Time Transfer Through GPS, and the Harmonization of GPS, GLONASS and Galileo for Timing

The Global Positioning System GPS has become a key component of the world technology, and one measure of its success is that it has inspired enhancements and similar systems. GPS can therefore be considered the prototype of several existing or planned Global Navigation Satellite Systems GNSS, with which term we also include supporting satellite-based enhancement systems. This paper briefly describes how GPS derives its timing, how GNSS systems are used for several means of precise time transfer, how carrier phase can assist time transfer, and how GNSS systems can cooperatively create improved products. Significant benefits in terms of robustness and precision can result from combining GNSS systems. Applications which may require them are situations involving interference, limited reception as in urban canyons or indoors, or where there is limited time to acquire a signal.