Global Positioning System Reference
In-Depth Information
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Figure 2.7
UT1-UTC variation during 1999.
(Data from 1999 IERS Annual Report.)
[28
the earth's rotation. These forced variations are computable. The currently adopted
model includes terms with periods up to thirty-five days.
The five corrections, UT1
Lin
—
0.9
——
Sho
PgE
−
UTC, polar motion
x
p
, and
y
p
, and the celestial pole
ψ
offsets
d
and
dε
, are required to transform the terrestrial reference frame to the
celestial one and vice versa. The IERS monitors and publishes these values. They
are the earth orientation parameters (EOP). Modern space techniques allow these
pa
rameters to be determined with centimeter accuracy.
Various laboratories and agencies operate several atomic clocks and produce their
ow
n independent atomic time. For example, the time scale of the U.S. Naval Observa-
to
ry is called UTC(USNO), and the National Institute of Standards and Technology
(N
IST) produces UTC(NIST). The IERS, which uses input from 200 plus clocks from
sixty plus different laboratories scattered around the world, computes TAI. UTC and
TAI differ by the integer leap seconds. TAI is not adjusted, but UTC is adjusted for
leap seconds.
The GPS satellites follow GPS time (GPST). This time scale is steered to be within
one microsecond of UTC(USNO). The initial epoch of GPST is 0
h
UTC January 6,
1980. Since that epoch, GPST has not been adjusted to account for leap seconds. It
follows that GPST
[28
19
s
, i.e., equal to the offset of TAI and UTC at the
initial GPST epoch. Each satellite carries several atomic clocks, including the spare
clock. These clocks establish the space vehicle time. The control center synchronizes
the clocks of the various space vehicles to GPST.
The Julian day date (JD) used in (2.29) is but a convenient continuous counter
of mean solar days from the beginning of the year 4713 B.C. By tradition, the Julian
day date begins at Greenwich noon, i.e., 12
h
UT1. As such, the JD has nothing to
do with the Julian calendar, which was created by Julius Caesar. It provided for
the leap year rule that declared a leap year of 366 days if the year's numerical
designation is divisible by 4. This rule was later supplemented in the Gregorian
calendar by specifying that the centuries that are not divisible by 400 are not leap
years. Accordingly, the year 2000 was a leap year but the year 2100 will not be. The
Gregorian calendar reform also included that the day following October 4 (Julian
−
TA I
=−
