Study Of UWB SAR Image Autofocus Algorithms Based On Real Data

Ultra-WideBand Synthetic Aperture Radar (UWB SAR) enhances radar's information acquisition capability and information awareness ability by obtaining high-resolution imageries. Now, UWB SAR has been used in airborne radar system as a new function of modern radar. But its flight path and motion attitudes are hardly controlled in the influence of airflow during the period of recording data. The Trajectory deviation of the aircraft is the main cause of phase errors in airbone UWB SAR. So, motion compesation is the premise to realize high-resolution iamge. It is one of the key technologies in UWB SAR.The motion compesation based on image data is one of the important steps in the UWB SAR compesation system. In this paper, we emphasize study the autofocus algorithms based on UWB SAR real image data. Several new autofocus algorithms have been proposed, which were better in focusing UWB SAR real image. And quality of UWB SAR images have been improved greartly.The phase gradient autofocus (PGA) algorithm, used in SAR community is proven to be a superior phase error correction method for fine resolution SAR. Based on the traditional PGA algorithm, a modified method—the contrast optimized PGA algorithm was proposed in this paper. The modified method greatly improved the ability of the PGA algorithm for focusing the P band UWB SAR image. In this paper, the introduced course of the new linear phase error in the traditional PGA algorithm has been deducted in detailed. And the reference scatterer coregistration method was proposed to remove the shift distortion (along the azimuth) in the focused image.In this paper, the model of the range-dependent phase error in the UWB SAR that was caused by the platforms off-track was constructed,and we analysize the spatial variant characteristic of the phase error along the range by simulation. Based on the characteristic, the Non-Uniform Segmentation PGA (NUSPGA) Algorithm has been proposed, which can be realized well focusing for the UWB SAR images that have been blurred by serious range-dependent phase error.After the traditional PGA algorithm had been proposed, another autofocus algorithm—stripmap PGA (SPGA) algorithm was proposed for stripmap mode SAR imaging. After studying the SPGA algorithm in real earnest, some modification has been proposed to realization process of the original algorithm. And which can help improving the precision of the estimated azimuthal spatial variant phase error in SPGA algorithm. In this paper, we have proposed to estimate the range spatial invariant factors, by which the range-dependent phase errors will be got. And this make up the shortcoming in the traditional SPGA algorithm, in which it didn't consider the range-dependent phase error. At last, we analyzed the performance of the autofocus algortihtms and modified autofocus algorithms that have been mentioned in this paper from the aspect of focusing ability and applied conditions. So it will help for choosing the right autofocus algorithm to focus the UWB SAR images, and the needed focus result will be got.