Simulating Future GPS Clock Scenarios with Two Composite Clock Algorithms

Matthias Suess, Demetrios Matsakis, and Charles A. Greenhall

Abstract:	Using the GPS Toolkit, the GPS constellation is simulated using 31 satellites (SV) and a ground network of 17 monitor stations (MS). At every 15-minute measurement epoch, the monitor stations measure the time signals of all satellites above a parameterized elevation angle. Once a day, the satellite clock estimates are uploaded to the satellites. Two composite clock algorithms are applied to estimate the station and satellite clocks. The first composite clock (B) is based on the Brown algorithm [1], and is now used by GPS. The second one (G) is based on the Greenhall algorithm [2]. The composite clock of G and B performances are investigated using three ground-clock models. Model C simulates the current GPS configuration, in which all stations are equipped with cesium clocks, except for masers at USNO and Alternate Master Clock (AMC) sites. Model M is an improved situation in which every station is equipped with active hydrogen masers. Finally, Models F and O are future scenarios in which the USNO and AMC stations are equipped with fountain clocks instead of masers. Model F is a rubidium fountain, while Model O is a more precise but futuristic Optical Fountain. Each model is evaluated using three performance metrics. The timing-related user range error having all satellites available is the first performance index (PI1). The second performance index (PI2) relates to the stability of the broadcast GPS system time itself. The third performance index (PI3) evaluates the stability of the time scales computed by the two composite clocks. A distinction is made between the “Signal-in-Space” accuracy and that available through a GNSS receiver.
Published in:	Proceedings of the 42nd Annual Precise Time and Time Interval Systems and Applications Meeting November 15 - 18, 2010 Hyatt Regency Reston Town Center Reston, Virginia
Pages:	481 - 502
Cite this article:	Suess, Matthias, Matsakis, Demetrios, Greenhall, Charles A., "Simulating Future GPS Clock Scenarios with Two Composite Clock Algorithms," Proceedings of the 42nd Annual Precise Time and Time Interval Systems and Applications Meeting, Reston, Virginia, November 2010, pp. 481-502.
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