Simulation of radar sounder echo from lunar surface layer and detection of lunar subsurface structure

In lunar exploration, spaceborne high frequency (HF) radar sounder is an effective tool for investigation of lunar subsurface structure in lunar exploration [1] [2]. The primary strategy for radar sounder detection of subsurface structure is through the time delay and intensity difference of the lunar surface and subsurface nadir echoes (Figure 1). It is important to fully understand electromagnetic wave propagation, scattering and attenuation through the lunar media in order to retrieve information of lunar layering structure from the weak subsurface nadir echoes, which in general is interfered by strong off-nadir surface clutters at the same time. Based on Kirchhoff approximation (KA) of rough surface scattering and ray tracing of geometric optics, an effective simulation approach of radar echo from lunar layering structure is developed. According to the lunar surface feature, the topography of mare and highland surface is generated numerically (Figure 2), and the triangulated network is applied to making digital elevation of lunar surface [3]. Then scattering from the lunar surface and subsurface are numerically calculated using KA approach [4] [5]. Radar echoes of mare regions and radar images of highland regions are numerically simulated (Figure 3). The simulation results show: (1). The lower the altitude of radar orbit, the easier to distinguish the subsurface echo. (2). Broadening the bandwidth of the transmission pulse of radar sounder can enhance the range resolution. However, increment of bandwidth requires the increase of the centre frequency of radar sounder, which will reduce the penetration depth of radar wave in lunar subsurface layer. Therefore, selection of bandwidth for radar sounder should be a compromise between the range resolution and penetration depth.