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GPS Changes Before and After Implementation of the Architecture Evolution Plan



Marc A. Weiss


We characterize the performance of GPS signals before, and after Architecture Evolution Plan (AEP) implementation. We use the period from May 20 through September 13, 2007, MJDs 54240-54356, as the period before AEP. The interval from September 14, 2007 through December 31, 2007, MJDs 54357- 54465, we use as 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 the Time Deviation (TDEV) of various data after AEP implementation divided by the TDEV before 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) the difference of pseudoranges computed using broadcast minus National Geospatial-Intelligence Agency s (NGA) post-processed estimates ephemerides at NIST, Boulder, and 2) the comparison of each satellite's broadcast ephemeris and clock model against the NGA estimates. Generally we find evidence of some worsening in ephemerides from before to after AEP. RMS 15-minute stabilities across satellite blocks worsened by 8-30%. 12-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 AEP. In the process we found a periodic variation in all satellites that lasts about 27 days. The periodic variation is in the magnitude of the deviation of the broadcast radial ephemeris, using the NGA post-processed values as a reference. The periodic effect does not appear in the value of the broadcast minus NGA radial ephemeris. Nor is the periodic effect significant in the deviation of this difference. Rather, the periodic effect appears clearly in the deviation of the broadcast radial velocity and radial acceleration terms. Since TDEV is a second difference variance, this periodic effect appears clearly in the evolution of the TDEV of the radial broadcast minus NGA ephemerides.
Proceedings Title
Proc. 2008 PTTI Conf.
Conference Dates
December 1-4, 2008
Conference Location
Reston, VA


Global Positioning System (GPS), Architecture Evolution Plan (AEP), Time Deviation (TDEV), periodic variation


Weiss, M. (2008), GPS Changes Before and After Implementation of the Architecture Evolution Plan, Proc. 2008 PTTI Conf., Reston, VA, [online], (Accessed March 2, 2024)
Created December 1, 2008, Updated February 19, 2017