Page 72 - Fundamentals of Radar Signal Processing

compression, clutter filtering, and Doppler processing.

Beamforming is applicable when the radar antenna is an array, i.e., when
there are multiple phase center signals, or channels, available to the signal
processor. Fixed beam-forming is the process of combining the outputs of the
various available phase centers to form a directive gain pattern, similar to that
shown in Fig. 1.6. The high-gain mainlobe and low sidelobes selectively
enhance the echo strength from scatterers in the antenna look direction while

suppressing the echoes from scatterers in other directions, typically clutter. The
sidelobes also provide a measure of suppression of jamming signals so long as
the angle of arrival of the jammer is not within the mainlobe of the antenna. By
proper choice of the weights used to combine the channels, the mainlobe of the
beam can be steered to various look directions, and the tradeoff between the
sidelobe level and the mainlobe width (angular resolution) can be varied.
Adaptive beamforming takes this idea a step further. By examining the

correlation properties of the received data across channels, it is possible to
recognize the presence of jamming and clutter entering the antenna pattern
sidelobes and design a set of weights for combining the channels such that the
antenna not only has a high-gain mainlobe and generally low sidelobes, but also
has a null in the antenna pattern at the angle of arrival of the jammer. Much
greater jammer suppression can be obtained in this way. Similarly, it is also

possible to increase clutter suppression by this technique. Space-time adaptive
filtering (STAP) combines adaptive beamforming in both angle and Doppler for
simultaneous suppression of clutter and jammer interference. Figure 1.20
illustrates interference suppression using STAP, allowing a previously invisible
target signal to be seen and perhaps detected. The two vertical bands in Fig.
1.20a represent jammer energy, which comes from a fixed angle of arrival but is
usually in the form of relatively wideband noise; thus it is present at all Doppler

frequencies observed by the radar. The diagonal band in Fig. 1.20a is due to
ground clutter, for which the Doppler shift depends on the angle from the radar
to the ground patch contributing energy. Figure 1.20b shows that the adaptive
filtering has created nulls along the loci of the jammer and clutter energy,
making the target at 0° angle of arrival and 400 Hz Doppler shift apparent.

Adaptive interference suppression will be introduced in Chap. 9.