Timing in GNSS - its importance and challenges

Heart of satellite based navigation system is timing by precise clocks. In Global Navigation Satellite System (GNSS,) mainly consisting of GPS, GLONASS, Galileo and Beiduo, timing plays a vital role also. Coordination of the time scales of all these systems is very important for the optimum utilization of GNSS. It has some stringent requirements. Coordination among different constellations is also a serious issue. This paper details all these aspects.