Target Tracking - MATLAB Simulations for Radar Systems Design - page 403

sequentially. For this purpose, a special antenna feed is utilized such that the
four beams are produced using a single pulse, hence the name Ðmonopulse.Ñ
Additionally, monopulse tracking is more accurate and is not susceptible to
lobing anomalies, such as AM jamming and gain inversion ECM. Finally, in
sequential and conical lobing, variations in the radar echoes degrade the track-
ing accuracy; however, this is not a problem for monopulse techniques since a
single pulse is used to produce the error signals. Monopulse tracking radars can
employ both antenna reflectors as well as phased array antennas.

Fig. 9.7 show a typical monopulse antenna pattern. The four beams A, B, C,
and D represent the four conical scan beam positions. Four feeds, mainly
horns, are used to produce the monopulse antenna pattern. Amplitude
monopulse processing requires that the four signals have the same phase and
different amplitudes.

A

D

B

C

Figure 9.7. Monopulse antenna pattern.

A good way to explain the concept of amplitude monopulse technique is to
represent the target echo signal by a circle centered at the antennaÓs tracking
axis, as illustrated by Fig. 9.8a, where the four quadrants represent the four
beams. In this case, the four horns receive an equal amount of energy, which
indicates that the target is located on the antennaÓs tracking axis. However,
when the target is off the tracking axis ( Figs. 9.8b-d) , an imbalance of energy
occurs in the different beams. This imbalance of energy is used to generate an
error signal that drives the servo-control system. Monopulse processing con-
sists of computing a sum

Σ and two difference ∆ (azimuth and elevation)

antenna patterns. Then by dividing a ∆ channel voltage by the Σ channel volt-

age, the angle of the signal can be determined.

The radar continuously compares the amplitudes and phases of all beam
returns to sense the amount of target displacement off the tracking axis. It is
critical that the phases of the four signals be constant in both transmit and
receive modes. For this purpose, either digital networks or microwave compar-
ator circuitry are utilized. Fig. 9.9 shows a block diagram for a typical micro-
wave comparator, where the three receiver channels are declared as the sum
channel, elevation angle difference channel, and azimuth angle difference
channel.