GPS Changes Before and After Implementation of the Architecture Evolution Plan

M. A. Weiss and A. Masarie

Abstract:	We characterize the performance of GPS signals before, and after Architecture Evolution Plan (AEP) implementation. We use the period from 20 May through 13 September 2007, MJDs 54240-54356, as the period before AEP. We use the interval from 14 September 2007 through 31 December 2007, MJDs 54357- 5446 as the period after the implementation of AEP. We use a variance analysis to characterize performance, allowing us to characterize periodic behavior in the presence of power-law noise types. Computing the ratio of TDEV of various data after AEP implementation divided by the TDEV before the implementation allows us to see changes in deviation due to both power-law noise and periodic effects. We analyze two types of data for changes between these time periods: 1) error in broadcast ephemerides and clock corrections as combined for pseudoranges at NIST, Boulder, using National Geospatial-Intelligence Agency’s (NGA) postprocessed estimates as the reference, and 2) the error of each satellite's broadcast ephemeris and clock model separately against the NGA estimates. Generally, we find evidence of some worsening in ephemerides from before to after the implementation of AEP. RMS 15-minute stabilities across satellite blocks worsened by 8-30%. Twelve-hour and 24-hour periodic behavior generally worsened. Though results vary by satellite block and ephemeris component, the best was a 13% improvement, and the worst was a worsening of 29%. Results vary significantly by individual satellite. By contrast, broadcast clock correction values generally showed little change from before to after the implementation of AEP. In the process of this study, we found evidence of an approximate 27-day periodic variation in the difference: broadcast minus NGA radial ephemeris of all satellites studied. The periodic variation is not evident in the difference data themselves, but in the broadcast minus NGA radial velocity and acceleration data. The effect is a periodic variation in the magnitude of the deviation. It can be seen in the 900-s Dynamic TDEV values of the radial differences, the evolution of the TDEV values at 900 s.
Published in:	Proceedings of the 40th Annual Precise Time and Time Interval Systems and Applications Meeting December 1 - 4, 2008 Hyatt Regency Reston Town Center Reston, Virginia
Pages:	315 - 334
Cite this article:	Weiss, M. A., Masarie, A., "GPS Changes Before and After Implementation of the Architecture Evolution Plan," Proceedings of the 40th Annual Precise Time and Time Interval Systems and Applications Meeting, Reston, Virginia, December 2008, pp. 315-334.
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