RADAR, Synthetic Aperture
Synthetic aperture RADAR (SAR) is used for high-resolution mapping of the ground from moving aircraft or spacecraft. A stationary RADAR system's angular resolution—that is, the clarity with which it can distinguish two small, side-by-side targets at a given distance—is determined by the physical width (aperture) of its antenna. By appropriate processing of the echoes received by a small, but moving antenna, an angular resolution equivalent to that of a much larger antenna can be synthesized—hence the term "synthetic aperture RADAR" for such a system.
SAR exploits the Doppler effect, a property of waves reflected (or emitted) by moving objects. If a wave is reflected or emitted by an object approaching a receiver, its frequency as perceived by the receiver is raised; if the object is receding, its frequency is perceived by the receiver as lowered. Most people have noticed the Doppler effect when a vehicle blowing its horn passes them at high speed; the sound of the horn is high-pitched when the vehicle is approaching, then drops when the vehicle passes by. Basic SAR works as follows: first, a narrow, fan-shaped radar beam is projected at right angles to the forward motion of an aircraft (or other platform). Distant objects cut across this side-looking beam as the aircraft moves in a straight line. As an object first enters the beam, its relative motion has a component that is toward the aircraft and which Doppler-shifts its RADAR reflection to higher frequencies. As the object passes through the centerline of the beam, it ceases to get closer to the aircraft. At this fraction of a second, its reflection ceases to be Doppler shifted. Next, as the object passes through the trailing half of the beam, it begins to move away from the aircraft, which Doppler-shifts its reflection to lower frequencies. Thus, although reflections from all objects at a given distance from the RADAR return to its antenna at the same moment, reflections from objects ahead of the aircraft are Doppler shifted to higher frequencies, and those from objects trailing the aircraft are shifted to lower frequencies. This effect can be used to distinguish objects inside the beam, achieving an angular resolution that is higher than the beam's physical width.
SAR mapping was first demonstrated in 1953 and has since been widely used by the military (with various refinements) for airborne battlefield surveillance. SAR has also been used for satellite-based radar mapping of the Earth and Venus.
█ FURTHER READING:
Edde, Byron. RADAR: Principles, Technology, Applications. Englewood Cliffs, NJ: PTR Prentice Hall, 1993.
Fitch, J. Patrick. Synthetic Aperture RADAR. West Hanover, MA: Springer-Verlag, 2001.