TABLE OF CONTENTS We have so far concentrated on a single pulse. Radar receivers (but not racons, Section 12.9) make their final detection decision only after receipt of at least one scan packet of several pulses, affecting detectability in two main ways:
successive echoes always occur at target range but noise spike ranges are random, improving effective SNR;
echo strength may fluctuate from pulse to pulse and/or scan to scan, depending on the Swerling Case of the target, affecting the observation time necessary to obtain a truly representative sample.
Figure 12.27(a) depicts the echoes of the targets of Figure 12.10(a), here considered constant from sweep to sweep (Case 0). Figure 12.27(b) (e) are the noisy narrow-band IF returns from N successive sweeps within a single scan. Although N has been taken as 4 for illustration, in practice the packet more likely contains 6 40 returns depending on prf (hence on range scale) and on scanner azimuth beamwidth and rotation rate. All the sweep returns are the same in general form, random noise making them differ in detail. The P D of each would be generally as Figure 12.10(e) or (f), with low P D or high P FA according to the selected threshold. Waveform (f) shows the sum of the four returns. At (g) this waveform is scaled down by a factor of 4 to give the average return. Comparison shows its echoes are much more apparent than those within any single return,...
