Global Positioning System Reference
In-Depth Information
TABLE 8.7. Date for introduction of leap seconds to be added to UTC to get GPST
Total number
Date introduced
of leap seconds
1
1 July 1981
2
1 July 1982
3
1 July 1983
4
1 July 1985
5
1 Jan. 1988
6
1 Jan. 1990
7
1 Jan. 1991
8
1 July 1992
9
1 July 1993
10
1 July 1994
11
1 Jan. 1996
12
1 July 1997
13
1 Jan. 1999
14
1 Jan. 2006
to precede the historical record, chosen to be at 12 h UT on January 1, 4713 BC.
The Julian day number denotes a day in this continuous count, or the length of
time that has elapsed at 12 h UT on the day designated since this epoch. The JD of
the standard epoch of UT is called J2000.0, where
5 d UT
.
The astronomic year commences at 0 h UT on December 31 of the previous year
so that 2000 January 1.5 d UT = 2000 January 1 12 h . JD is a large number, so often
it is replaced by the Modified Julian Date MJD:
J2000
.
0
=
JD 2,451,545.0
=
2000 January 1
.
MJD
=
JD
2,400,000.5
.
Hence J2000.0 = MJD 51,544.5. Note an MJD starts at midnight.
Because GPS Time (GPST) is a continuous-time scale, it does not maintain
synchronization with the solar day since the Earth's rotation rate is slowing by an
average of about 1 s per year. This problem is solved by defining Universal Time
Coordinated (UTC), which runs at the same rate as GPST but is incremented
by leap seconds periodically. Leap seconds are introduced by the IERS so that
UTC does not vary from UT1 by more than 0.9 s. UT1 is UT corrected for polar
motion. (IERS is an acronym for the International Earth Rotation Service. This
service is also responsible for maintaining continuity with earlier data collected
by optical instruments.) First preference is given to the end of June and December,
and second preference to the end of March and September.
The time signals broadcast by the GPS satellites are synchronized with the
atomic clock at the GPS Master Control Station in Colorado. GPST was set to 0 h
Search WWH ::




Custom Search