Variable-Resolution SAR Imaging Mode With the Principle of Maximum Mutual Information

A generalized synthetic aperture radar (SAR) modality of operation named variable-resolution (VR) SAR is proposed, which explores the diversity of antenna patterns and inhomogeneity of pulse repetition frequencies (PRFs) for adaptive imaging. Based on the relationship between the azimuth resolution and the corresponding integration angle, it uses dynamic beam patterns along the trajectory to illuminate different regions. We formulate the optimization problem of VR SAR based on the principle of maximum mutual information. First, the information content of a specific scene is defined by modeling its distributed scattering as a stochastic process, and the mutual information between scenes and the observed SAR image can be derived. Then, we construct an optimization problem to maximize the mutual information by solving for the optimal beam manipulation scheme of the VR SAR mode. Further optimization of PRF is conducted to obtain a relatively larger swath width and smaller data volume by compromising the resolution of some regions with less information while ensuring there are no azimuth ambiguities. The potential advantages of VR mode are: 1) it simultaneously provides higher resolution for the high-information regions and a larger imaging area than would otherwise be possible in strip map and spotlight SAR modes and 2) it optimizes the efficiency of data acquisition while extracts as much information from scenes as possible. A mathematical model of VR SAR mode with the principle of maximum mutual information is established, and the feasibility and merits of the method are demonstrated through a series of simulations and an equivalent experiment using RADARSAT-1 raw data.

I. Introduction

Since its inception, synthetic aperture radar (SAR) has proven to be a reliable technology for spaceborne remote sensing of high-resolution global imaging under all weather conditions, day and night. A SAR system consists of a side-looking radar mounted on a moving platform, which forms an equivalent synthetic aperture via motion [1]. To obtain an image of a given region of interest (ROI), the SAR system measures the ROI with pulses transmitted at a set of azimuth sampling positions. The bandwidth of the transmitted pulses dictates the range resolution of the system [2], given by

$$\begin{equation*} \delta _{r}\approx \frac {c}{2B}\tag{1}\end{equation*}$$ where denotes the speed of light. The azimuth resolution is likewise determined by the Doppler bandwidth, which is proportional to the observation integration angle in the azimuth direction, and the relationship between and can be given by $$\begin{equation*} \delta _{a}\approx \frac {\lambda }{4\sin \frac {\bigtriangleup \theta }{2}}\tag{2}\end{equation*}$$ where is the wavelength of the carrier frequency. And the synthetic aperture length is expressed as $$\begin{equation*} L\approx \frac {R_{0}\lambda }{2\delta _{a}}\tag{3}\end{equation*}$$ where is the reference slant range. Note that the integration angle is related to the equivalent synthetic aperture length given the nominal range from radar to target and the nominal azimuth incident angle.