TABLE OF CONTENTS Efficient target detection in clutter and noise is central to radar operation. Several strategies are used to maximise probability of detection ( P D). The following general summary omits a lot of secondary ifs and buts from the detailed treatments within the previous sections.
At given range, mean echo strength depends on RCS and environmental effects (atmospheric refraction and ducting at long range, multipath interference, atmospheric attenuation).
RCS of ships is difficult to measure and aspect-dependent. It conventionally lies within 5 dB of numerical tonnage, gt, but is less for small, streamlined craft. When large ships and coastlines overflow scanner beamwidth or pulselength, effective RCS increases with range.
Range-dependent multipath interference alternately raises and reduces echo strength of point targets as indirect ray phasing changes and is most severe in calm water.
Extended targets show little multipath. Effective height is usually 
to 
tip height, affecting critical range where echo strength law changes from R -4 to R -8.
Pulse-pulse or scan-scan echo fluctuation as the radar and/or target move in the seaway is approximated by a few Swerling case mathematical models. Large targets usually approximate Case 1, small ships Case 3 and fixed reflectors Case 0 (non-fluctuating).
Case 1 fluctuates slowly enough for a whole scan's echoes to be unusually small, needing exceptionally high mean SNR to assure high P D. Case 3 is better, Case 0 best. Case differences become small when P D ~ 0.3.
