SAR Introduction
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Introduction to SAR
SAR (synthetic aperture
radar) is a type of instrument placed on board various satellites and
aircraft for various purposes. SAR can be used for mapping and observing
different phenomena, both natural and unnatural. The instrument simply
emits radiation at a specific wavelength, and measures how much radiation
is received. The image made up of reflectivity data, can then be analyzed
for specific purposes. Different bands of radiation are more appropriate
for measuring different phenomena. The most common bands of radiation
used by SAR devices are the L and C bands.
How Radar works
Radar is an active imaging microwave sensor. Active means that it is sending out microwaves (waves with a wavelength between 1 cm and 1 m) and determining how much is reflected rather than measuring the amount of radiation the target is emitting. Microwaves are used because their long wavelength allows them to pass through cloud cover. Imaging means that the radar device is trying to create an image of the target area. Radar devices send out a pulse of radiation and wait to see when the pulse gets back and how intense the reflected pulse is. By doing this on a larger scale, you can create an image of the area you send pulses at.
History of SAR
The first satellite with a SAR was the Seasat, deployed by the U.S. in June 1978. Although it was only operational for 3 months, the L band SAR onboard Seasat demonstrated the usefulness of SAR in viewing various processes on the Earth. In the early '80s, two shuttle missions were launched (SIR-A and SIR-B) which used an L band SAR. These shuttle missions demonstrated SAR's usefulness in mapping of the Earth's surface. More recently, in 1991 the European Space Agency (ESA) launched the European Environmental Remote Sensing Satellite (ERS-1). It uses a single frequency and has a fixed angle of incidence, meaning radiation always hits the Earth at a specific angle, in this case 23 degrees. The Japanese have experimented with SAR technology, launching the JERS-1 in 1992. The JERS-1 contains an L band SAR, but it's transmitter no longer functions properly. The Russian Space Program had a SAR satellite called Almaz, launched in 1987. The program was kept mostly secret, but Russia apparently has plans to do a follow up program. In 1995, NASA launched the Canadian Radarsat satellite. Radarsat contains a C band SAR which can take images with ranges of resolutions, incidence angles, and swath widths.At JHU/APL, we mostly use data acquired in the ScanSAR Beam Mode of the SAR on Canada's Radarsat, which provides fairly high resolution data over a wide swath of ocean.
How Radarsat's SAR Works
The C band radiation that the Radarsat SAR uses is the most sensitive to changes in wind speed. When the SAR passes over a body of water it measures surface roughness, that is the roughness of the surface of a body of water. When the body of water is rough, it reflects more of the SAR's radiation. The idea is that when the wind blows, it makes the sea surface a rougher, meaning it reflects more radiation. The more wind there is, the more the surface reflects. Basically, by knowing how much the surface is reflecting and by knowing what direction the wind is blowing in (provided by climate models), the windspeed can be determined.
The scientists at APL convert surface roughness measurements to wind speed measurements using a computer algorithm called CMOD4.