A high-precision horizon-based illumination modeling method for the lunar surface using pyramidal LOLA data

Abstract

Precise illumination prediction is a necessary preparatory task for exploration activities on the lunar surface, while altimetry data obtained by the Lunar Orbiter Laser Altimeter (LOLA) is a common basis for illumination modeling. In this paper, a new high-precision horizon-based illumination modeling method is proposed. The method owns the following features: (1) pyramidal LOLA data in a unified Cartesian space is used as the basis for modeling, where each layer has a different accuracy and coverage, allowing the laser terrain to be described with an accurate triangular network and several interpolated rasters; (2) a database describing the horizon elevation angles is pre-calculated and stored, enabling high efficiency in the simulation at each time step; and (3) the Sun is treated as a 2-dimensional disc, the horizon is treated as line segments, and the solar disc's visible area is calculated by integration.Several simulations are performed at a site near the lunar south pole to verify the effectiveness and performance of the method. Under the test environment and precise horizon parameters, the average time cost per frame in a single thread is ∼0.0076 s for ∼15.7 k query cells (covering a 2 × 2 km area). These simulations are cross-validated with a Lunar Reconnaissance Orbiter Narrow-Angle Camera image and a previous study. The effects of interpolating the LOLA point clouds to a topographic grid, as well as adopting a coarser horizon, are discussed quantitatively with respect to illumination modeling. Finally, we applied the proposed method to perform a brief analysis of illumination conditions for some regions of interest near the Shackleton and Shoemaker craters.