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Time determination for spacecraft users of the Navstar Global Positioning System /GPS/Global Positioning System (GPS) navigation is performed by time measurements. A description is presented of a two body model of spacecraft motion. Orbit determination is the process of inferring the position, velocity, and clock offset of the user from measurements made of the user motion in the Newtonian coordinate system. To illustrate the effect of clock errors and the accuracy with which the user spacecraft time and orbit may be determined, a low-earth-orbit spacecraft (Seasat) as tracked by six Phase I GPS space vehicles is considered. The obtained results indicate that in the absence of unmodeled dynamic parameter errors clock biases may be determined to the nanosecond level. There is, however, a high correlation between the clock bias and the uncertainty in the gravitational parameter GM, i.e., the product of the universal gravitational constant and the total mass of the earth. It is, therefore, not possible to determine clock bias to better than 25 nanosecond accuracy in the presence of a gravitational error of one part per million.
Document ID
19790026426
Acquisition Source
Legacy CDMS
Document Type
Conference Proceedings
Authors
Grenchik, T. J.
(NASA Goddard Space Flight Center Greenbelt, Md., United States)
Fang, B. T.
(Washington Analytical Services Center Inc., Rockville, Md., United States)
Date Acquired
August 9, 2013
Publication Date
January 1, 1977
Subject Category
Space Communications, Spacecraft Communications, Command And Tracking
Meeting Information
Meeting: Annual Frequency Control Symposium
Location: Atlantic City, NJ
Start Date: June 1, 1977
End Date: June 3, 1977
Accession Number
79A10439
Distribution Limits
Public
Copyright
Other

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