Real Time Signal Processing For FMCW Synthetic Aperture Radar

Frequency-modulated continuous-wave(FMCW) synthetic aperture radar(SAR) is a light-weight,cost-effective,high-resolution imaging radar,which is suitable for small platforms such as unmanned aerial vehicles.Compared with pulsed SAR,there are four problems in FMCW SAR real-time signal processing.Firstly,the continuous antenna motion will induce serious dilation in the received signal and the dilation can cause serious distortions in the reconstructed images.Secondly,dechirp-on-receive technique can be used to reduce the bandwidth of the received signal before analog-to-digital conversion,so the system sampling frequency is low.Thirdly,sweep frequency non-linearity in the transmitted signal deteriorates the range compression seriously.Fourthly,the motion compensation should be easily compatible with the image formation algorithm.A modified frequency scaling(MFS) algorithm is proposed to perform the range processing of side-looking or low squint FMCW SAR signal with the continuous antenna motion.Based on the frequency scaling algorithm,which is suitable for dechirped signal,the MFS algorithm gives the rigorous expression of range-Doppler domain signal.The relation between the phase resulting from the continuous motion and the azimuth frequency is analyzed.The MFS algorithm compensates the continuous antenna motion by introducing a new phase term in range-Doppler domain without extra computation load.Two methods are proposed to perform the range processing of squint FMCW SAR. The first is the extention to the MFS(EMFS) algorithm.According to the negligibility of the residual video phase(RVP) of FMCW SAR,the EMFS algorithm removes the range frequency aliasing by adding a skewing operation and modifying relative phase factors.The second is the chirp transform imaging(CTI) algorithm.Also based on the negligibility of the RVP,the algorithm transforms the problem of range cell migration (RCM) correction to the problem of a non-standard Foruier transformation.The CTI algorithm performs the non-standard Fourier trandformation by a chirp transform with low sampling frequency.The performances of the EMFS algorithm and those of the CTI algorithm are equivalent.The CTI algorithm is more efficency than the EMFS algorithm. A Doppler centroid dependent frequency scaling(DCDFS) algorithm is proposed to perform the range processing of squint FMCW SAR signal with non-linearity.The impact of transmitted signal non-linearity is analyzed.The linearity requirement for two different type phase en,ors in SAR imaging is given.The DCDFS algorithm introduces a factor dependent Doppler centroid to alleviate the impact of the squint angle when the nonlinearity and the RCM are compensated at the same time.At the position,the range frequency aliasing is removed.The algorithm can also be extended to pulsed SAR.A range varying step transform(RVST) algorithm is proposed to perform the azimuth processing of FMCW SAR signal.The algorithm performs the azimuth compressing by using the dechirping method from the characteristic of accomplishing the second-order motion compensation in azimuth time domain.Due to the large variation of the look angle in the small platforms' FMCW SAR,the Doppler centroid can vary several hundred hetz from the near to the far range.The algorithm accommodates the variation of the Doppler centroid with the range distance by using a range-varying phase factor.The nonlinear chirp rate of the azimuth signal is also considered.The complete derivation of aforementioned algorithms is given.The computation load and processing error of the algorithms are analyzed.The algorithms performances are shown by point target simulation results.