Intrapulse Continuous Azimuth Frequency Scanning-Based Spotlight SAR

The azimuth extent of imaged scene is relatively small under staring spotlight synthetic aperture radar (SAR) mode. The sliding spotlight SAR mode can enlarge the extent of the azimuth scene, but at the cost of azimuth resolution loss. In this article, an intrapulse azimuth frequency scanning (AFS) spotlight SAR mode is presented. Through AFS, the proposed spotlight SAR mode can significantly enlarge the extent of the azimuth scene without causing any loss in the azimuth resolution. Moreover, the proposed mode can separate the azimuth ambiguities in the range-frequency domain. Therefore, the extent of the azimuth scene can be enlarged without increasing the pulse repetition frequency (PRF). Then, the range swath width can be maintained. However, to overcome the reduction in the effective range-frequency bandwidth caused by frequency scanning, an ultrawideband transmitting signal is required. The signal model of the proposed mode is derived. Based on the analysis of the characteristics of the echo spectrum, the design criteria for this mode are developed, followed by a design example. Finally, an imaging algorithm for this mode is presented. Numerical simulations verify the effectiveness of the proposed mode.