Spaceborne SAR Terrain Matching Curved Imaging Method and Satellite Demonstration: (II) Time-Varying Azimuth Sampling

The spaceborne synthetic aperture radar (SAR) terrain matching curved imaging is characterized by using a swath that always matches the terrain, requiring the implementation of 2-D beam steering. This leads to complex ambiguity variations, causing the conventional time-varying azimuth sampling design based on iterative optimization to often fail to achieve satisfactory ambiguity performance. In this case, this article proposes a noniterative method of time-varying azimuth sampling design for achieving low ambiguity data acquisition. The main technical contributions are twofold: first, the ambiguity and blockage constraints are analyzed in the “time−pulse repetition frequency (PRF)” domain, providing prior guidance for azimuth sampling design and hence avoiding the iteration. In particular, the ambiguity constraint is indirectly constructed by adopting a threshold, i.e., an upper bound of ambiguity, which is derived by a two-step scaling of the ambiguity. Second, a corresponding PRF selection method is proposed, in which the designed PRF sequence achieves the minimum number of PRF switchings while satisfying the ambiguity and blockage constraints. The effectiveness of the presented approaches is verified via the computer simulations and the real data experiment conducted by the SmartSat-X1 satellite.