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2.2     The Modified RD Algorithm

In the RD algorithm, the inverse transform part of the azimuth compression stage extracts target energy from selected stationary parts of the Doppler spectrum.   In the modified algorithm, the IFFTs are placed so as to follow the specific time-frequency pattern of the burst mode cycles shown in Figure 2.   The IFFT length is chosen so that if one full burst of a target is captured, no other energy of that target is present.   The choice of IFFT length involves an efficiency/oversampling tradeoff, with zero padding and decimation used as appropriate [5].

Returning to the previous example, let DF be the frequency excursion of each target in a burst.   Also, let four short IFFTs, each of duration 2DF, be positioned as in Figure 2.   The IFFTs are denoted by S1 to S4.

In S1, Targets T1 to T5 and T9 to T13 have one contiguous set of Doppler frequencies (one burst bandwidth) within the IFFT.   After the matched filtering and IFFT, these complete targets will be kept as ``good" ones and the other incomplete targets thrown away.   Specifically, the good points to keep are: T1 to T4 and T9 to T12 in S1 and S3, and T5 to T8 and T13 to T16 in S2 and S4.

Note that only two IFFTs are needed to give complete coverage of the ground in this case, for example, the sets S1 and S2, S2 and S3, S3 and S4, or S4 and S1.   The data of the chosen pair is then stitched together to form a final Single Look Complex (SLC) product.   At this point, three possibilities of data utilization are available:

  1. use one set only for an SLC product,
  2. coherently add each set to achieve interferometric phase smoothing, or
  3. detect and add each set to form a look-summed product for non-interferometric use.

The placement of the IFFTs and the IFFT length can be adjusted to accommodate the various RADARSAT and ENVISAT burst arrangements.

The short IFFTs have a time varying band pass filtering effect, but without having to implement this time consuming filter in the time domain.   Also, since the IFFTs capture the full burst energy of each good target in the Doppler spectrum there is no additional radiometric artifact due to the processor.


next up previous
Next: ANALYSIS OF RESULTS Up: ALGORITHM DESCRIPTION
Previous: Spectrum of ScanSAR Targets

ian cumming      ;  Fri Sep 5 16:48:53 PDT 1997