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CrossTrack Velocity Error of the Moving TargetIn most practical situations, the crosstrack velocity of the moving target cannot maintain constant during the entire data acquisition T_{a}. If the crosstrack velocity is inconstant, the energy of the moving target will spread on multiple Doppler centroids. The crosstrack velocity error has a significant impact on the Doppler centroid of the moving target. For example, consider a typical Kuband airborne SAR system, in which k is 0.0194 m, R_{0} is 19000 m, and V_{a} is 100 m/s, the Doppler centroid of the moving target can be calculated as
Suppose V_{x} is 10 m/s, then f_{ac} is 1031 Hz. However, if V_{x}(t_{a}) varies from 9 to 11 m/s, f_{dc} changes between 927.9 and 1134.1 Hz, as shown in Fig. 6.8. In other words, in the existence of the crosstrack velocity error, the moving target cannot be focused into a single Doppler centroid, even though the highorder phase errors are compensated. Therefore, the crossvelocity error of the moving target must be compensated in the real data processing. Fig. 6.7 Impact of the platform velocity error on the Doppler centroid of the moving target Fig. 6.8 Doppler centroid of a target with crosstrack velocity error in timefrequency domain 
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