Ensemble time and frequency stability or GPS satellite clocks

In anticipation of cross-link ranging in the Global Positioning System (GPS) and the establishing of GPS ensemble time, the frequency stability characteristics of the clocks in the constellation were investigated. Once those characteristics were ascertained, the clock readings were optimally combined using the NBS time-scale-algorithm software to generate a GPS space clock ensemble. Frequency drifts of some parts in 10¹⁶ per day were observed in the GPS cesium beam frequency standards on board the space vehicles. The GPS space clock ensemble frequency stability reached about 2 parts in 10¹⁴ . This is comparable to some of the best clock ensembles at major timing centers throughout the world