Lunar surface navigation is an important area of research that needs to address existing capabilities. Decisions need to be made in regards to how the current communication and tracking infrastructure will be used for lunar surface navigation. One idea that was investigated at the NASA Glenn Research Center was the use of the Deep Space Network (DSN), geostationary orbit satellites (GEO), and combinations thereof (DSNGEO).
The Earth-Based Lunar Surface Navigation Analysis characterized the performance of various types of Earth-based tracking assets. Ground stations located at the DSN locations were analyzed with multiple minimum-elevation-angle requirements. Satellites located in GEO at the Tracking and Data Relay Satellite System (TDRSS) orbit locations were analyzed with various nadir and zenith beam-width patterns. Finally, combinations of the GEO satellite assets with the DSN ground stations were analyzed.
Two forms of navigation were investigated: one-way and two-way navigation. One-way navigation systems use signals that are only broadcast in one direction. The user receives broadcast pseudorange signals from satellites and determines their own position and time bias on the basis of pseudorange and range-rate measurements. Two-way navigation systems use systems that are transponded back to the initiator. The user receives ranging signal measurements from the initiator and determines their own position on the basis of range and range-rate measurements.
Two metrics used to characterize the navigation performance of the Earth assets were system availability (SA) and system latency (SL), both of which utilize the Dilution of Precision (DoP) technique. DoP is a high-level geometrical analysis of the view angles from the user to the satellites in view, which is used to determine the effect of geometrical spacing on solution error. This can take several forms depending on the state variables being analyzed. One-way navigation methods, called the Geometrical DoP (GDoP), require solving for a topocentric Cartesian position and time. In comparison, two-way navigation methods, called Positional DoP (PDoP), only solve for a topocentric Cartesian position.
The SA and SL metrics compare the DoP results with a threshold. In the SA analysis, the DoP results, with a given integration latency, were compared with a threshold of 10, and the number of instances where the DoP was less than the threshold was weighted spatially to determine the overall SA. In the SL analysis, integration latency was increased until the SA was 90 percent of the DoP results, from which the integration latency was spatially weighted to determine the overall SL. These metrics were analyzed with a 5° minimum elevation angle from the lunar surface points. Various dynamic integration time periods were used, ranging from 15 min to 12 hr.
SA results for Earth assets for an integration period of 1 hr.
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Earth-based assets of any type can work well if long solution integration periods are acceptable, but only in the Apollo landing zone region on the lunar surface. Earth-based assets do not perform well for regions such as the lunar south pole or far side, and are not recommended for these regions. The study concluded that the system that worked best was the GEO constellation with the largest view angles (GEO 150/150). See reference 1 for an evaluation of lunar-based navigation.
SA results for Earth assets for an integration period of 12 hr.
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SL results for Earth assets for an integration period of 1 hr.
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The Earth-Based Lunar Surface Navigation Analysis effort is managed under the Space Communications and Data Systems Project at Glenn. The work was performed in-house by members of the Communications Systems Integration Branch in Glenn’s Communications Technology Division.
Find out more about the research of Glenn’s Communications Technology Division: http://ctd.grc.nasa.gov
Glenn contacts:
Bryan W. Welch, 216-433-3390, Bryan.W.Welch@nasa.gov
Dr. O. Scott Sands, 216-433-2607, Obed.S.Sands@nasa.gov
Joseph W. Connolly, 216-433-8728, Joseph.W.Connolly@nasa.gov
Author:
Bryan W. Welch
Headquarters program office:
Space Communications Technology Program
Programs/projects:
Space Communications and Data Systems Project, Space Communication Architecture Working Group
Last updated: December 14, 2007
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