Satellite navigation systems provide user coordinates derived from distance measurements that are based on the propagation time of the transmitted signals. Thus, all these systems rely on very accurate clocks and time standards. To meet the needs of internal time synchronization and dissemination, each GNSS maintains a specific system time. The time systems of the four global navigation satellite systems, Global Positioning System (GPS), GLONASS, Galileo, and BeiDou, are all based on the SI second and atomic time similar to TAI. However, they are realized by different clock ensembles and have different origins and offsets with respect to TAI [1].
GPS time (GPST) is the system time employed by the Global Positioning System. Since 1990, it is formed as a composite clock from atomic clocks within the GPS Control Segment as well as the atomic frequency standards onboard the GPS satellites. Each of these clocks contributes to the resulting time scale with a specific weight based on the observed variance of the respective clock [8]. Using common view time transfer, GPS time is steered to deviate by at most 1s from UTC(USNO). In practice, the GPS – UTC(USNO) offset is much smaller than the specified range and achieves representative values at the level of 20 ns. In order to provide GPS users with access to UTC, a forecast value of the offset between both time scales is transmitted as part of the navigation message. GPS time is not adjusted by leap seconds to slow down with UT and it is thus permanently trails TAI by a constant amount
| t (GPS) = TAI – 19s |
Note that above relationship describes only the nominal (integer second) offset between GPS time and TAI, but neglects additional fractional offsets (typically at the level of tens of nanoseconds) related to different realization of the two time scales. GLONASS Time (GLST) is the only GNSS time scale that actually follows the ITU recommendation to align a disseminated time scale with UTC. Its origin is chosen as January 1, 1996 in the UTC (SU) time system, that is, the Russian (formerly Soviet Union, SU) realization of UTC maintained by the Institute of Metrology for Time and Space in Moscow. Besides incorporating leap seconds, GLST is always 3h ahead of UTC because of the time zone difference between Greenwich and Moscow. Thus,
| t (GLONASS) = UTC + 3h |
Again, this relation does not account for fractional second offsets resulting from the independent realization of both time scales. GLST is obtained from an ensemble of hydrogen-masers in the GLONASS ground segment and synchronized to UTC (SU) using two-way time transfer with a specified tolerance of 1s. Following a consolidated effort to improve the alignment of GLST with UTC, the difference of the two time scales has improved from several hundred ns to a few tens of ns as of the second half 2014.
Both the Galileo System Time (GST) and BeiDou time (BDT) exhibit a constant offset from TAI. The origin for Galileo time, for consistency, is defined to be identical to that of GPS Time, but the origin for the BeiDou time system has been chosen as January 1.0, 2006 UTC. Thus
| t (Galileo) = TAI – 19s | |
| t (BeiDou) = TAI – 33s |
Both time scales are generated from atomic clocks in the respective control segments and steered to UTC via time transfer and clock comparison with other UTC laboratories. GST is specified to differ by less than 50 ns (2s) from UTC while a maximum offset of 100 ns applies for BeiDou.
Similar to Galileo, continuous time scales with a fixed −19 s offset from TAI are also adopted by the Japanese Quasi-Zenith Satellite System (QZSS) and the Indian Regional Satellite Navigation System (IRNSS/NAVIC) [2].
References
- W. Lewandowski, E.F. Arias: GNSS Times and UTC, Metrologia 48(4), S219–S224 (2011).
- Teunissen, Peter JG, and Oliver Montenbruck, eds. Springer Handbook of Global Navigation Satellite Systems. Vol. 10. Cham, Switzerland: Springer International Publishing, 2017.
