The Apollo missions were limited to lunar equatorial regions that were in continuous view of the Earth because they utilized Earth-based assets for navigation. Lunar surface navigation is now a pressing issue with the prospect of astronauts exploring the lunar poles, where direct line-of-sight to the Earth is poor. However, if a "GPS-like"1 constellation was deployed at the Moon, then the communication and navigation requirements for continuous global coverage would be met.
In fiscal year 2005, a lunar radiometric navigation analysis at the NASA Glenn Research Center characterized the performance of several types of satellite constellations that could be placed around the Moon. The constellations were designed to implement continuous communication and navigation over the entire lunar surface. Therefore, users could obtain communication access and navigation position information at any location. Radiometric navigation provides users with the ability to obtain position information by processing navigation data provided within the communications channel. Constellations ranging from 5 to 12 satellites were analyzed in several different orbit formations (inclined Walker, inclined elliptical, and polar). All the constellations provided continuous coverage of at least one satellite, whereas the 12-satellite constellation provided continuous twofold coverage.
Illustration of polar 6/2/1 SMA9250.2 Satellite numbers represent various planes, and satellite letters represent different satellites.
Long description of figure 1.
A GPS-like solution could take two different forms: one-way and two-way navigation. The current GPS system is considered to be a one-way navigation system because signals are broadcast in only one direction. Two-way navigation would eliminate the need to solve for time because the time-stamped signal would be retransmitted back to the source for processing. Consequently, only three satellites would need to be in view for a user to obtain a position fix. In contrast, one-way solutions require four satellites to be in view.
The metric used to characterize the navigation performance of the constellations was Dilution of Precision (DOP). DOP is a high-level geometrical analysis of the view angles from the user to the satellites that are in view for the effect on solution error. DOP analyses can take several forms, depending on the variables being analyzed. One-way navigation methods, called Geometrical DOP (GDOP), solve for a topocentric Cartesian position and time. Two-way navigation methods, called Positional DOP (PDOP), only solve for a topocentric Cartesian position.
DOP results for the polar 6/2/1 SMA9250, showing system availability. Top: GDOP results. Bottom: PDOP results.
Animation of results.
The Lunar Radiometric Navigation Analysis effort is managed under the Space Communications & Data Systems project at Glenn. The work was performed in-house by members of the Communications Systems Integration Branch in Glenn’s Communications Division.
Find out more about the research of Glenn’s Communications Division: http://ctd.grc.nasa.gov
Glenn contacts: Dr. Obed S. Sands, 216-433-2607, Obed.S.Sands@nasa.gov; Bryan W. Welch, 216-433-3390, Bryan.W.Welch@nasa.gov; and Joseph W. Connolly, 216-433-8728, Joseph.W.Connolly@nasa.gov
Authors: Dr. Obed S. Sands, Bryan W. Welch, and Joseph W. Connolly
Headquarters program office: Space Communications Technology Program
Programs/Projects: Space Communications & Data Systems Project
Last updated: October 16, 2006
Responsible NASA Official:
Gynelle.C.Steele@nasa.gov
216-433-8258
Point of contact for NASA Glenn's Research & Technology reports:
Cynthia.L.Dreibelbis@nasa.gov
216-433-2912
SGT, Inc.
Web page curator:
Nancy.L.Obryan@nasa.gov
216-433-5793
Wyle Information Systems, LLC
