Study On High Squint Imaging Algorithm Of Airborne Synthetic Aperture Radar

Squint airborne synthetic aperture radar(SAR) is a kind of SAR whose antenna points forward or backward and the squint angle can be as much as several tens of degrees. For it has the potential to provide information about surface structure through the measurement of the azimuthal angle dependence of backscatter and could also increase the flexibility with which a desired area on the surface is imaged within a single pass of the platform, squint airborne SAR has important applications in military reconnaissance and target positioning.However, since the special characteristics of the squint mode contain a large linear range cell migration(RCM) and Doppler centroid frequency, the imaging algorithm becomes more complicated with the increase of the squint angle. Although there have been many imaging algorithms proposed for the squint mode SAR, such as range Doppler(RD) algorithm and its modification, chirp scaling(CS) algorithm, etc, yet all these algorithms are only suitable for processing the SAR data when the squint angle is low or moderate due to the serious range-azimuth coupling in high squint mode SAR. Other algorithms like omega-K algorithm, high squint sub aperture(HSS) algorithm, nonlinear chirp scaling(NLCS) algorithm can process the high squint SAR data, but they have their own disadvantages. Omega-K algorithm can be applied to process any squint angle SAR data theoretically, but its complex Stolt interpolation operation limits its practical application. HSS algorithm is a good choice for high squint mode SAR data focusing, but its sub aperture processing increase the complexity of the imaging procedure. Besides, omega-K and NLCS algorithm both need a higher pulse repetition frequency(PRF) to avoid azimuth ambiguities.In this paper, we analysis the geometry and the echo characteristic of the airborne squint mode SAR, and propose a imaging algorithm which can adapt to the high squint case. Due to the large range-azimuth coupling, the spectrum of range-azimuth is titled and a larger(PRF) is needed to avoid azimuth ambiguities. This will increase the amount of calculation and decrease the swath width. Furthermore, the change of PRF will lead to spectrum folding. In this algorithm, a linear range cell migration correction(RCMC) is performed to correct the spectrum and a azimuth demodulation is taken to move the spectrum in the baseband. Finally, we expand the two-dimensional imaging algorithm to three-dimension and give the simulation results.The main work of this dissertation and innovation points are as follows:1. The research of imaging geometry model, data acquisition model and system properties of the airborne squint mode SAR. On the basis of the normal imaging geometry, we consider the difference of the squint angle and the projection of the squint angle on the ground and build a more accurate imaging geometry model. So the acquisition model will be accurate based on the geometry model. Furthermore, we analyze the Doppler properties and the RCM of the squint airborne SAR in detail.2. The research of two-dimensional imaging algorithm of squint airborne SAR. The echo properties of the airborne squint whose antenna pointing direction is not perpendicular to the flight path is not same as the traditional SAR’s, which has a large RCM and Doppler centroid. So the imaging algorithms proposed for the broadside SAR may be not applicable for squint SAR. In the algorithm, a linear RCMC is performed to move the spectrum and a azimuth demodulation is taken to move the spectrum in the baseband, which decrease the PRF and the amount of calculation.3. The research of three-dimensional imaging algorithm of squint airborne SAR. We expand the two-dimensional imaging algorithm and can extract the height information of the target. Here we obtain two SAR images of the interested terrain form two parallel flight tracks, utilizing the geometry to calculate the height of the target. So the accuracy of the single SAR image becomes very important. We consider the third order of the RCM, and the results will be more accurate.