Principles of Modern Radar - Volume 2 1891121537

17.3 Direct Signal and Multipath/Clutter Cancellation Techniques 763noise floor estimation inaccuracy, a more conservative stop condition can be used bycomparing PNFR | dB PSLR | dB + ε| dB .4. If the PNFR is higher than the PSLR, a constant threshold is applied on the 2D-CCFfrom Step 1. This threshold is set at η dB below the main peak of the 2D-CCF, whereη is a given value in the range [0, PSLR] dB. This allows detection of the strongestpeaks in the Doppler-range plane due to disturbance residuals and strong targets whileavoiding false alarms from sidelobes. Most of the detections associated with disturbanceresiduals usually appear at very short ranges and can be discarded at a postprocessingstage. The detected target positions are stored and added to the final detection resultsin a later stage.5. The Doppler-range bins at the point a detection was declared in Step 4 are used toupdate the cancellation mask, and a new stage of ECA-B is performed over the extendeddisturbance subspace basis. This is likely to include the main contributions still limitingthe detection of the smaller targets. To obtain an effective cancellation of the strongestpeaks, a small area around the corresponding Doppler and range bins is considered forcancellation. The Doppler and range extents can be set adaptively using the resolutionvalues evaluated from Step 2. Subsequently, the resulting 2D-CCF is evaluated and thePNFR value updated.6. Steps 3–5 are repeated until the algorithm stops. As the number of stages increases, thedimension of the mask over which cancellation is performed grows. As a consequence,the PNFR decreases for two reasons. First, the strongest peaks present in the 2D-CCF atthe previous stage have been removed; thus, the actual main peak has a lower absolutevalue. Second, the noise floor level in the output data increases slightly since a thermalnoise contribution is added that comes from the reference signal replicas used forcancellation.Figure 17-10 shows the results obtained for the same data file of Figure 17-7 at thedifferent stages of the ECA-B&S algorithm (using the parameters listed in Table 17-3).Three stages of cancellation are performed, within which the cancellation mask isadaptively extended by adding the bins where a peak in the 2D-CCF was found. Specifically,the reference signal AF PSLR is 24 dB, while the estimated PNFR at the firststage is 35.5 dB. Thus, a constant threshold is applied that detects the strong target (Figure17-10b). At the second stage the estimated PNFR is 32.2 dB (Figure 17-10c), andthe constant threshold applied over the 2-D map results in many detections, probablyrelated to disturbance residuals as they are at relative bistatic ranges smaller than 2 km(Figure 17-10d). After the third stage the PNFR decreases to 26.6 dB (Figure 17-10e),and the detection results obtained after the application of the CFAR threshold show anincreased number of detections with bistatic ranges of up to 150 km (see Figure 17-10f).TABLE 17-3 ECA-B&S ParametersAdopted in Figure 17-10ParameterValueNumber of batches b 10Initial number of DOF K 50ε3dBη5dBP fa (for the CFAR threshold) 10 −4