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GMTI and Parameter Estimation Algorithm Based on Multiple RevisitsThe alongtrack velocity of a moving target mainly causes the azimuth secondorder phase error, and the impact can be neglected in DBS imaging. Therefore, the crosstrack velocity is analyzed in the GMTI of mechanic scanning mode. The geometry of a moving target in mechanic scanning mode is shown in Fig. 4.5. Fig. 4.5 Geometry of a moving target in the mechanic scanning mode In Fig. 4.5, the crosstrack velocity of moving target P is V_{r}. After azimuth time t_{a}, the platform flies from O to O_{1}, while the target moves to P_{1}. The instantaneous slant range R(t_{a}) can be expressed as
After Taylor expansion, R(t_{a}) can be approximated as
Substitute Eq. 4.4.2 into Eq. 4.2.8, the Doppler centroid of a moving target can be expressed as Compared with Eq. 4.2.1, in the existence of V_{r}, the Doppler centroid of a moving target is shifted from that of a stationary target. The shift will result in the dislocation of a moving target in the azimuth direction in the DBS image. Therefore, the moving target can be indicated by using the Doppler filtering algorithm. The specific processing steps are as follows. Step 1: Perform DBS imaging of the raw data to obtain the false image; Step 2: Estimate the Doppler centroid of the clutter in the azimuthfrequency domain, and derive the LOS of the antenna; Step 3: Eliminate the spectrum of the clutter by using the bandpass filter; Step 4: Perform CFAR detection of the image to detect the moving targets. The four steps above are used in the images of each frame. In order to increase the possibility of detection and decrease the false alarm rate, detection results from each frame can be correlated to further judge the existence of the moving targets. After the indication of the moving targets, the motion parameters are estimated by using the multiple revisits. The specific processing steps are as follows. Step 1: Perform GMTI of each DBS image to locate each moving target; Step 2: Read the scanning angle from the subsidiary data to obtain two DBS images of neighboring visit; Step 3: Perform image registration of the two images. There are mainly two methods: first, operate crosscorrelation of the two images to search for the peak; second, use the feature point match method; Step 4: Calculate the location change AN_{r} of the target in two images. Suppose the range sampling interval is p_{r}, the time interval of the imaging of the two images is At, the crosstrack velocity can be estimated as
Step 5: By using the estimated V_{r}, the Doppler centroid shift Af_{dc} can be calculated according to Eq. 4.4.3. Suppose coherent pulses is M, the azimuth dislocation AN_{a }can be calculated as
Step 6: Relocate the target into its actual location. If there are multiple targets, repeat these steps. 
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