Global Positioning System Reference
In-Depth Information
TABLE 3.3. Galileo system time parameters
Parameter No. of bits
Scale factor Unit
WN
12
—
week
TOW
20
1
s
TABLE 3.4. Galileo clock correction parameters
Parameter No. of bits
Scale factor Unit
t
oc
14
60
s
2
−
33
a
o
28
s
2
−
45
a
1
18
s/s
2
−
65
s/s
2
a
2
12
k
Knowing the travel time
τ
this quantity can be converted to the so-called pseu-
i
dorange
P
i
by multiplication with the speed of light
c
:
P
i
k
=
c
τ
i
.
(3.17)
However, clocks do not work perfectly. So we introduce the receiver clock off-
set
dt
i
and the satellite clock offset
dt
k
:
t
Gal
t
i
=
+
dt
i
t
k
k
Gal
dt
k
=
(
−
τ
i
)
+
.
t
The receiver clock offset has to be estimated from the observed pseudoranges
while the satellite clock offset can be computed from
dt
k
2
=
a
0
+
a
1
(
t
−
t
0
c
)
+
a
2
(
t
−
t
0
c
)
,
(3.18)
where
t
is the transmit time. The constants
a
0
,
a
1
,and
a
2
are parameters transmit-
ted according to Table 3.4.
The
basic computational equations for time
are
t
k
,
G
al
t
Gal
i
P
i
/
=
−
c
(3.19)
−
a
0
+
2
.
k
Gal
t
k
(
t
−
τ
i
)
=
a
1
(
t
−
t
0
c
)
+
a
2
(
t
−
t
0
c
)
(3.20)
Additionally, a signal in space accuracy (SISA) parameter is planned. It is en-
coded as 8 bits.
An
ionospheric correction service
is planned. An effective ionization-level pa-
rameter is computed from three broadcast coefficients.
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