Evaluation of NIMA and Air Force GPS Satellite Ephemerides Using NASA Laser Ranging Data

James W. O’Toole

Abstract:	This paper describes the use of Satellite Laser Range (SLR) data in assessing the accuracy of the Global Positioning System (GPS) precise ephemerides produced by the Department of Defense (DoD) and civilian organizations. The SLR data were collected by the National Aeronautics and Space Administration (NASA) during 1996, 1997, and 1998. During the three month period October-December, 1996, NASA launched an intensive tracking campaign for the two GPS satellites which carry laser retro-reflectors. The satellites, Satellite Vehicle Numbers (SVN) 35 and 36, were to be tracked on a higher than normal priority level with nineteen NASA affiliated tracking stations, Figure 1B, for the duration of the campaign. The campaign afforded an excellent opportunity to assess the accuracy of the GPS ephemerides and establish a baseline from which future ephemeris improvements could be measured. The procedure utilized ephemerides provided by the International GPS Service (IGS), the Air Force GPS Operational Control Segment (OCS), and the National Imagery and Mapping Agency (NIMA). Data in the form of NASA "quick-look" normal points, the principal NASA SLR data product, were obtained from the NASA Crustal Dynamics Data Information System (CDDIS). The normal point data have better than centimeter accuracy and were used here to calculate SLR residuals. SLR Residuals were computed from the NIMA and the IGS ephemerides during the three month campaign period in 1996. During a subsequent three month period (December, 1997 - February, 1998), SLR residuals were calculated from the OCS, NIMA, and IGS ephemerides. The Mean and RMS values for the SLR residuals over each three month tracking period are provided.
Published in:	Proceedings of the 54th Annual Meeting of The Institute of Navigation (1998) June 1 - 3, 1998 The Adams Mark Hotel Denver, CO
Pages:	601 - 609
Cite this article:	O’Toole, James W., "Evaluation of NIMA and Air Force GPS Satellite Ephemerides Using NASA Laser Ranging Data," Proceedings of the 54th Annual Meeting of The Institute of Navigation (1998), Denver, CO, June 1998, pp. 601-609.
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