Global Positioning System Reference
In-Depth Information
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Fig. 7. L2C Partial acquisition using matched filter
of spans
i
(1
1) is used, in the following, the notations for the two techniques will
be GDC(
i
) and MGDC(
i
).
≤
i
≤
M
−
4.2 Application of technique to L2C signal
In this Section, the MGDC technique is used to acquire GPS L2C signal. This signal is chosen
because it employs a long PRN code period, which can be used to generate partial correlator
outputs with the same sign. Hence, there is no combination loss due to data bit transitions in
differential accumulation (see Section 3.2.3).
4.2.1 L2C signal acquisition
The L2C signal has advantages in interference mitigation due to its advanced PRN code
format. This signal is composed of two codes, namely L2 CM and L2 CL. The L2 CM
code is 20-ms long containing 10230 chips; while the L2 CL code has a period of 1.5 s with
767250 chips. The CM code is modulo-2 added to data (i.e. it modulates the data) and the
resultant sequence of chips is time-multiplexed (TM) with CL code on a chip-by-chip basis.
The individual CM and CL codes are clocked at 511.5 kHz while the composite L2C code has
a frequency of 1.023 MHz. Code boundaries of CM and CL are aligned and each CL period
contains exactly 75 CM periods. This TM L2C sequence modulates the L2 (1227.6 MHz) carrier
(GPS-IS, 2006). The original L2C data rate is 25 bps but a half rate convolutional encoder is
employed to transmit the data at 50 sps. Consequently, each data symbol matches the CM
period of 20 ms.
With these specifications, the common signal representation in (1) is changed to
]=
√
2
C
[
{
[
]
[
+
τ
]+
c
l
[
+
τ
+
]
}
[
π
(
f
IF
+
f
D
)
nT
S
+
ϕ
]+
n
W
[
]
r
n
d
n
c
m
n
n
kP
cos
2
n
(42)
[
]
and
c
l
[
]
where
c
m
is the
received signal delay;
P
refers to the number of samples in a full CM code period (i.e. 20 ms),
0
n
n
are the received CM and CL codes respectively (samples);
θ
≤
k
≤
74 is an integer that gives the CL code delay relative to CM code.
Fig. 7 shows an architecture of the partial acquisition suitable for L2C CM signal. A segmented
matched filter (MF) is used as a correlator (Dodds & Moher, 1995; Persson et al., 2001). The
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