Global Positioning System Reference
In-Depth Information
TABLE 3.6. GST to GPST conversion
Parameter No. of bits
Scale factor Unit
2
−
35
A
0G
16
s
2
−
51
A
1G
12
s/s
t
0G
8
3600
s
We need to distinguish among three different cases:
3
4
5
-
When
t
E
>
WN
LSF
and
t
E
>
DN and DN
+
<
t
E
and
t
E
<
DN
+
4
,
then
t
UTC
=
mod
(
t
E
−
t
UTC
,
86
,
400
),
(3.22)
3
4
5
-
when DN
+
<
t
E
<
DN
+
4
,then
W
=
mod
(
t
E
−
t
UTC
−
43
,
200
,
86
,
400
)
+
43
,
200
(3.23)
t
UTC
=
mod
(
W
,
86
,
400
+
t
LSF
−
t
LS
),
(3.24)
-
when WN
LSF
<
t
E
and DN
>
t
E
,then
t
UTC
=
(
t
E
−
t
UTC
,
,
).
mod
86
400
(3.25)
The
difference between GST and GPST
is determined as follows. Let
t
Gal
be GST
estimated by the user receiver; then the offset between GST and GPST at time
t
Gal
is
t
systems
=
A
0G
+
A
1G
(
t
Gal
−
t
0G
).
(3.26)
Table 3.6 describes the parameters concerned.
3.4.3 Service Parameters
The satellite identification SV
ID
is a number between 1 and 128. A parameter,
issue of data (IOD), identifies the set of data. This allows a receiver to compare
batches of data received from different satellites. IOD is transmitted in each page
of ephemeris and clock correction (9 bits) and almanac (2 bits).
Signal and data health status referring to the transmitting satellite is planned as
well.
The six Keplerian elements
of any active satellite are con-
tained in the
almanac
. The elements are given with less precision than the ephe-
meris. Clock correction parameters are given for computation of satellite clock
offset
(
a
,
e
,ω,,
i
,µ)
dk
k
=
a
f0
+
a
f1
(
−
t
0a
).
t
(3.27)
The almanac reference time
t
0a
refers to the almanac reference week WN
a
.
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