Global Positioning System Reference
In-Depth Information
sampling clock, the sampling timescale can be defined as:
x
LO
n
f
S
n
f
S
+
n
f
S
+
y
0
n
f
S
(
)
f
S
=
=
+
t
S
s
x
0
(33)
n
=
t
=
n
0, 1, 2, ...
where
n
/
f
S
is the ideal sampling instant and
f
S
is the sampling frequency.
The sampled
version of the IF signal is:
r
n
y
0
n
f
S
r
[
n
]=
f
S
+
x
0
+
(34)
and it is affected by a time-variant delay with respect to the ideal case. This gives rise to an
equivalent Doppler effect, as previously discussed, and hence to an additional correlation loss.
Oscillators typically used in GNSS receivers are mostly Crystal Oscillators (XOs) with some
degree of frequency stabilization, e.g.
Thermally-Compensated Crystal Oscillator (TCXO),
10
−
6
∼
with typical accuracy
y
LO
and Oven-Controlled Crystal Oscillator (OCXO), with
10
−
8
typical accuracy
y
LO
∼
(Vig, 2005).
Table 2 shows how a constant offset on the LO
frequency may impact both on
α
mean
,
α
max
and on the accuracy of the code-delay estimation
in case ofa1scoherent integration.
f
D
/
f
LO
α
max
(
dB
)
α
mean
(dB) Code-phase error (chips)
0
22
31
0
10
−
6
0.5
·
18
31
0.75
10
−
6
·
1.5
0
27
1.5
Table 2. Constant offset on LO frequency.
T
int
=
1s,
C
/
N
0
=+
∞
3.4 Channel combining approach:
•
Channel Combining on Different Carrier Frequencies
In a new or upgraded GNSS, there are several civil signals broadcast in different frequencies.
This fact assures a future for civil GNSS dual-frequency receivers, which are now used only
in high-value professional or commercial applications such as survey, machine control and
guidance, etc. Beside the predictable advantages, such as ionosphere error elimination and
carrier phase measurement improvement, civil dual-frequency receivers also offer sensitivity
improvement by making possible combined acquisition strategies. The combined acquisition
on different carrier frequencies is guaranteed by the fact that the signal channels belonging to
a common GNSS are time synchronized, and the Doppler shifts of these channels are related
by the ratio among the carrier frequencies. In literature, (Gernot et al., 2008) uses this approach
for combined acquisition of GPS L1 C/A and L2C signals.
•
Channel Combining on a Common Frequency:
New GNSS signals are composed of data and pilot (data-less) channels. These two channels
can be multiplexed by Coherent Adaptive Subcarrier Modulation (e.g. Galileo E1 OS), Time
Division Multiplexing (GPS L2C) and Quadrature Phase-Shift Keying (Galileo E5; GPS L5,
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