The principle of radar is accomplished by developing a pulse of microwave energy that is transmitted from the aircraft and is reflected by objects in its path. The reflected pulse is amplified and converted by the receiver for display on the display. The timing unit, or synchronizer, synchronizes all the actions in the set. To this basic unit, improvements are added for special purposes, such as weather avoidance, filtering, and terrain following.
The receiver and transmitter are usually one unit (the R/T) with separate functions that, for this description, are dealt with separately. [Figure 7-1]Radar System Components
The transmitter produces the radio frequency (RF) energy using magnetrons. A magnetron generates radar pulses by bunching electrons using alternately charged grids that the electrons travel past. The spurts of energy are of high power and short duration. The energy is released at intervals (the pulse recurrence rate) determined by the selected operating range.
The generated pulse travels through either coaxial cable or, more frequently, a hollow tube called the wave guide. The wave guide requires pressurization to ensure the maintenance of conditions for proper microwave conduction. The energy passes an electronic switching device that directs outgoing pulses to the antenna and incoming pulses from the antenna to the receiver.
The antenna is a parabolic dish with a protruding wave guide. It is gimbal-mounted to allow rotation of the dish and, in most cases, to allow stabilization of the dish relative to the earth’s surface when the aircraft turns. Rotation of the antenna could be through 360° or in a sector (either variable or preset). The 360° rotation, or scan, is usually for mapping, whereas a sector is used in aircraft with limited space for the antenna or where the intent is to concentrate energy in a small area. The antenna assembly is either permanently locked to the longitudinal axis of the aircraft (boresighted) or only so aligned when stabilization units are inactive. When not caged, the antenna stabilization is accomplished by using gyroservo mechanisms. A sensor system that provides information to a computer keeps the antenna radiation plane parallel to the earth even when the aircraft is in a climb or a bank.
There are two radiation patterns popular in airborne radar design: fan and pencil beams. The fan beam is a wide pattern that distributes the RF energy across the beam in proportion to the distance it must travel. [Figure 7-2] The fan beam is best for general mapping. To concentrate the energy emitted, the pencil beam antenna is used. The pencil beam dish allows scanning for weather or aircraft while eliminating ground clutter. It can be used to put more energy on a section of ground to increase returns.The antenna can be manipulated to aim the emissions through a control that tilts the dish from the horizontal plane. At cruising altitudes, in the mapping mode, it is sufficient to slightly tilt the dish down, but tilt should be constantly adjusted for optimum returns.
After transmission, the reflected energy is directed back to the wave guide where it travels past the switching device that directs the returns to the receiver. The receiver converts the microwave returns to electrical signals that are amplified and sent to a display called the planned position indicator (PPI). The amplification of the returns is controllable through a gain circuit. Depending on the type of return desired on the display, the operator adjusts the receiver gain. Other booster circuits, such as sweep intensity or video gain, are available but operation of the receiver is most important. If adequate receiver amplification of weak returns is not applied, no amount of later stage adjustments put the target on the scope.
The display or scope, offers both range and azimuth information about targets to the operator. This information is relative to the aircraft’s position, which can be referenced at either the center of the scope or offset to the side of the screen. [Figure 7-3] The controls manipulates the display so that returns can be presented on the scope in their correct position relative to the observer. [Figure 7-4]Applying a polarization to the signals going to a display produces the actual presentation of the return. The null return has a predominantly positive charge; therefore, the trace is suppressed. A polarization shift is produced in the current to produce a blooming of the trace corresponding to the strength and position of the received signal.
Range is determined by the travel time of a pulse from and back to the R/T unit. Knowing that RF energy travels at the constant speed of light, range determination is simple. The synchronizer coordinates its display on the display.
At the same instant that the timer triggers the transmitter, it also sends a trigger signal to the indicator. Here, a circuit is actuated that causes the current in the deflection coils to rise at a linear (uniform) rate. The rising current, in turn, causes the spot to be deflected radially outward from the center of the scope. The spot traces a faint line on the scope; this line is called the sweep. If no echo is received, the intensity of the sweep remains uniform throughout its entire length. However, if an echo is returned, it is so applied to the display that it intensifies the spot and momentarily brightens a segment of the sweep relative to the size of the target. Since the sweep is linear and begins with the emission of the transmitted pulse, the point at which the echo brightens the sweep is an indication of the range to the object causing the echo.
The progressive positions of the pulse in space also indicate the corresponding positions of the electron beam as it sweeps across the face of the display. If the radius of the scope represents 40 miles and the return appears at three-quarters of the distance from the center of the scope to its periphery, the target is represented as being about 30 miles away.
In the preceding example, the radar is set for a 40-mile range operation. The sweep circuits operates only for an equivalent time interval so that targets beyond 40 miles do not appear on the scope. The time equivalent to 40 miles of radar range is only 496 microseconds (496 × 10–6 seconds). Thus, 496 microseconds after a pulse is transmitted (plus an additional period of perhaps 100 microseconds to allow the sweep circuits to recover), the radar is ready to transmit the next pulse. The actual pulse repetition rate in this example is about 800 pulses per second. The return, therefore, appears in virtually the same position along the sweep as each successive pulse is transmitted, even though the aircraft and the target are moving at appreciable speeds.
At times, the display does not display targets across the entire range selected on the scope. In these cases, atmospheric refraction and the line of sight (LOS) characteristics of radar energy have affected the effective range of the set. The following formula can determine the radar’s range in these situations where D is distance and h is the aircraft altitude:
D = 1.23 √h
Azimuth measurement is achieved by synchronizing the deflection coil with the antenna. In the basic radar unit, when the antenna is pointed directly off the nose of the aircraft, the deflection coils are aligned to fire the trace at the 12 o’clock position on the scope. As the antenna rotates, the deflection coil moves at the same rate. Relative target presentations are displayed as the sweep rotation is combined with the range display.