Interoperability of the GPS and Galileo timescales for positioning and metrology

Today, GPS plays an important role in positioning and the dissemination of time and frequency on a global scale. In Europe, Galileo is under development and will enter service some time after 2008. Both systems will have their own timescales: GPS Time for GPS and Galileo System Time for Galileo. There are enormous benefits to be gained if GPS and Galileo can be used together as one "super-constellation" of over 50 satellites. There is therefore a need for a clear understanding of the factors that will contribute to, and ultimately limit interoperability of the GPS Time and Galileo System Time. In an ideal world, both GPS and Galileo timescales would be aligned with TAI/UTC and interoperability would be assured, however, the systems each have their own timescale and TAI/UTC is not available in real time; offsets between the two are inevitable. Section 2 of this paper details the GNSS interoperability considerations with respect to system time. Section 3 outlines the timing concept and requirements for Galileo, and Section 4 presents the implications for positioning receivers of various time offsets for a mixed constellation in an urban environment. Section 5 presents a GPS / Galileo traceability model from a user receiver to TAI/UTC. In Section 6, a range of conclusions are drawn on how Galileo could interact with GPS and World Timekeeping, and recommendations are made for avenues of research that may further reduce the problem and so enhance GNSS interoperability.