Study Of Coherent Methods On Dechirping Echoes In Wide-Band Tracking Radar System

The general approach to ISAR imaging is Range-Doppler (RD) imaging approach. For this approach, the translational motion compensation (TMC) is firstly obtained by envelope alignment and autofocus, so the target can be treated as a rotating target for the next processing. But in this method, scatterers' migration through resolution cells (MTRC) caused by rotational motion is neglected. However in practice, MTRC exists with the improvement of resolution or for big target. For MTRC compensation, the keystone transformation in SAR is used. Before the keystone transformation, it is demanded that the rawdata is coherent.In theory, the coherent echoes of ISAR can be obtained by coherent receiver. In practice, in order to increase transmit power, and decrease the receiver bandwidth, the radar usually transmits LFM signal, and then dechirps the received signal. For the sampling bandwidth to be reduced, ISAR echoes are often dechirped at the receiver with the reference range varying according to the target position which is measured by the narrow-band radar each pulse. Due to the measuring precision of the narrow-band radar, the echoes are non-coherent after dechirping.The non-coherence can be eliminated by autofocus processing when in high Signal-to-Noise ratio condition. But when the Signal-to-Noise ratio is low, it's impossible to make rawdata coherent. On the other hand, it'll take a lot of time for the autofocus processing and be hard to satisfy the real-time imaging needs. In this paper, we discuss the echoes coherent methods for a varying reference range dechirping ISAR system. We will bring out new solutions from software, hardware and combining software and hardware aspects. These solutions will improve ISAR imaging speed greatly and be of special value in ISAR imaging when Signal-to-Noise ratio becomes low.