Moving target indication enhancement in FMCW SAR using deramped randomized stepped-frequency signals

A new technique to enhance the indication of moving targets is proposed, which makes use of randomized Stepped Frequency Continuous Wave (SFCW) modulations, where the frequency order is changed from pulse to pulse. This kind of signals gives the possibility to discriminate target responses whose Doppler spectrum is folded back into the clutter region. The discrimination is performed directly during the range compression, so avoiding spectrum aliasing in the Doppler domain. For the range compression, the deramping technique, commonly used with linear SFCW, is extended to the case of non linear stepped modulations. In this way, the sampling constraints can be relaxed also when using randomized SFCW signals, allowing the same sampling frequency as with deramped linear SFCW, and reducing therefore the system complexity. Compared to linear modulations, randomized SFCW signals give also the advantage to suppress range ambiguities and therefore the capability to look at further range.