Principles of Modern Radar - Volume 2 1891121537

320 CHAPTER 7 Stripmap SARLet us represent this relaxation of the azimuth null-to-null PRF constraint with anazimuth sampling factor, K AZ , that takes on a value between 1 and 2. The PRF constraintin (7.115) can be writtenThe impact on B SAR is easily found asPRF ≥B SAR = B 0 SAR4vK AZ D az(7.116)( ) 2− 1K AZ(7.117)where BSAR 0 is the achievable Doppler bandwidth for unambiguous sampling of the mainbeam.Reduction of the SAR Doppler bandwidth increases (spoils) stripmap resolution bythe same amount, so that (7.3) may be writtenCR(stripmap) ≥ D ( )az K AZ(7.118)2 2 − K AZIf K AZ equals 1, the full Doppler bandwidth of the mainbeam is available for imagingand the finest stripmap resolution is possible. If K AZ equals 2, the PRF limit is relaxedby a factor of 2 but there is no unaliased bandwidth for SAR processing and cross-rangeresolution is infinite.The PRF restriction in (7.116) establishes a limit on swath depth and area coverage rate.If the PRF is too high, returns from multiple pulses will arrive at the radar simultaneously.These range-ambiguous returns are avoided if the swath depth is such that returns from theup-range edge on the current pulse arrive at the radar after returns from the down-rangeedge on the previous pulse:T Swath ≤ 1PRF(7.119)The swath extent in slant range is cT Swath /2, and this interval is projected onto the ground,L Swath , by the cosine of the local grazing angle, θ g ,L Swath = cT Swath2 cos θ g(7.120)Combining (7.119) and (7.120) establishes a limit on the ground swath depthL Swath ≤c2 cos θ g PRF(7.121)Combining (7.116) and (7.121) yieldsL Swath ≤ 8K AZ( cv)Dazcos θ g(7.122)Deeper swaths are afforded by slower platform speeds and larger antennas.Area coverage rate (ACR) is equal to the swath depth times the velocity of the platformACR = vL Swath (7.123)