Timescales for the GPS and Galileo

GPS Time, the time scale internal to the Global Positioning System, is a uniform scale of time without leap seconds having an initial epoch of midnight, January 5/6, 1980 UTC. Galileo proposes to use a realization of TAI, another scale without leap seconds, as its internal time scale. However, TAI is ahead of GPS Time by 19 seconds, a constant value, and in 2004, GPS Time is ahead of UTC by 13 seconds. Thus GPS Time, UTC, and TAI have different epochs, which could lead to potential timing errors at the level of tens of seconds. The rationale for using either GPS Time or TAI is that it is required for those who cannot tolerate leap seconds. The international timing community is considering the adoption of a single time scale free of leap seconds to address operational concerns, as well as safety issues related to multiple time scales with different epochs. This paper provides a brief history and technical background for TAI and UTC and proposes options for the unification of all civil atomic time scales, including those of the GPS and Galileo.