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

Abstract

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.