Global Positioning System Reference
In-Depth Information
where
t
UTC
is in seconds and
WN
t
)
] seconds
(
5
.
5
)
t
E
: GPS time as estimated by the user on the basis of correcting
t
SV
for
factors given in the subframe 1 clock correction discussion as well as for
ionospheric and satellite (dither) effects.
t
SV
: effective satellite pseudorange code phase time at message of trans-
mission time.
t
LS
: delta time due to leap seconds.
A
0
,A
1
: constant and first-order terms of polynomial.
t
ot
: reference time for UTC data.
WN
: current week number (derived from subframe 1).
WN
t
: UTC reference week number.
The estimated GPS time
(t
E
)
is in seconds relative to end/start of week.
The reference time for UTC data
(t
ot
)
is referenced to the start of that week
whose number
(W N
t
)
is given in bits (227 - 234) of page 18 subframe 4
representing the 8 LSB of the week. The user must account for the truncated
nature of the week number.
Whenever the user's current time falls within the time span of
DN
t
UTC
=
t
LS
+
A
0
+
A
1
[
t
E
−
t
ot
+
604
,
800
(W N
−
+
3
/
4to
DN
+
5
/
4, proper accommodation of the leap second event with a
possible week number transition is provided by the following expression
for UTC:
t
UTC
=
+
t
LSF
−
W
[modulo
(
86
,
400
t
LS
)
] seconds
(
5
.
6
)
where
W
=
(t
E
−
t
UTC
−
43
,
200
)
[modulo 86
,
400]
+
43
,
200 seconds
(
5
.
7
)
The definition of
t
UTC
given in Equation (5.4) applies throughout the
transition period. Note that when a leap second is added, unconventional
time values of the form 23:59:60.xxx are encountered. Some user equip-
ment may be designed to approximate UTC by decrementing the running
count of time within several seconds after the event, thereby promptly
returning to a proper time indication. Whenever a leap second event is
encountered, the user equipment must consistently implement carries or
borrows into any year/week/day counts. Table 5.12 gives the past history
of the difference between the GPS and the UTC times.
(
8
)
In 19 years the
difference is 13 seconds.
The tendency is that most of the modern navigation equipment uses GPS
time as the time base. Therefore, the translation from GPS time to UTC
time may no longer be needed in modern equipment.
5.
Ionospheric data
: In page 18 subframe 4, there are eight ionospheric data:
α
0
(69 - 76),
α
1
(77 - 84),
α
2
(91 - 98),
α
3
(99 - 106),
β
0
(107 - 114),
β
1
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