Global Positioning System Reference
In-Depth Information
TABLE 3.5. GST to UTC conversion
Parameter No. of bits
Scale factor Unit
2
−
30
A
0
32
s
2
−
50
A
1
24
s/s
t
LS
8
1
s
t
0t
8
3600
s
WN
t
8
1
week
WN
LSF
8
1
week
DN
3
1
...
7
day
t
LSF
8
1
s
3.4.2 Conversion of GST to UTC and GPST
Compared to the present GPS, Galileo offers some advantages for the timing com-
munity. For example, data for real-time estimation of Universal Time Coordinated
(UTC) are available. Likewise for the difference between GST and GPST. How-
ever, in case the user disposes over a combined GPS and Galileo receiver, it is
likely that an estimate based on Equation (8.45) turns out to be more accurate.
The relation between GST and UTC is established via the time scale Temps Atom-
ique International (TAI). UTC and TAI differ by an integer number of seconds.
On January 1, 2003, the difference was
TA I
−
UTC
2003
=+
32 s
.
UTC is a uniform time scale, and it tries to follow variations in the Earth's ro-
tation rate; this is accommodated for by introducing leap seconds in the UTC.
Consequently, this changes the difference between UTC and GST in steps of 1 s
(see Table 3.5).
Let the estimated epoch time in GST, relative to the start of the week, be de-
noted by
t
E
.Let
A
0
denote the offset between GST and TAI at the time
t
E
.The
time derivative of
A
0
is called
A
1
. Let the difference between TAI and UTC
be
t
LS
, and the validity time
t
0t
for the UTC offset parameters.
Leap seconds are always introduced on January 1 or/and July 1. The day num-
ber in the week in which the leap second is introduced is called DN. Days are
counted from 1 to 7 (Sunday is 1) and is rounded to an integer.
The week number, modulo 256, in which DN falls is denoted WN
LSF
.Fi-
nally, the offset due to the introduction of a leap second at WN
LSF
and DN is
called
t
LSF
.
The following equations are in unit of s. We start by introducing the correction
A
1
t
E
−
WN
t
)
.
t
UTC
=
t
LS
+
A
0
+
t
0t
+
604
,
800
(
WN
−
(3.21)
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