Global Positioning System Reference
In-Depth Information
FIGURE 8.4
Outputs from conventional tracking method of sv17.
changes with time; this is the transit effect of the tracking loop. Finally, the
amplitude reaches a steady state.
8.8 CARRIER FREQUENCY UPDATE FOR THE BLOCK ADJUSTMENT
OF SYNCHRONIZING SIGNAL (BASS) APPROACH
(
8,9
)
The purpose of introducing the BASS method is to present a different tracking
program from the conventional method. This program is used for the software
GPS receiver discussion in the next chapter. In this program, once the C/A code
is generated it is used all the time. No initial phase adjustment is required such
as in the conventional phase-locked loop discussed in previous sections. The fine
time resolution can be obtained from the early and the late outputs of the code
loop, which will be discussed in Sections 8.11 and 8.12.
This discussion is based on reference 8. The operation is on 1 ms of data for
simplicity; however, other data lengths can be used. The concept is based on
discrete Fourier transform (DFT). If the digitized input signal is
x
(
n
), the DFT
output
X
(
k
) can be written as
N
−
1
x(n)e
−
j
2
πnk/N
X(k)
=
(
8
.
40
)
n
=
0
where
k
represents a certain frequency component and
N
is the total number of
data points. If
x
(
n
) is obtained from digitizing a sinusoidal wave, the highest
amplitude
|
|
represents the frequency of the input signal. The real (Re) and
imaginary (Im) parts of
X(k
1
)
can be used to obtain phase angle
θ
as
X(k
i
)
=
tan
−
1
Im[
X(k
i
)
]
Re[
X(k
i
)
]
θ
(
8
.
41
)
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