Time Window Selection Algorithm for ISAR Ship Imaging Based on Instantaneous Doppler Frequency Estimations of Multiple Scatterers

Selecting a suitable time window is an important aspect of the ship imaging process for inverse synthetic aperture radar (ISAR) systems. One type of existing algorithm involves finding the time period during which a ship's rotation is relatively stable by extracting the instantaneous Doppler frequency variation caused by the ship's rotation. However, existing algorithms extract either the instantaneous Doppler frequency of only a single scatterer or the average Doppler frequency of the entire ship. A novel time window selection algorithm for ISAR ship imaging is proposed through extraction of the instantaneous Doppler frequencies of multiple scatterers. The instantaneous Doppler frequency of a scatterer caused by a ship's rotation is verified to behave approximately as a linear frequency modulation signal over a short period. The formula for calculating the length of such a short period is derived. A parameter estimation method based on compressed sensing after data extrapolation is presented to extract the centroid and frequency modulation rate for short-time chirp signals. The instantaneous Doppler frequency variations in the multiple scatterers over the entire acquisition period are obtained by associating the data from adjacent short time periods. The suitable time window during which the instantaneous Doppler frequency changes are all relatively stable in the multiple scatterers is then selected. The results of the performed simulation and real data processing verify the effectiveness of the proposed algorithm. The proposed algorithm achieves better image quality than the image-contrast-based algorithm and algorithms based on instantaneous Doppler frequency estimation of a single scatterer.