Global Positioning System Reference
In-Depth Information
Carrier
NCO
IF
(
I
,
Q
)
E, P, L correlator
data
Loop
discrimin-
ators/
filters
E, P, L correlator
pilot
BOC
generator
PRN
generator 1
PRN
generator 2
Secondary
code
Code
NCO
FIGURE B.7. Generic L1 OS tracking architecture.
minimum correlation time (dwell time) of 4 ms for a complete code period
of correlation for both acquisition and tracking.
The Galileo acquisition and tracking might need additional techniques to
acquire/track the right peak of the ACF if the L1 OS signals will use the
proposed CBCS coding scheme, confer Hein et al. (2005). Depending on
the configuration of the CBCS coding scheme, ACFs with multiple, local
extreme might occur.
A relatively simple false lock detection consisting of checking the ACF
signal power in the vicinity of the tracked point could be implemented. This
could be done with either extra correlator arms, situated “very early” and
“very late” with respect to the early, prompt, and late arms, or it could be
done by periodically jumping to neighboring code phases with the existing
early, prompt, and late correlators.
Acquisition of Galileo Signals
The acquisition of Galileo signals can be carried
out as with GPS signals except from the differences mentioned in the pre-
vious subsection. In the case of the multiple peak correlation function, the
local extremes could be ignored in the acquisition process due to the lim-
ited code phase resolution. The check for false locks should then be carried
out when the handover from acquisition to tracking has taken place and the
DLL has converged.
The dwell time dictates the maximum step for the frequency search. For the
minimum length of 4 ms the maximum frequency step is 250 Hz (was 1 kHz
for the 1 ms dwell time). For an improved performance a 125 Hz would be
a better choice.
Tracking
The high level architecture of the tracking block is almost exactly the
same in the basic BOC(1,1) case as in GPS. The main differences are the
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