2.1    Spectrum of ScanSAR Targets

This subsection examines the target spectrum for burst-mode data, and explains why the basic RD algorithm is not optimal in this case.   Understanding the spectrum characteristics leads to the proposed algorithm.

In this discussion, the burst arrangement of the ENVISAT AP mode is used as an example.   Let T_b be the burst duration as well as the burst separation, and let the beamwidth be 5T_b.   Consider 16 equally-spaced targets that lie in the same range gate after RCMC, denoted by T₁ to T₁₆ in Figure 1.   The frequency excursions of these targets are shown in Figure 2.   It is assumed that the PRF equals the full target bandwidth. Because of the data gaps, the spectrum of each target is segmented.

  

Figure 1: Burst and Target Arrangements

  

Figure 2: Target Spectrum

To use the standard single-look RD algorithm, the data gaps must be zero padded. However, the following problems are encountered:

• The impulse response exhibits a ``spiky" appearance [4] due to the rectangular modulation of the input signal. The modulation is caused by processing across the zero padding, and the phase changes by +/- pi whenever the amplitude crosses zero.
• The range of frequencies covered by each target is different as shown in Figure 2. In the RD algorithm, the same weighted matched filter is applied to all the targets, resulting in each target being weighted differently.

With modifications to the basic RD algorithm, these problems can be solved.