Digital Signal Processing Techniques and Applications in Radar Image Processing

8

RANGE–DOPPLER PROCESSING ON SAR IMAGES

In Chapters 6 and 7, we discussed the basics of radar images and provided a system model for analyzing and processing them. The M × N array of 2D radar image raw data is formed by M bursts of radar pulses with N samples each. The radar signal itself is a pulsed LFM waveform, which can be used to measure the target's range or identify its location through the matched filtering. The phase variation for various pulsed LFM waveforms in the azimuth direction leads to the concept that another LFM type of signal exists in the azimuth direction. This signal is not directly generated from the radar, but derived indirectly from the many LFM pulses sent and received by the radar within the synthetic aperture length. This azimuth-direction-based LFM signal plays a key role in fine-tuning the target function in the azimuth direction, and greatly improves the radar image quality.

Also mentioned previously is that radar image processing can be independently processed in the range and azimuth direction if range migration can be neglected. However, as we described in Section 6.6 (of Chapter 6), the range migration could become severe for radar with large synthetic aperture length (or wide antenna beamwidth), or when the radar beam has a nonzero squint angle toward the target. Therefore, the range cell migration correction is needed to decouple the azimuth process from that of the range.

The SAR image processing therefore can be summarized as three major ...