Estimation and correction of complex target motion effects in inverse synthetic aperture imaging of aircraft

Inverse synthetic aperture radar imaging may be seen as a 3D-to-2D projection of the target. The projection plane is shown to be determined by the effective rotation of the target as seen from the radar. Two images formed from similar radar data are discussed. One is well focused and the other is blurred. A time-frequency analysis applied to the radar data shows that the blurring is caused by changes in the Doppler shifts of the separate scatterers of the target. The results indicate that the changing Doppler shifts may be caused by both rotation rate variation and projection plane changes. A comparison with motion reference data verifies that both effects are present. The time-frequency analysis method is used to form well-focused images of the target, even when the motion is complicated.