Effects of the "Stop-and-Go" Approximation on the Lunar-Based SAR Imaging

The Lunar-Based SAR (LBSAR) has plenty of advantages, for example, large-scale mapping, high temporal resolution, and long-period operation, for earth's observation. However, because of the extremely high orbit, there is a round-trip propagation time delay on the order of several seconds for the LBSAR signal. Consequently, the conventional "stop-and-go" approximation used for the synthetic aperture radar (SAR) signal modeling is no longer applicable in the LBSAR. Generally, such an approximation raises two groups of effects. One is the effect of the antenna displacement during the pulse duration, which gives rise to the center frequency shift and the frequency modulation (FM) rate variation in the chirp signal. The other one is correlated with the range history and manifests as the Doppler error. These two effects might, respectively, give rise to image distortions in LBSAR imaging. This letter quantitatively analyzes the effects of the "stop-and-go" approximation on the LBSAR imaging performance. Theoretical analysis shows that the impact of the antenna displacement during pulse duration contributes little to LBSAR imaging, and thus it can be reasonably ignored in signal processing. In contrast, the azimuth imaging is sensitive to the Doppler error owing to the "stop-and-go" approximation. Under this effect, there is a severe position deviation in the azimuth direction, although the focusing quality is almost uninfluenced. To compensate for the Doppler error adequately, an effective method is proposed to retrieve the range history of the LBSAR with "nonstop-and-go" configuration. Finally, point target responses are simulated for verifying the theoretical analysis and proposed method.