Monopulse Radar

a method of using radar to measure the angular coordinates of an object based on determination of the angular error in the location of a radar antenna aimed at the object from a single pulse signal received by the radar installation. The signal may be reflected or reradiated by the object. Monopulse radar is used in monopulse tracking.

As compared with other radar methods using continuous signals or several sequentially received pulse signals, the monopulse method offers the advantage of higher measurement accuracy (reduction of errors to tenths of an angular minute), because monopulse radar is not sensitive to fluctuations in the amplitude of the received signals. However, practical operation of monopulse radar requires a more complicated design of the receiving circuit in the radar station because of the necessity of using several receiving channels. Because of this feature, monopulse radar is also called multichannel radar.

There are two main types of monopulse radar stations; one is based on amplitude comparison of the signals, and the other, on phase comparison. Their operation is based on use of the dependence of the amplitude or phase of the signals (received simultaneously through several channels) on the direction of the incident waves. In the amplitude-comparison method for determining one angular coordinate (see Figure 1) the signals are received through two identical reception channels and the antenna has two radiators displaced from its focus. As a result, the directions of their maximums in the radiation pattern of the antenna cross at an angle. Signals detected by the receivers are fed to an amplitude comparator. The ratio of the amplitudes at its output determines the magnitude and polarity (sign) of the displacement of the direction toward the object as compared with the lobe direction. The resultant signal is called the error signal; it is often used to turn the antenna automatically toward the object.

Figure 1. Diagram of the determination of one angular coordinate using the amplitude-comparison method: (1) reflector of rotating antenna, (2) radiation pattern of second channel (arbitrary representation), (3) radiator of first channel, (4) receiver of first channel, (5) amplitude comparator, (6) output to device for automatic tracking of the object by the antenna, (7) receiver of the second channel, (8) radiator of the second channel, (9) radiation pattern of first channel (arbitrary representation). The solid line shows the direction of the signal coming from the object; the dot-and-dash line shows the lobe direction. The length of the segment AO is directly proportional to the amplitude of the signal in the first channel, and that of the segment BO, to the signal in the receiver of the second channel.

To determine one angular coordinate of the object by the phase comparison method (see Figure 2), a system is used that consists of two antennas separated by a distance l (the base). The signal from the object arrives at the antennas with a phase shift

where λ is the wavelength and θ is the angle between the direction to the object and a direction normal to the base. The received signals are amplified and then fed to a phase meter, which determines the phase difference, characterizing the direction to the object in one plane. To determine two angular coordinates of the object in orthogonal planes by the monopulse method, with either amplitude or phase comparison, it is necessary to use at least three radiation patterns (usually four) and a corresponding number of receiving channels.

Combined phase-amplitude signal comparison is used for angular tracking of an object by determining its angular coordinates in two orthogonal planes with the aid of only two radiation patterns. Information describing the angular position of the object in one plane, such as the azimuthal plane, is obtained by the phase-comparison method with the aid of two separate antennas located side-by-side along a horizontal line. Information on its position in some other plane (the elevation plane) is obtained by the amplitude-comparison method by deflecting the radiation pattern of one antenna upward and that of the other antenna downward. This method is called the mixed amplitude-phase method of comparing signals.

Figure 2. Diagram of determination of one angular coordinate of an object using the phase-comparison method: (1) reflector of first antenna, (2) radiator of first antenna, (3) receiver of first channel, (4) phase meter, (5) output to equipment for automatic tracking of the object by the antenna system, (6) receiver of second channel, (7) reflector of second antenna, (8) radiator of second antenna. The broken line shows the direction of signals coming from the object; the dot-and-dash line shows the direction of the maximums of the antenna radiation pattern.

In practice, monopulse radar installations using phase comparison of the received signals are less common than those using amplitude comparison. This results from the detrimental effect of the side lobes of radiation patterns on the performance of real antennas and from the less efficient use of the total antenna aperture.