Simulating Future GPS Clock Scenarios With Two Composite Clock Algorithms

Kurzfassung

Using the GPS Toolkit, the GPS constellation is simulated using thirty-one satellites (SV) and a ground network of seventeen 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, Model F is a future scenario in which the USNO and AMC stations are equipped with fountain clocks instead of masers. 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.