Global Positioning System Reference
In-Depth Information
Early gate correlation
e
y
e2
y
e1
y
e3
d
3
d
2
0
e + 1
P
e + 1 − 0.32
e + 1 + 0.34
Time in unit of sample time
FIGURE 11.16
Peak correlations from three early gates.
one second. For the example above,
d
1
=
0,
d
2
=
0
.
34, and
d
3
=−
0
.
32. Equ-
ation (11.10) can be written with this notation as
n
=
1
y
en
/N
y
e
1
y
e
2
=
= ··· =
(
11
.
11
)
(
n
=
1
d
n
/N)
]
P
−
(ε
−
1
)
P
−
(ε
−
1
+
0
.
34
)
P
−
[
ε
−
1
+
=
where
N
1000 representing 1000 ms.
The average values of the early peak correlation
y
esa
and late peak correlation
y
lsa
are
N
1
N
y
esa
=
y
en
n
=
1
(
11
.
12
)
N
1
N
y
lsa
=
y
ln
n
=
1
(
n
=
1
d
n
/N)
, and the position of
y
lsa
is at 1
−
+
+
+
The position of
y
esa
is at
1
ε
(
n
=
1
d
n
/N)
because the early and late gates are separated by two sampling
units. The values of
y
esa
and
y
lsa
can be used to calculate the fine time
x
by way of
Equation (11.8). It is desirable to make
y
esa
and
y
lsa
in Equation (11.12) equal, as
discussed in Section 11.11 and shown in Figure 11.13. Then the position of
y
psa
can be found accurately. In order to accomplish this goal, the carrier frequency
must be measured and the mismatch predicted after every ms in one second.
At the end of each second the C/A code will be generated with the proper
ε
+
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