Page 124 - Fundamentals of Radar Signal Processing

different aspect angle will decorrelate the RCS, i.e., result in a significantly

different measured value. Figure 2.14 illustrates the ability of frequency agility
to force RCS variations. A 20 scatterer, 5 m by 10 m random target was
observed from a fixed aspect angle of about 54°, making its effective depth
approximately 10sin(54°) = 8.1 m. If the same RF frequency was used for each
pulse, the RCS and thus received power would be exactly the same on each
pulse. However, in this case the RF frequency was increased by 18.5 MHz

[calculated from Eq. (2.63)] from one pulse to the next, starting at 10.0 GHz.
The resulting relative RCS measurements vary by 38 dB, a factor of about 6300.

FIGURE 2.14 Variation in RCS due to frequency agility for a constant viewing
angle. See text for details.

It will be seen in Chap. 6 that in certain cases detection performance is
improved when successive target measurements are uncorrelated. For this
reason, some radars use a technique called frequency agility to force
decorrelation of successive measurements (Ray, 1966). In this process, the
radar frequency is increased by ΔF Hz or more between successive pulses,
where ΔF is given by Eq. (2.63), ensuring that the target echo decorrelates from

one pulse to the next. Once the desired number of uncorrelated measurements is
obtained, the cycle of increasing frequencies is repeated for the next set of
measurements.
Equation (2.63) is based on a highly simplified target model and an
assumption about what constitutes the correlation interval. A different
definition, for example defining the interval by the point at which the correlation
function first drops to 1/2 or 1/e of its peak, would result in a smaller estimate

of the required change in angle or frequency to decorrelate the target. Also,