Introduction to Radar Basics - MATLAB Simulations for Radar Systems Design - page 28

where is the azimuth antenna beamwidth, is the scan time, and is the θ a T sc f r

radar PRF. The number of reflected pulses may also be expressed as

θ a f r
ß scan

n P = ------------ (1.81)

ß scan where is the antenna scan rate in degrees per second. Note that when

using Eq. (1.80), is expressed in radians, while when using Eq. (1.81) it is
expressed in degrees. As an example, consider a radar with an azimuth antenna
beamwidth

θ a

ß scan

, antenna scan rate (antenna scan time,

θ a = 3° = 45° sec ⁄

), and a PRF . Using either Eq.s (1.80) or (1.81)

T sc = 8 sec onds f r = 300 Hz

yields pulses. n P = 20

The process of adding radar returns from many pulses is called radar pulse
integration. Pulse integration can be performed on the quadrature components
prior to the envelope detector. This is called coherent integration or pre-detec-
tion integration. Coherent integration preserves the phase relationship between
the received pulses. Thus a build up in the signal amplitude is achieved. Alter-
natively, pulse integration performed after the envelope detector (where the
phase relation is destroyed) is called non-coherent or post-detection integra-
tion.

Radar designers should exercise caution when utilizing pulse integration for
the following reasons. First, during a scan a given target will not always be
located at the center of the radar beam (i.e., have maximum gain). In fact, dur-
ing a scan a given target will first enter the antenna beam at the 3-dB point,
reach maximum gain, and finally leave the beam at the 3-dB point again. Thus,
the returns do not have the same amplitude even though the target RCS may be
constant and all other factors which may introduce signal loss remain the same.
This is illustrated in Fig. 1.18, and is normally referred to as antenna beam-
shape loss.

a ntenna 3-dB beamwidth

time

Figure 1.18. Pulse returns from a point target using a rotating
(scanning) antenna