Role of International Time Reference UTC in the Definition of Galileo System Time
Lewandowski, W.; Arias, F.
BIPM

The Bureau International des Poids et Mesures (BIPM), located in Sèvres near Paris, is on charge of computing and publishing international reference time scale Coordinated Universal Time (UTC). Whenever necessary a leap second is added to UTC to keep it in step with the slightly irregular rotation of the Earth. UTC is based on the uniform scale International Atomic Time (TAI) to which leap seconds are not applied. TAI is a paper time scale to be used in scientific applications and not for the coordination of human activities. UTC is the sole reference time scale for world-wide time coordination. Approximations to UTC are realized by clocks in national laboratories. It serves as the basis of legal time in many countries.

The Global Navigation Satellite Systems (GNSS) are using internal reference time scales. For example GPS uses GPS time, which was set in 1980 to have zero second difference with UTC. However, GPS time is a continuous, uniform time which does not follow leap seconds. GPS time is 19 seconds behind TAI, and until the end of 2007 at least, 14 seconds ahead of UTC. GLONASS uses GLONASS time which does follow leap seconds.

At the early stages of the definition of Galileo system it was considered to use TAI as a reference for Galileo System Time (GST). However, the final decision is that GST will be set to have zero second difference with GPS time, and will be a continuous time scale so not following leap seconds. This solution should enhance the interoperability between the two systems.

Each of these systems is programmed to broadcast a prediction of UTC. The requirements of time keeping evolved and the need of a continuous reference time scale increases. It happens that for some applications it is suggested to use TAI or uniform system times as GPS time at present, Galileo System Time in the future, instead of UTC ‘suffering’ from leap seconds. However, such proliferation of various time scales may cause confusion among users. Due to above and other considerations the utility of leap seconds is under discussion within International Telecommunications Union (ITU).

Our paper provides detailed analysis of the definition of Galileo System Time and its relation to the system times of other GNSS: GPS, GLONASS, and upcoming Japanese QZSS, Chinese BEIDOU, Indian GAGAN.