Global Positioning System Reference
In-Depth Information
65
64
63
62
61
60
59
58
0
500
1000
1500
2000
2500
3000
Frequency bins
(c) Compensated by outputs separated by 1 KHz and 100 Hz.
FIGURE 10.10
(
continued
)
If ten 10 ms of data are processed, the overall data length is 100 ms. The
outputs are ten 199 by 2500 matrices. Noncoherent integration adds all of these
matrices together to form one 199 by 2500 matrix. If there is Doppler frequency
shift in the input signal, this approach will misalign some data, and the result will
be degraded. The goal of this section is to remedy this degradation by aligning
the time domain with different Doppler frequencies.
The Doppler phenomenon can be explained as follows. If a satellite sends
a 1 ms signal at the L1 frequency of 1575.42 MHz, it contains 1575
.
42
×
10
3
cycles of RF wave. The corresponding wavelength
λ
is
c
f
L
1
λ
=
(
10
.
19
)
where
c
is the speed of light and
f
L
1
=
1575
.
42 MHz. If an observer receives
this signal with a positive Doppler frequency shift of
f
d
, the wavelength received
by the observer changes to
λ
d
, which can be expressed as
c
f
L
1
+
λ
d
=
(
10
.
20
)
f
d
The observer will receive the same number of cycle in less than 1 ms because
of the positive Doppler frequency shift. The time required to receive the same
number of cycles will be
f
d
≈
10
−
3
1
−
10
−
3
λ
d
λ
10
−
3
f
L
1
f
L
1
+
f
d
f
L
1
t
=
=
(
10
.
21
)
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