Study On Continuous Wave SAR Imaging Exploiting Doppler Resolution

The traditional synthetic aperture radar(SAR) uses wideband pulsed waveforms. In order to meet the increasing demands of miniaturization, light-weight and low power consumption of the SAR platform, a SAR system using frequency modulated continuous waves(FMCW) arises. Both of these two SAR systems depend on the high range resolution provided by wideband waveforms. This thesis focuses on studying a novel continuous wave(CW) SAR imaging method. The new method is not dependent on the bandwidth of the transmitted waveforms. Instead, it exploits the high Doppler resolution of CW signals. Consequently, the method studied in this thesis is suitable for narrowband CW SAR. Besides, it also has good applications in the field of passive radar imaging with illuminators of opportunity.In Chapter 1, the history CW SAR development is reviewed. Several traditional CW SAR imaging methods are introduced. At the end of the chapter, the research background and the main content of this thesis are prsented.In Chapter 2, a general received signal model is presented based on the scalar wave equation. Then, the data model/forward model for imaging is formed, which can utilize the Doppler resolution. The characteristics of this data model are also analyzed. Afterwards, a filtered-backprojection imaging method is presented using this data model. By analyzing the Point Spread Function(PSF), the nature of this Doppler resolution based imaging approach is revealed, and the parameters that influence the image reconstruction quality are specified. When separately increasing the frequency of the transmitted waveform, signal integrated time and the movement speed of SAR platforms, the image resolution is improved. When increasing the distance between SAR platforms and targets, the image resolution is decreased. Finally, the comparision between the new method and the traditionl SAR imaging method are presented from the perspectives of imaging look-direction and image resolution.In Chapter 3, numerical simulations are conducted to verify the performance of the CW SAR imaging method exploiting Doppler resolution. In these simulations, a single frequency CW signal is used as the transmitted signal. Under a variety of platform configurations, both a point target imaging and a region imaging are considered. For a given configuration, the point target imaging simulation is performed with different parameters. Finally, the analysis of the results is presented.In Chapter 4, the imaging method exploiting Doppler resolution is applied to FMCW SAR. First, the FMCW SAR imaging algorithm is presented. Then, in the monostatic configuration, FMCW signals with different bandwidths are used to perform simulations. The results show that the Doppler based imaging algorithm is not restricted by signal bandwidth. When a narrowband signal is used, the presented FMCW imaging algorithm can also obtain good range resolution.In Chapter 5, Global Positioning System(GPS) signals are used as the tranmitted signals, in order to discuss the performance of the Doppler resolution based imaging method in the field of psssive SAR imaging using illuminators of opportunity. First, the ambiguity function of GPS signals are analyzed to obtain its delay and Doppler resolution property.Then, in the configuration where GPS satellite is used as a transmitter and an airborne radar is used as a receiver, the imaging simulations are conducted. The results demonstrate that the imaging method exploiting Doppler resolution is suitable for the passive SAR area.Chapter 6 summerizes the work of the thesis, and points out the issues that need to be further studied.