Principles of Modern Radar - Volume 2 1891121537

7.3 Doppler Beam Sharpening Extensions 275−500DBS SAR (dB)0−500DBS + Az Dechirp SAR (dB)0−400−5−400−5−300−10−300−10−200−15−200−15Crossrange (m)−1000100−20−25−30Crossrange (m)−1000100−20−25−30200−35200−35300−40300−40400−45400−45500−500−400−300−200−100 0 100 200 300 400 500−50 500−500 −400 −300 −200 −100 0 100 200 300 400 500Downrange (m)Downrange (m)(a)(b)FIGURE 7-13 Simulated imagery for (a) DBS, and (b) DBS-AD.−50Figure 7-13a is a DBS image with scatterer returns exhibiting clear smearing in crossrangedue to significant quadratic phase over the dwell, while Figure 7-13b demonstratesthe benefit of DBS-AD with well-focused point target responses in the image.The quadratic phase function, the conjugate of which is employed by the AD stage, isshown in Figure 7-14. Image color maps to radians of phase, the maximum phase deviationis about 2π. The ordinate corresponds to platform aperture (along-track sample) number,where the total collection length D SAR was about 80 m. The abscissa is frequency numberand covers the 15 MHz waveform bandwidth centered at 9.5 GHz.A subtle point in Figure 7-14 is that the dechirp function does not depend on frequency.Instead, AD is performed using the quadratic phase as determined by the center RFonly. Technically, the chirp function does depend on frequency, as suggested by the wavelengthterm in (7.18). One can show that applying a frequency-dependent phase correction1020Azimuth Dechirp Phase32FIGURE 7-14Quadratic phaseterm used in theazimuth dechirpstage of DBS-AD.30401Position #50607080900−1−210020 40 60 80 100Frequency #−3