Global Positioning System Reference
In-Depth Information
N
pq
km
N
pq
km
K
k
K
k
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
=
+
−
(7.217)
is estimated instead of
N
pq
km
.
The telemetry load can be further reduced if it is possible to increase the time
between transmissions of the carrier phase corrections. For example, if the change in
the discrepancy from one epoch to the next is smaller than the measurement accuracy
at the moving receiver, or if the variations in the discrepancy are too small to affect
adversely the required minimal accuracy for the moving receiver's position, it is
possible to average carrier phase corrections over time and to transmit the averages.
It might be desirable to transmit the rate of correction
∂
/∂t
.If
t
0
denotes the
reference epoch, the user can interpolate the correctors over time as
∆Φ
p
k
∂
∆Φ
p
k
(t)
p
k
(t
0
)
∆Φ
= ∆Φ
+
(t
−
t
0
)
(7.218)
[29
∂t
One way to reduce the size and the slope of the discrepancy is to use the best
av
ailable coordinates for the fixed receiver and a good satellite ephemeris. Clock
er
rors affect the discrepancies directly, as is seen in Equation (7.210). Connecting
a
rubidium clock to the fixed receiver can effectively control the variations of the
re
ceiver clock error
d
t
k
. Prior to its termination, selective availability was the primary
ca
use of satellite clock error
d
Lin
—
-
——
No
PgE
¯
t
p
and was a determining factor that limited modeling
lik
e (7.218).
In the case of pseudorange corrections, we obtain similarly
p
k
p
k
P
k
= ρ
−
(7.219)
[29
P
k
p
k,
0
−
P
k
∆
= ρ
− µ
k
(7.220)
P
m
(t)
P
k
(t)
P
m
(t)
=
+ ∆
(7.221)
P
q
m
P
m
−
P
m
= ρ
I
pq
km,P
T
pq
km
d
pq
km,P
q
p
(t)
≡
m
(t)
− ρ
m
(t)
+
+
+
(7.222)
The approach described here is applicable to the L1 and L2 carrier phases and to all
three codes.
7.
9.2 Local Network Corrections
The impact of the troposphere, the ionosphere, and orbital errors on the single- and
double-difference observables are at the same level as the carrier phase measurement
resolution for short baselines or less. In fact, the definition of short baselines is directly
linked to this cancellation of tropospheric and ionospheric effects and orbital errors on
the single-difference observables, i.e.,
T
km
≈
0
,I
km
≈
p
km
≈
0. It is common
practice for short baselines to fix the double-difference ambiguities to integers using
the LAMBDA procedure, yielding centimeter-accurate baselines. Traditionally, RTK
techniques are applied to short baselines involving one base station and one roving
0, and
d
ρ
