Lunar Surface Topographical and Illumination Analyses Performed at GRC

Supporting the Design and Operational Planning of Power, Communications, and Thermal Subsystems in Lunar Surface-Based Landers, Rovers and Bases

Tools have been developed at NASA’s Glenn Research Center (GRC) to analyze lunar digital elevation models (DEMs) to enable the characterization of lunar polar illumination for power and thermal system design.  These tools have been modified to assess communications antenna visibilities of Earth stations and have been used in human activity operations planning.

The lunar poles are considered highly attractive sites for unmanned and manned space activity.  Apart from scientific research and anticipated in situ material resources, a critical reason for deployment at the lunar poles is the favorable thermal and solar energy conditions at specific highly illuminated locations.  Usually, spacecraft deployed to lunar sites require power provided by solar power generation and energy storage, using either batteries or fuel cell type systems. Most unobstructed locations on the Moon’s surface have about 15 days of illumination followed by 15 days of darkness.  However, near the poles, it is possible to have greater than 15 days of illumination with a subdivided illumination-darkness period due to the combination of shadowing caused by high terrain and the Sun being very close to the horizon over extended time periods.  A higher number of subdivisions reduces the maximum shadow time – the worse case design point.  This results in lower deployed power system mass, especially for the usually heavy energy storage.  Typical lunar polar illumination-darkness cycling appears somewhat random throughout the year, with illumination being nearly continuous during a number of months and highly variable for the remaining months.  This is quite different from the 92-minute orbit period of the International Space Station or Space Shuttle, which have a gradually changing, non-subdivided period of darkness lasting from 0 to 36 minutes.

Representative of the analyses performed so far at GRC are those documented for the Lunar South Pole region (Ref. 1).  Image 1 shows a composite overlay of all available Clementine spacecraft images of a region near the Lunar South Pole for a month close to the worse case illumination period.  The large crater in the center is Crater Shackleton on whose rim candidate lunar base sites have been proposed (Sites A1-A3).

Image 1: Lunar South Pole Sites

This animation (size is 14.5 MB) is the sequential depiction of these overlaid Clementine images.  There are about 12 hours between images.  Most images are from the first month of Clementine spacecraft's imaging while a few missing images were replaced with analogous ones from the following month.  These second month images were replaced to enhance the continuity of the animation and are indicated by the image revolution number appearing out of sequence.  Some dark animation frames are due to complete shadowing of the area by the horizon terrain.  The arrow shows the direction of the Sun's rays.  In order to create the overlays, a complex, individualized series of image adjustments/alignments had to be performed.  The overlays and animation were performed to support validation of the illumination analysis tools as well as the digital elevation models.

Elsewhere on the Internet, animations are presented that show images from the second month of operation (http://www.psrd.hawaii.edu/WebImg/shadowmovie1.gif ).  However, because there is more illumination during that month and lower resolution due to the increased spacecraft distance, it is of less interest than the time period closest to the worse case illumination (the month prior to the first month of the spacecraft polar imaging operation).  Currently, Clementine has the only publicly available image database.  The SMART-1 image database of images taken from 2005 to 2006 has not been released yet.

The GRC illumination paper (Ref. 1) contains technical illumination assessments directly suitable for use by power, thermal, and communication system designers for various sites near the Lunar South Pole.  These analyses were performed using digital elevation models.  These models (Ref. 2, 3) were created using Earth-based radar which reflected radar beams off the lunar surface to measure the heights of terrain.  Other more recent and higher resolution, Earth-based radar data has been collected by JPL, but DEMs are still being developed.  The lunar orbiting spacecraft, Clementine, has obtained laser profiles of the lunar surface, except near the poles.  Other spacecraft, which will have the potential to provide enhanced lunar terrain profile data, include Japan's Selene, the United States’ Lunar Reconnaissance Orbiter (LRO), and India’s Chandrayaan-1.  Selene, which was launched this year, is equipped with a laser ranger and a stereo camera.  The LRO uses a laser altimeter while Chandrayaan-1 uses a terrain mapping radar.  Both are scheduled to launch in 2008. When data becomes available from any of these spacecraft, it can be utilized with the GRC illumination/topographical software for enhanced analysis and predictions.

To support mission planners and designers, animators of the lunar polar area have made generalizations of the lunar horizon and terrain.  By using DEMs, it is possible to get a better impression of how the terrain appears.  The lunar surface, as viewed from a prospective Shackleton Crater Rim lunar base site (Site A1 in the above image), was animated (size is 7.65 MB) using the radar DEM.  The animation was generated using the ray-traced shadow casting feature of the 3ds Max® modeling software with 3.5 million polygons, 10 million vertexes, and 2.7 GB memory.

The view has one-hour time steps and covers the entire lunar day (i.e. one Earth month).  The dots are the Sun's position for the worse month and the best month, based on the Moon’s axis tilt relative to it’s solar orbital plane.  Illuminations/shadows are based on the best case sun position since the lower Sun position would result in mostly dark images.  The animation begins pointing at the 0 degrees longitude direction which is toward Malapert Mountain (and toward Earth).  The camera is level with the horizon as it rotates around tracking the Sun's azimuth and the camera site location was 50 meters above the Site A1 DEM surface.  

The terrain was purposely not smoothed to show the need for higher resolution data for nearby terrain, which is currently limited to an average height over 600 meter by 600 meter surfaces.  Interpolations between height points, while aesthetically more pleasing, are not valid since almost any shape may exist within each surface element.  The value of this dataset and animation is that it shows fairly well-defined, distant horizon terrain, even at the relatively low resolution of the DEM. Better height resolution or smaller pixel sizes would not be significantly beneficial for distant terrain.  However, nearby height surfaces can play a critical role in shadowing/illumination since small variations in height of very close terrain can easily block the sunlight or conversely may raise the site out of the shadow.  Also, decreasing the area of height surfaces for nearby shadowing terrain can reduce the duration of shadows into smaller intervals, rather than be forced to assume that an entire area is one average height blocking sunlight for a longer duration. 

Analysis of the radar DEM (as shown in Ref. 1) has shown some erroneous height surfaces, which were not removed from the animation because it was desired to show the DEM data in its original state.  The erroneous points were identified by comparing the overlay images with the detailed illumination analysis results. When a potential site appeared illuminated using DEM analysis, but the overlay Clementine imagery did not show such illumination, the DEM data point was considered erroneous.  In the animation, these points appear as extremely steep, oddly jagged terrain points in relation to adjacent terrain.  The raw higher resolution DEM dataset was averaged by its author (Ref. 2, 3) to remove these kinds of erroneous points (due to data processing), which apparently was not entirely successful. Also, inherent DEM height data errors of +-50 m were not included in the animation since it was desired to show the DEM data in its original state.  These types of errors are based on the instrument design which broadcasted and received the radar data and assumes perfectly flat surfaces at a lunar distance.  Finally, note that some regions of the Lunar South Pole are not visible from Earth (i.e. behind lunar mountains, in deep craters) and since they are missing in the DEM, it is assumed they are very low terrain in the animation and, thus, appear as completely black regions. 

Future Work

Future lunar illumination activities will include the following:

1) Acquire/utilize higher resolution radar DEMs
2) Acquire/analyze SMART-1 imagery of the lunar polar regions for the entire year, especially for the worse case lunar day
3) Acquire/analyze/utilize any relevant polar imagery/topographical data from future missions
4) Merge Earth-based radar DEMs with stereo-imagery derived DEMs to address missing height data regions
5) Analyze the northern lunar polar regions
6) Perform a more detailed validation assessment of DEMs using imagery to eliminate critical erroneous height points
7) Utilize topographical and illumination data for power beaming using reflected light
8) Assess illumination implications for high latitude rover paths through valleys
9) Generate worse case average illumination and energy storage maps for each lunar pole

Conclusions

This webpage has summarized some of the work being done at NASA GRC in the lunar topographical and illumination area in support of Constellation activities.  The GRC technical paper and animations documented here are a unique resource for lunar mission planners and designers to assist in understanding the lunar environment as well as to obtain detailed data to help better quantify proposed designs/operations.

References

1) Fincannon, J, “Lunar South Pole Illumination: Review, Reassessment, and Power System Implications”, 5th International Energy Conversion Engineering Conference and Exhibit,
Jun. 2007, AIAA 2007-4700.

2) Margot, J. L., Campbell, D. B., and Slade, M. A., “Digital Elevation Models of the Moon From Earth-Based Radar Interferometry”, IEEE Transactions on Geoscience and Remote Sensing, Vol. 38, No. 2, Mar. 2000, pp. 1122-1133.

3) Margot, J. L., Campbell, D. B., Jurgens, R. F., and Slade, M. A., “Topography of the Lunar Poles from Radar Interferometry: A Survey of Cold Trap Locations”, Science, Vol. 284, Issue 5420, Jun. 1999, pp. 1658-1660.