Advanced illumination modeling for data analysis and calibration. Application to the Moon

Abstract

We present a new illumination modeling tool, called IllumNG, developed at NASA Goddard Space Flight Center (GSFC). We describe its capabilities to enhance the analysis and calibration of science data collected by planetary missions. We highlight these with examples making use of lunar data, particularly the topographic and radiometric measurements collected by the Lunar Orbiter Laser Altimeter (LOLA) instrument, with applications to radiometric measurements from other LRO instruments as well. The unique features of IllumNG are its accuracy and flexibility to handle multiple types of observers and light sources, and its ability to accurately model both singly-and doubly-scattered radiation to an observer.

New Illumination and Temperature Constraints of Mercury’s Volatile Polar Deposits

“…After the high-resolution DEMs were produced for all eight craters, solar illumination models (e.g., Figure 3(b) ) were made at 125 m pixel −1 following the methodology of Mazarico et al (2011 , 2018 ) for lunar craters. These solar illumination models employed a double-precision ray-tracing method to determine the fraction of a Mercury solar day (176 Earth days) in which Mercury’s polar surface is directly illuminated by any portion of the Sun ( Mazarico et al 2011 , 2018 ). These illumination models treated the Sun as a disk, not a point source, due to the large areal coverage of the Sun in Mercury’s sky ( Mazarico et al 2018 ).…”

“…These solar illumination models employed a double-precision ray-tracing method to determine the fraction of a Mercury solar day (176 Earth days) in which Mercury’s polar surface is directly illuminated by any portion of the Sun ( Mazarico et al 2011 , 2018 ). These illumination models treated the Sun as a disk, not a point source, due to the large areal coverage of the Sun in Mercury’s sky ( Mazarico et al 2018 ). We found that ~100 elements are sufficient, after evaluating several simulations with up to several thousand elements to discretize the solar disk.…”

“…We found that ~100 elements are sufficient, after evaluating several simulations with up to several thousand elements to discretize the solar disk. The crater terrain was input from the local DEMs as an unstructured set of triangular elements at 125 m pixel −1 , and then solar rays could be traced to each point on the model to determine direct illumination throughout a Mercury solar day ( Mazarico et al 2018 ). Lower-resolution DEMs covering the whole polar region were used to capture occlusion from far-field obstacles up to 350 km away.…”

“…The high-resolution DEMs were also used to simulate the MDIS WAC broadband images used in this study. These simulated images used the same approach as the solar illumination models by modeling scattered sunlight into the craters’ shadowed regions by assuming a surface of uniform albedo ( Mazarico et al 2018 ); but unlike the solar illumination model that was run for a full Mercury solar day, the time and conditions of each simulated image were chosen to match each specific WAC broadband image. We assumed a Lambertian surface and phase function ( Lambert 1760 ).…”

Ground and In-Flight Calibration of the OSIRIS-REx Camera Suite

“…For other processing tasks, such as producing filter-specific albedo maps of the surface and calculating the corresponding albedo distribution , the absolute radiometric uncertainty is paramount. To assess the absolute radiometric accuracy, we perform an independent calibration with an illumination model of the Moon from NASA Goddard Space Flight Center (Mazarico et al 2018), based on SELENE (Kaguya) Multiband Imager (MI) global mosaics (Lemelin et al 2019;Ohtake et al 2013). We simulate images of identical observation geometry, as shown in Fig.…”