# 10.8 Monopulse Radar

On the basis of information extracted from the echo due to a single pulse, a monopulse radar can track the direction of a target with an angular accuracy equal to a fraction of its antenna beamwidth. To track a target in both elevation and azimuth, a monopulse radar uses an antenna (such as a parabolic dish) with four separate small horns at its focal point (Fig. 10-18). Monopulse systems are of two types. The first is called amplitude-comparison monopulse because the tracking information is extracted from the amplitudes of the echoes received by the four horns, and the second is called phase-comparison monopulse because it relies on the phases of the received signals. We shall limit our present discussion to the amplitude-comparison scheme.

# Figure 10-18 Antenna feeding arrangement for an amplitude-comparison monopulse radar: (a) feed horns and (b) connection to phasing network.

Figure 10 18 Full Alternative Text

Individually, each horn would produce its own beam with the four beams pointing in slightly different directions. Figure 10-19 shows the beams of two adjacent horns. The basic principle of the amplitude-comparison monopulse is to measure the amplitudes of the echo signals received through the two beams and then apply the difference between them to repoint the antenna boresight direction toward the target. Using computer-controlled phase shifters, the phasing network shown in Fig. 10-18 can combine the signal delivered to the four-element horn array by the transmitter or by the echo signals received by them in different ways. Upon transmission, the network excites all four feeds in phase, thereby producing a single main beam called the sum beam. The phasing network uses special microwave devices that allow it to provide the desired functionality during both the transmit and receive modes. Its equivalent functionality is described by the circuits shown in Fig. 10-20. During the receive period, the phasing network uses power dividers, power combiners, and phase shifters to generate three different output channels. One of these is the sum channel, corresponding to adding all four horns in phase, and its radiation pattern is depicted in Fig. 10-21(a). The second channel, called the elevation-difference channel, is obtained by first adding the outputs of the top-right and top-left horns (Fig. 10-20(b)), then adding the outputs of the bottom-right and bottom-left horns, and finally subtracting the second sum from the first. The subtraction process is accomplished by adding a 180° phase shifter in the path of the second sum before adding it to the first sum. The beam pattern of the elevation-difference channel is shown in Fig. 10-21(b). If the observed target is centered between the two elevation beams, the receiver echoes will have the same strength for both beams, thereby producing a zero output from the elevation-difference channel. If it is not, the amplitude of the elevation-difference channel will be proportional to the angular deviation of the target from the boresight direction, and its sign will denote the direction of the deviation. The third channel (not shown in Fig. 10-20) is the azimuth-difference channel, and it is accomplished through a similar process that generates a beam corresponding to the difference between the sum of the two right horns and the sum of the two left horns.

# Figure 10-19 A target observed by two overlapping beams of a monopulse radar.

# Figure 10-20 Functionality of the phasing network in (a) the transmit mode and (b) the receive mode for the elevation-difference channel.

(a) Transmit mode

Figure 10 20-a Full Alternative Text

(b) Receiver mode for elevation

Figure 10 20-b Full Alternative Text

In practice, the output of the difference channel is multiplied by the output of the sum channel to increase the strength of the difference signal and to provide a phase reference for extracting the sign of the angle. This product, called the angle error signal, is displayed in Fig. 10-21(c) as a function of the angle error. The error signal activates a servo-control system to reposition the antenna direction. By applying a similar procedure along the azimuth direction using the product of the azimuth-difference channel and the sum channel, a monopulse radar provides automatic tracking in both directions. The range to the target is obtained by measuring the round-trip delay of the signal.

# Figure 10-21 Monopulse antenna (a) sum pattern, (b) elevation-difference pattern, and (c) angle error signal.

(a) Sum pattern

(b) Elevation-difference pattern

(c) Angle error signal

Concept Question 10-5: How is the PRF related to unambiguous range?

Concept Question 10-6: Explain how the false-alarm probability and the detection probability are related to the noise level of the receiver.

Concept Question 10-7: In terms of the geometry shown in Fig. 10-17, when is the Doppler shift a maximum?

Concept Question 10-8: What is the principle of the monopulse radar?