一种基于距离向入射角的月表雷达圆极化比影像校正模型

A rectification model for lunar surface circular polarization ratio(CPR) images based on range-dependent incidence angle

搭载在月球侦察轨道器(Lunar Reconnaissance Orbiter,LRO)上的微型无线电频率(Miniature Radio Frequency,Mini-RF)合成孔径雷达(Synthetic Aperture Radar,SAR)观测系统主要通过以固定入射角观测所得的圆极化比(Circular Polarization Ratio,CPR)来研究月球表面的物理性质.然而值得注意的是,在许多Mini-RF CPR影像中都可以发现沿距离向存在着明显的梯度.对于一个具有15 km刈宽的典型Mini-RF CPR影像,CPR沿距离向的系统性增长会达到0.2–0.3,这可能会对雷达影像的地质解译带来误导.更进一步的研究表明,这种系统性的CPR梯度有可能是由原本被认为固定的、但实际上沿距离向变化的局部入射角引起的.为了消除这种由入射角引起的CPR梯度,本文首先提出了三种模型用以拟合CPR与入射角的变化关系,接着再用一种基于沿距离向入射角的校正模型进行影像校正.以月球北极地区Baillaud撞击坑坑底的CPR影像为例,校正结果表明本文所提出的方法在保证原有CPR影像统计特征不变的情况下有效减少了系统性的梯度.这一方法可以为较大区域CPR影像的无缝镶嵌以及更为准确地解译月表的地球物理性质提供帮助.

Miniature Radio Frequency （Mini-RF） system is a synthetic aperture radar （SAR） onboard Lunar Reconnaissance Orbiter （LRO）, with the principle objective to study lunar surface geophysical properties using the observed radar circular polarization ratio （CPR） at a fixed nominal incidence angle. However, it is noteworthy that, in most Mini-RF CPR images there are obvious CPR gradients along the range direction. For a typical S band CPR image with a swath width of 15 km, a systematical increase in CPR is generally between 0.2 and 0.3, and this will result in incorrect interpretations in CPR data deciphering. Further analyses show that CPR gradient is most probably caused by the increased incidence angle along range direction that was thought to be constant previously. To eliminate this incidence-angle-induced CPR gradient, three models were firstly proposed to fit CPR with respect to local incidence angle. Then, an incidence-angle-based CPR rectification model was presented for image correction. Experiment results for a region in the northern floor of Baillaud crater showed that our rectification model can effectively reduce the CPR gradient while retain its statistical characteristics. The proposed model in this study can be used to produce seamless CPR mosaics for large regions and help to better interpret the geophysical properties of the Moon.