Principles of Modern Radar - Volume 2 1891121537

7.3 Doppler Beam Sharpening Extensions 285Crossrange (m)DBS + Az Dechirp SAR (dB)DBS + AD + RMC SAR (dB)–50–400–5–50–40–30–10–20–15–10–200–2510–3020–3530 –404050–50 –40 –30 –20 –10 0 10 20 30 40 50–45–504050–50 –40 –30 –20 –10 0 10 20 30 40 50Downrange (m)Downrange (m)(a)(b)FIGURE 7-28 Simulated imagery for (a) DBS-AD, and (b) DBS-AD-RMC.Crossrange (m)–30–20–1001020300–5–10–15–20–25–30–35–40–45–507.3.3 Notes on Doppler Beam SharpeningAs capable as the DBS family of techniques may appear, they suffer from a number ofdrawbacks and are rarely employed in high-fidelity imaging. For example, the outputimage is warped because the algorithm operates on circles of slant range and hyperboliciso-Doppler lines. The resulting geometric distortion makes geographic registration ofDBS imagery difficult. In addition, imaging at very fine resolutions over wide scenesgenerates higher-order phase and migration histories the DBS paradigm cannot readilyaccommodate.Worse yet, DBS has fatal flaws for stripmap applications. Stripmap acquisitions aretypically long affairs with many scatterers moving through the mainbeam of the realantenna over an extended tract of terrain. Imagine a fairly benign collection whereinhigher-order phase effects and range migration are negligible so that DBS-AD will sufficefor image formation. We might begin by processing a subset of the data, applying a blockof along-track records to DBS-AD. If the block size is equal to the illumination intervalfor a point target, DBS-AD will focus a return at broadside to full resolution. However,returns at other cross-range locations will have been illuminated over only a fraction ofthe block, so their returns will be focused to a degraded cross-range resolution. Block sizecould be increased, thereby ensuring full resolution over a finite along-track extent, butthe problem comes in the azimuth dechirp stage. Referring to Figure 7-10, dechirp mapsalong-track position to spatial frequency. Larger block sizes include more along-trackreturns and generate higher and lower frequencies after dechirp. However, the data aresampled discretely along-track, so these signals will soon begin to wrap in frequency. Theend product will be highly aliased in cross-range.Finally, the DBS approach is rather ad hoc: as damaging affects appear we tack onadditional stages to fix them. Such a strategy is not sustainable. The next section developsa family of techniques that are much more intellectually pleasing.