Global Positioning System Reference
In-Depth Information
The satellite manufacturer initially determines the offset
T
p
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
GD
for each satellite
during factory testing. The offset is available to users through the broadcast message.
The code emission times on both frequencies are related as follows:
GD
1
− α
f
t
L
1
−
t
L
2
=
T
p
(5.71)
The symbol
α
f
is given in (5.13). The offset can be treated as an additional clock
correction for code phase observations,
c
d
T
GD
+
P
k,
1
(t)
p
k
(t
p
)
I
k,
1
,P
(t)
T
k
(t)
t
p
= ρ
−
cd
t
k
+
¯
+
+
(5.72)
p
k,
1
,P
(t)
+ δ
+
ε
1
,P
[La
c
d
f
T
GD
+
P
k,
2
(t)
p
k
(t
p
)
I
k,
2
,P
(t)
T
k
(t)
t
p
= ρ
−
cd
t
k
+
¯
+ α
+
[18
(5.73)
p
k,
2
,P
(t)
+ δ
+
ε
2
,P
Lin
—
1
——
No
PgE
Fo
llowing the sign convention of (5.39), L1 users can modify the computed satellite
clo
ck correction by subtracting
T
GD
; L2-only users must subtract
f
T
GD
. The offset
T
G
D
can be combined with and is indeed inseparable from the satellite hardware delay
denoted by
d
1
,P
above. The same is true for L2.
The
T
GD
offsets change with time. For example, changes in satellite configuration
du
e to use of backup hardware might impact the offsets. The offsets are routinely
es
timated from ground observations. See Sardón et al. (1994) and Sardón and Zarraoa
(1
997) for details on the observation and processing techniques. As an example, the
re
sults of a recent determination (Wilson et al., 1999) show a spread in the offset
be
tween the various satellites from close to zero to 12 ns, corresponding to about 4
m
. The broadcast values are updated approximately four times per year. The actual
bi
ases are monitored daily to identify any abrupt changes (B. D. Wilson, JPL, private
co
mmunication).
In addition to the P1-P2 interfrequency P-code bias, there is a bias between C/A
an
d P-code pseudoranges. The C/A-P1 biases can also reach several nanoseconds.
Cu
rrent value estimates of these biases are available on the Internet. For example,
JP
L estimates these biases as part of the real-time Internet-based global differential
GP
S solution.
α
[18
