A synthetic aperture radar (SAR) image is a two dimensional representation of the spatially varying radar cross-section of a particular scene on the ground. The image is made from radar echos emitted and received by the SAR which is being flown by a manned or unmanned aircraft or by a spacecraft along a linear track. The image consists of pixels that are located at a specific range from the airplane (cross-track direction) and at a particular azimuthal location (along-track direction). Thus, each pixel has associated with it a radar cross-section, a range value and an azimuthal value. The physical size of the pixel is approximately given by the resolution of the SAR. For LYNX, this value can be as small as 4 inches. The range value of the pixel is essentially determined by the time delay between the emitted and received radar pulse. Non-imaging radars also determine range in this way. What makes SAR unique among radars is hoe it determines the azimuthal value of the pixel. A conventional radar determines azimuthal according to how much of the scene is illuminated by the antenna beam. Thus, in a conventional radar the azimuthal resolution is the size of beam on the ground. The beam will spread out due to diffraction and the amount of this spreading is inversely proportional to the size of the antenna, that is, the antenna aperture. It is possible to create fine resolution imagery with a very large antenna aperture. In SAR a huge antenna aperture is created artificially by continuously storing the received pulses as the plane flies its track. During this synthetic aperture, the radar must keep track of the magnitudes and phases of the received radar pulses and must correct for the aircraft motion during the track. The size of this synthetic aperture can be longer than a kilometer to make a 4 inch image at the ranges that LYNX operates. |
