Navigation Upload Performance

Greg Hatten and Jack Taylor

Abstract:	The navigation data message broadcast from each GPS satellite allows the user to obtain accurate positioning and timing information. This navigation data message is created from measurements gathered from the worldwide GPS Monitoring Station (MS) network and is uploaded to the satellites from the GPS Master Control Station (MCS) in Colorado Springs, Colorado. The quality of the navigation data upload depends on three broad factors: 1. The ability of the MCS to accurately model the clock and ephemeris states. 2. The stability of these states, which allows the MCS to predict them accurately. 3. The time that these estimates were provided to the satellite for broadcast (Age of Data). Each of these factors has limits to further improvement. In the first case, numerous improvements to the MCS estimation process have been implemented over the years, the most recent being the Accuracy Improvement Initiative (AII) funded by the GPS Joint Program Office. Although further improvements are on-going, these processes are yielding less substantial results. In the second case, the atomic clocks aboard the satellites are unavailable for hardware upgrades. Also, some satellites are of an advanced age and must be run in a manner that degrades their ephemeris estimates. In these instances, no improvement should be expected. In the third case, the MCS has control over the timing of the upload data. The upload can be timed so that its creation coincides with the best possible state estimate and subsequent prediction quality. The upload data can also be replaced when changes in the clock or ephemeris states make the errors associated with the old upload unacceptable. This paper will examine the upload timing issue from two perspectives. First, the aging of the upload will be examined. This data will show that certain satellites perform better than others as their uploads age. The possible causes of poor performance will also be discussed. Next, the timing of the upload will be examined to see if there exists an optimal time to upload a specific satellite.
Published in:	Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000) September 19 - 22, 2000 Salt Palace Convention Center Salt Lake City, UT
Pages:	425 - 431
Cite this article:	Hatten, Greg, Taylor, Jack, "Navigation Upload Performance," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, UT, September 2000, pp. 425-431.
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