GPS time
The GPS time (GPST) is the time system of the navigation satellites of the Global Positioning System (GPS). It is defined as International Atomic Time (TAI) minus 19 seconds. The monitoring and the necessary correction due to the drift of the atomic clocks is carried out by comparison with the ground station of the United States Naval Observatory .
As part of the GPS system, the GPS time or its elements are supplied or used for the following purposes:
- It is necessary to determine the position, whereby only the TOW (Time-of-Week, see below) is used (for details see GPS technology ).
- The time information can also be used as a time standard .
- Based on the position, the local time can be calculated using a time zone database .
Difference from UTC
| When? | The GPS time is | |
|---|---|---|
| on January 6, 1980 | UTC +0 s | TAI −19 s |
| from 1981 to 1998 | UTC +1 ... +12 s leap second every 1 ... 2 years |
TAI −19 s |
| from January 1, 1999 | UTC +13 s | TAI −19 s |
| from January 1, 2006 | UTC +14 s | TAI −19 s |
| from January 1, 2009 | UTC +15 s | TAI −19 s |
| from July 1, 2012 | UTC +16 s | TAI −19 s |
| from July 1, 2015 | UTC +17 s | TAI −19 s |
| from January 1, 2017 | UTC +18 s | TAI −19 s |
The GPS time started on January 6, 1980 at 00:00:00 (see also GPS week ) and was then identical to the Coordinated Universal Time (UTC). At this point in time, UTC had a difference to International Atomic Time (TAI) of 19 seconds. Since then, a leap second has been added to UTC every 1–6 years in order to keep world time in sufficient agreement with the Earth's rotation . This leap second is not taken into account in the GPS time .
Elements of the GPS time indication
The GPS time is transmitted encoded with the user data. These include the GPS week (10 bits, counting from 0 to 1023) and the Time-of-Week (TOW) , i.e. the time that has elapsed since the start of the current GPS week (specification not in seconds, but in 1.5 second steps, 19 bits, 0-402199).
Since a complete subframe is transmitted every 6 seconds, 4 steps can be dispensed with, so that only 17 bits are transmitted. By limiting the weekly counter to 1023, the GPS time can only be determined within a period of 19.7 years without the aid of additional information ("roll-over", see GPS week ).
In addition, the current difference between the GPS time and UTC is transmitted in a separate data field. GPS receivers can also use it to calculate the UTC time. If the transit time of the satellite signal is precisely determined, the GPS system guarantees a deviation from UTC of a maximum of one microsecond.
See also
literature
- Günter Seeber : Satellite Geodesy. Basics, methods and applications . de Gruyter-Verlag, Berlin et al. 1989, ISBN 3-11-010082-7 , p. 36ff.
- Albert Schödlbauer : Geodetic Astronomy. Basics and concepts . de Gruyter-Verlag, Berlin et al. 2000, ISBN 3-11-015148-0 , pp. 363-364.
Web links
- IERS time bulletin
- Atomic time scales , information from PTB
- Uni Münster GPS in the geosciences