The GeoSAR radar system is a dual frequency design using both P- and X-band wavelengths. The longer P-band wavelength will penetrate deeper into the canopy and, coupled with computer modeling, map beneath the vegetation canopy. When combined with other remote sensing data such as Landsat multi-spectral information, it will be possible to not only determine land cover type such as tree species, but also tree height and perhaps even width, such as crown diameter. Maps created with the GeoSAR data will be used to assess potential goelogic/seismic hazards, such as landslides, classify land cover, map farmlands and urbanization, and manage forest harvests. This system will become operational in early 2000.