In document 23724318 Electronic Warfare Fundamentals (Page 50-56)



The most common type of radar design is the pulse radar system. The name describes a process of transmitting discrete bursts of RF energy at the frequency of the radar system. The time that pulses are transmitted determines the pulse repetition frequency (PRF) of the radar system. A pulse radar system can figure out range and azimuth. Range is determined by the time that it takes a pulse to go to a target and return. Target azimuth is determined by the relative position, or antenna orientation, when the pulse strikes the target. Figure 4-1 is a basic block diagram of a simple pulse radar system.

Figure 4-1. Pulse Radar Block Diagram

a. The purpose of the transmitter is to deliver a series of high-energy bursts of radio frequency (RF) energy to the antenna. The transmitter group of a modern pulse radar normally consists of a pulse generator or waveform generator, a modulator, and some kind of power amplifier (Figure 4-2).

(1) The purpose of the waveform generator is to generate the proper waveform or pulse, normally at a low power level, before delivery to the modulator. It is much easier to generate complex waveforms at a lower power level. These complex waveforms are required for coherent systems employing digital moving target indicator techniques and for pulse Doppler radar operations.

Figure 4-2. Transmitter Group

(2) The modulator is a major portion of the transmitter. The modulator provides an extremely powerful, very short pulse of direct current (DC) voltage to the power amplifier. This is similar to the ignition system of an automobile but with very stringent requirements. The modulator has an energy storage device and a switch. Between pulses, during the resting time of the transmitter, energy is accumulated and stored in the storage device. When keyed by the master timer, all this energy is switched to the power amplifier as a pulse. The waveform of this pulse is determined by the waveform generator.

(3) The power amplifier for a modern radar is normally a klystron, traveling wave tube, cross field amplifier, or solid state amplifier. Most common pulse radars use a klystron power amplifier. No matter what power amplifier is used, the purpose of the transmitter group is to produce a series of pulses at the correct amplitude, at the proper interval, with the exact waveform, and at the operating frequency of the radar.

b. A duplexer is required when both the transmitter and receiver use the same antenna. The duplexer acts as a rapid switch to protect the sensitive receiver from damage when the high-power transmitter is on. When the transmitter is off, the duplexer directs the weak target signals to the receiver. The duplexer's main purpose is to minimize power loss and maximize isolation. Power lost in the duplexer during transmission reduces the maximum detection range of the radar.

Isolation refers to the amount of transmitter power that “bleeds through” the duplexer to the receiver during transmission. This “bleed through” must be extremely small to avoid receiver saturation or damage.

c. The capabilities of the receiver group are critical to radar performance (Figure 4-3). The ability of the radar receiver to detect the presence of the target return and extract the required information is limited primarily by noise. Noise can enter the receiver through the antenna along with the target return. This type of noise is called external noise. Noise generated within the receiver is called thermal noise. Radar noise can never be completely eliminated. Minimizing noise

is the most important consideration in the design of the sensitive receivers used with modern radars. In addition, relative immunity to noise makes a radar system more resistant to jamming.

Figure 4-3. Receiver Group

(1) The most common pulse radar receiver is the superheterodyne receiver. A superheterodyne receiver consists of an RF amplifier, a mixer and local intermediate frequency (IF) amplifier, a detector, and a video amplifier.

(2) Radar target returns enter the receiver group via the antenna and duplexer. Since these signals are normally very low power, the RF amplifier boosts the signal gain and filters out as much external noise as possible. The capability of the RF amplifier to minimize noise determines the receiver sensitivity. The boosted RF signal is sent to the mixer where it is converted to a lower IF. This is accomplished by mixing the RF signal with the signal from the local oscillator to produce an IF that is easier to process. The IF amplifier increases the IF signal level and includes a matched filter. The matched filter maximizes the signal-to-noise ratio which enhances detection of the target return.

The detector, which is usually a crystal diode, extracts the video modulation from the IF or converts the IF to a video signal.

d. The brain of a basic pulse radar is the master timer, or synchronizer, which coordinates the operation of the various parts of the radar (Figure 4-4). Exact timing within the radar is necessary to get accurate range. The master timer is an oscillator that triggers the transmitter to initiate transmission of a pulse.

Simultaneously, the master timer sends a signal to initialize the display to ensure that range and azimuth information is accurately displayed.

Figure 4-4. Master Timer/Synchronizer

e. The function of the antenna during transmission is to concentrate the radar energy from the transmitter into a shaped beam that points in the desired direction (Figure 4-5). During reception, or listening time, the function of the antenna is to collect the returning radar energy contained in the echo signals, and deliver these signals to the receiver. Radar antennas are characterized by directive beams that are usually scanned in a recognizable pattern.

Figure 4-5. Parabolic Antenna

f. The purpose of the radar display is to take the information derived from a radar target in the receiver group and present it to the operator in a usable format. There are many different types of scope displays depending on the purpose of the radar and how the radar information is to be used. There are four basic types of radar displays: the A scope, B scope, range height indicator (RHI) scope, and plan position indicator (PPI) scope.

(1) The A scope is used to display target range or velocity (Figure 4-6).

Threat systems using A scope displays include air interceptors (Als) with range-only radar, surface-to-air missiles (SAMs), and radar-directed antiaircraft artillery (AAA) systems. SAM and AAA systems may use the A scope for range or velocity information, and other radar displays for azimuth and elevation data. The A scope displays range or velocity in relation to amplitude. The operator must distinguish the target return from other returns, including ground return and noise.

Figure 4-6. A Scope Display

(2) The B scope is used to display target range and azimuth (Figure 4-7).

Threat systems using B scope displays include Al and SAM systems. The position of the target return to the right or left of the centerline of the screen shows the azimuth of the target. The position of the target return in relation to the bottom of the display, or zero range, shows target range.

Figure 4-7. B Scope Display

(3) The RHI scope is used to display range and elevation (Figure 4-8). The RHl scope is used with height finder radars, and a modified RHI scope is used for ground-controlled approach (GCA) radars. The sweep trace of the display produces a fan-shaped display with the vertex at the lower left of the scope. The antenna sweeps up and down and is synchronized with the display.

Figure 4-8. RHI Scope Display

(4) The PPI display is probably the best known radar display (Figure 4-9).

The display represents a map picture of the area scanned by the radar beam, usually 360 degrees. The PPI display is used by early warning, acquisition, ground-controlled intercept (GCI), and SAM radar systems. The target return's angular position shows target azimuth, while distance from the center of the display shows range.

Figure 4-9. PPI Scope Display

In document 23724318 Electronic Warfare Fundamentals (Page 50-56)