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 that alternates the signals involved between transmitted and received
(4) A directional antenna that shapes and focuses each pulse into a stream
(5) The same antenna which picks up returned pulses and sends them to a receiver that determines the time delays between their transmission and return, and converts (and amplifies) them into video signals
(6) A recording device which stores them for later processing
(7) An electronic signal processing system that analyzes the signal and prepares it for display
(8) A realtime analog display on a cathode ray tube (CRT), or a TVlike monitor, or on film exposed by a moving light beam to record the image.
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; the dish can rotate (sweep), commonly over a 360° range, or is fixed and looking outward.
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 radar system (RAR), also know as brute force radar, operates with an antenna that has a discrete physical length. (Aperture is analogous to Field of View.) For a SLAR radar this is 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.
The antennas can be large, although weight especially is a limit to the size put on an airplane. The antenna used in the SIR-C radar system was designed to fit into the cargo bay of its host Space Shuttle. This is the antenna before it was mated to the Shuttle:
So, for many operational setups there is an upper limit to the size of the antenna. But the system spatial resolution is proportional to length of the antenna, thus getting an antenna as long as practical is an objective in designing the radar. A "trick" can increase the effective length, and hence resolution, by simulating a real aperture through electronic means for integrating the pulse echos into a composite signal. This is the approach used by Synthetic Aperture Radar (SAR), which is always associated with moving platforms carrying the system. SAR utilizes both recording and processing techniques to generate a signal that acts as though it has an "apparent" length greater than the antenna itself. At any instant the transmitted beam propagates outward within a fan-shaped plane, perpendicular to the flight line; this amounts to a "broad beam" through which targets components advance. The movement of the antenna, be it on aircraft or spacecraft, is involved in increasing the effective length simulating a real aperture by integrating the pulse echos into a composite signal. Mathematical manipulation, including Fourier Analysis, is needed to handle the pulse stream associated with the forward motion. More information on SAR is found at the Wikipedia website on SAR.
SIR-A, B, C, Radarsat, ERS-1,2, JERS-1, and Envisat are the main examples of space satellites that use SAR.
Another type of radar system is exclusive to conditions in which there is relative motion between platform and target. 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 most know, when a target is moving towards the observer, and emits a signal such as a sound, the frequency continues to rise as the target moves closer - the sound increases in pitch; movement away lowers the frequency. Doppler radar is used in the so-called Radar Guns used by the police to gauge auto speeds based on the rate of frequency shift; in this case the radar beam is continuous rather than pulsed.
For a SLAR system, there is a Doppler component in the returned beam that results in changing frequencies, which give rise to variations in phase and amplitude in the returned pulses. 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. The system analyzes the moderated pulses and recombines them to synthesize signals. The radar records these data for later processing by optical (using coherent laser light) or digital correlation methods; Fast Fourier Transform analysis is also applied. More about Doppler radar is provided by Wikipedia.
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 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; at this distance the pulsed signal has the shortest roundtrip transit time. The pulsed ground point at the greatest distance normal to the flight path fixes the far range; the delay between the instant of pulse release and its return is longest at 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 β (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).
The duration of a single pulse determines the two types of resolution at a given slant range. This range resolution (effectively, the minimum distance between two reflecting points along the look direction at that range at which these may be sensed as separate and distinct) gets poorer outward for a specific pulse duration. The formula for range resolution is:
where τ (Tau, in Greek) is the pulse length (in microseconds), c is the speed of light (3 x 108 m/sec) and β (Beta) is the depression angle. There is a second measure, the azimuth resolution, which at any specific slant range point expresses the minimal size of an object along the direction of the flight path that can be resolved; it too varies with depression angle (i.e., slant distance outward). It is given by:
where S is the slant range (in km), λ (small lambda) is the system wavelength, and D is the effective length of the antenna in centimeters.
Pulse intensities of returned signals within the beam-swept strip are plotted in the lower half of the above figure. The pulses undergo varying degrees of backscattering when they reach an object. A smooth, or specular, surface at low angles to the look direction (subparallel) scatters most of the pulses away from the receiver, so that its image expression is dark. A rough surface, in contrast, scatters the pulse beam over many directions, a fraction of which returns to the radar. The amount of returned signal determines the relative lightness (related to signal amplitude) of the image tone. The quantity of returned energy (as backscatter echos) also depends on the size of the target relative to the signal's wavelength. Objects with dimensions similar to the wavelength appear bright (as though rough), while smaller ones are dark (smooth).
In the above diagram, note first the intensity peak in the tracing associated with the steep slope of the mountain side facing the passing aircraft. At this low incidence angle, a significant part of the transmitted pulses is reflected directly back to the receiver. However, the beam fails to reach (illuminate) the opposing mountain slope (back side) leading to no return (black) from this shadowed area or if the slope is so inclined as to receive some illumination at high incidence the returned signal is weak (dark gray). (For a mountain with some average slope and a given height, the shadow length increases with decreasing depression angle.) The next feature encountered is vegetation, which typically consists of irregular-oriented surfaces, with some leaves facing the radar and others in different positions. These objects in the plant together behave as somewhat rough and diffuse surfaces, scattering the beam but also returning variable signals of intermediate intensities, depending on leaf shape and size, tree shape, continuity of canopy, etc. The metal bridge, with its smooth surfaces, is a strong reflector (buildings, with their edges and corners, also tend to behave that way but the nature of their exterior materials somewhat reduces the returns). The lake, with its smooth surface, functions as a specular reflector to divert most of the signal away from the receiver in this far range position. Smooth surfaces at near range locations will return more of the signal. The behavior of a surface, whether smooth, intermediate, or rough, in terms of the height h of small surface objects, can be determined by a "less than" formula. Peake and Oliver give these criteria: For a surface to be smooth, this applies: h < wavelength/25 x sin β; to be rough: h > wavelength/4.4 x sin β (remember, β is the depression angle).
The signal trace shown in the figure represents a single scan line, which is composed of pixels, each corresponding to a resolution-determined area on the ground. The succession of scan lines produces an image by varying either the light intensities on a display (itself made up of screen-resolution pixels), or the density levels in a film, in proportion to the signal intensities. On either film or conventional black and white monitors, strong intensity peaks show as light tones and weak returned signals are dark.