How Radar Works


A typical radar system consists of the following components:

(1) a pulse generator that discharges timed pulses of microwave/radio energy
(2) a transmitter
(3) a duplexer
(4) a directional antenna that shapes and focuses each pulse into a stream
(5) returned pulses that the receive antenna picks up and sends to a receiver that converts (and amplifies) them into video signals
(6) a recording device which stores them digitally for later processing and/or produces a realtime analog display on a cathode ray tube (CRT) or drives a moving light spot to record on film.

Each pulse lasts only microseconds (typically there are about 1,500 pulses per second). Pulse length-an important factor along with bandwidth in setting the system resolution-is the distance traveled during the pulse generation. The duplexer separates the outgoing and returned pulses (i.e., eliminates their mutual interferences) by blocking reception during transmission and vice versa. The antenna on a ground system is generally a parabolic dish.

Radar antennas on aircraft are usually mounted on the underside of the platform so as to direct their beam to the side of the airplane in a direction normal to the flight path.** For aircraft, this mode of operation is implied in the acronym SLAR, for Side Looking Airborne Radar. A real aperture SLAR system operates with a long (about 5-6 m) antenna, usually shaped as a section of a cylinder wall. This type produces a beam of noncoherent pulses and uses its length to obtain the desired resolution (related to angular beamwidth) in the azimuthal (flight line) direction. At any instant the transmitted beam propagates outward within a fan-shaped plane, perpendicular to the flight line.

A second type of system, Synthetic Aperture Radar (SAR), is exclusive to moving platforms. It uses an antenna of much smaller physical dimensions, which sends its pulses from different positions as the platform advances, simulating a real aperture by integrating the pulse echos into a composite signal. It is possible through appropriate processing to simulate effective antenna lengths up to 100 m or more. This system depends on the Doppler effect (apparent frequency shift due to the target's or the radar-vehicle's velocity) to determine azimuthal resolution. As coherent pulses transmitted from the radar source reflect from the ground to the advancing platform (aircraft or spacecraft), the target acts as if it were in apparent (relative) motion. This motion results in changing frequencies, which give rise to variations in phase and amplitude in the returned pulses. The radar records these data for later processing by optical (using coherent laser light) or digital correlation methods. The system analyzes the moderated pulses and recombines them to synthesize signals equivalent to those from a narrow-beam, real-aperture system.

What is the main practical advantage over SAR over SLAR?


Let us now consider the beam characteristics of a typical radar system, as well as the nature and interpretation of the signal returns, as displayed on film or a monitor. The following illustration describes this process (from Sabins, 1987):

The upper half of this figure depicts a strip of land surface being scanned by the radar beam. The aircraft moves forward at some altitude above the terrain in an azimuthal direction, while the pulses spread outward in the range (look) direction. Any line-of-sight from the radar to some ground point within the terrain strip defines the slant range to that point. The distance between the aircraft nadir (directly below) line and any ground target point is its ground range. The ground point closest to the aircraft flight trace, at which sensing begins, is the near range limit. The pulsed ground point at the greatest distance normal to the flight path fixes the far range. At the radar antenna, the angle between a horizontal plane (essentially, parallel to a level surface) and a given slant range direction is called the depression angle ß (beta) for any point along that directional line (a mnemonic for that is "lowering your head down from staring forward when depressed"). We refer to the complementary angle (measured from a vertical plane) as the look angle (a good mnemonic is to think of looking up from staring at your feet [vertically downward]).

The incidence angle at any point within the range is the angle between the radar beam direction (of look) and a line perpendicular (normal) to the surface, which can be inclined at any angle (which varies with slope orientation in non-flat topography). The depression angle decreases outward from near to far range. Pulse travel times increase outward between these limits. The duration of a single pulse determines the resolution at a given slant range. This range resolution is effectively the minimum distance between two reflecting points along the azimuthal direction that the radar can identify as separate, at that range. Range resolution gets poorer outward for a specific pulse duration. Thus the resolution increases (gets better) with increasing depression angles (it's optimum, close-in).



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