Time Dissemination Alternatives for Future NASA Applications

Al Gifford, Robert A. Nelson, Richard S. Orr, A.J. Oria, Beryl L. Brodsky, James J. Miller, Barbara Adde

Abstract: This study describes an activity being undertaken by NASA. The Space Communication Architecture Working Group (SCAWG) was established to recommend the future architecture of NASA’s communication, navigation, and timing systems through studies coordinated across all NASA Centers and Mission Directorates. The architecture will support both science missions and the Vision for Space Exploration (VSE), which calls for returning of humans to the Moon, expanding human presence to Mars, and exploring the solar system. Foreseeable NASA applications span a wide range of demands for time services. Four examples include: (1) event logging by human explorers; (2) networking of cooperative robots on a planetary surface (e.g., the Moon or Mars); (3) time transfer for navigation local to a planetary body; and (4) time synchronization for communication and science. Documented accuracies range from 1 s for event logging down to 10 ns for local navigation and communication. Accuracy requirements below 1 ns for science missions are anticipated. Required time corrections due to the change in relative range between a time source and user during the light-time measurement interval are described. In addition, corrections for relativistic effects on clocks in Earth orbit or the Mars and Moon environments are necessary. The time dissemination alternatives identified thus far are based on existing time dissemination technologies, including radiometric ranging, network time dissemination, and stand-alone time dissemination techniques. The paper presents concept descriptions of: (1) DSN two-way ranging utilized for time transfer; (2) scalable oneway, two-way time dissemination using relay satellites; and (3) a GPS augmentation system, consisting of pseudolites or beacons distributed on the surface of the Moon and elsewhere in the solar system.
Published in: Proceedings of the 38th Annual Precise Time and Time Interval Systems and Applications Meeting
December 7 - 9, 2006
Hyatt Regency Reston Town Center
Reston, Virginia
Pages: 319 - 328
Cite this article: Gifford, Al, Nelson, Robert A., Orr, Richard S., Oria, A.J., Brodsky, Beryl L., Miller, James J., Adde, Barbara, "Time Dissemination Alternatives for Future NASA Applications," Proceedings of the 38th Annual Precise Time and Time Interval Systems and Applications Meeting, Reston, Virginia, December 2006, pp. 319-328.
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