Simulation of Pol-SAR Imaging and Data Analysis of Mini-RF Observation From the Lunar Surface

High circular polarization ratio (CPR) characteristics were found in permanently shaded regions (PSRs) near the lunar poles. High CPR was regarded as a water ice index. The compact-polarimetric (CP) miniature radio frequency (Mini-RF) radar transmits left-circularly polarized signals and receives horizontally polarized (
$S_{\text {HL}}$
) and vertically-polarized (
$S_{\text {VL}}$
) echoes from the lunar surface. Statistics of the CPR data show its relations with the relative phase (
$\delta$
) between
$S_{\text {HL}} $
and
$S_{\text {VL}} $
and the degree of polarization (
$m$
) but few interpretations were provided. The average CPR data reach the maximum and minimum at
$\delta =\pm 90^{\circ }$
, respectively. As
$m$
becomes very small, the CPR approaches 1. It has been found that CPR is also affected by surface roughness and incidence angle of radar waves. The CPR is now expressed in CP mode to explain the Mini-RF observation. Full-polarimetric radar echoes and CP parameters of the lunar surface are numerically simulated using the bidirectional analytic ray-tracing method. Single-bounce and multiple-bounce scattering components are included in the simulation. Radar images of the lunar crater are simulated with the digital elevation model (DEM) data. The
$H-\alpha $
decomposition derived from the full-polarimetric simulation is presented to analyze
$\delta $
and
$m$
. Simulated radar images with different surface roughness are analyzed statistically to study the functional dependences of
$\delta $
,
${m}$
, and CPR on incidence angle and roughness. Relationships among
$\delta $
,
$m$
, and CPR are used to analyze the effects of incidence angle, roughness, TiO
₂
, and rock abundance on the scattering components. The CPR,
$m$
, and
$\delta $
of PSR craters of different ages are compared with those of nonpolar craters. The results indicate that the CPR,
$m$
, and
$\delta $
are unlikely to be unambiguous evidence of water ice.