Global Positioning System Reference
In-Depth Information
combination. In fact, the coherent combination technique is equivalent to the
T
int
extension
with the advantage that in this stand-alone scenario
T
int
≤
T
b
. The squaring loss (Choi
et al., 2002) caused by the non-coherent combination makes this technique less competitive
than the others. However, its simplicity and moderate complexity make it suitable for
conventional GNSS receivers. Among the three techniques, the differential combination
can be considered as a solution trading-off sensitivity and complexity of an acquisition
stage (Schmid & Neubauer, 2004; Zarrabizadeh & Sousa, 1997). As an expanded view of
the conventional differential combination technique, generalized differential combination is
introduced for further sensitivity improvement (Corazza & Pedone, 2007; Shanmugam et al.,
2007; Ta et al., 2012).
In addition, modern GNSSes broadcast new civil signals on different frequency bands.
Moreover, these new signals are composed of two channels, namely data and pilot (data-less)
channels (e.g. Galileo E1 OS, E5, E6; GPS L5, L2C, L1C). These facts yield another approach,
usually named
channel combining acquisition
(Gernot et al., 2008; Mattos, 2005; Ta et al.,
2010) able to fully exploit the potential of modern navigation signals for sake of sensitivity
improvement.
This topic chapter strives to identify the issues related to HS signal acquisition and also to
introduce in details possible approaches to solve such problems. The remainder of the chapter
is organized as follows. Section 2 presents fundamentals of signal acquisition including
the common representation of the received signal, the conventional acquisition process.
Furthermore, definition of the the performance parameters, in terms of detection probabilities
and mean acquisition time are provided. HS acquisition issues and general solutions, namely
stand-alone, external-aiding and channel combining approaches, are introduced in Section
3. In Section 4, the stand-alone generalized differential combination technique is presented
together with its application to GPS L2C signal in order to show the advantages of such
a technique. Section 5 focuses on introducing a test-bed architecture as an example of the
external-aiding signal acquisition. The channel combining approach via joint data/pilot signal
acquisition strategies for Galileo E1 OS signal is introduced in Section 6.
Eventually, some
concluding remarks are drawn.
2. Fundamentals of signal acquisition
2.1 Received signal representation
The received signal after the Analog to Digital Converter in a Direct Sequence Code Division
Multiple Access (DS-CDMA) GNSS system can be represented as
]=
√
2
Cd
r
[
n
[
n
]
c
[
n
+
τ
]
cos
(
2
π
(
f
IF
+
f
D
)
nT
S
+
ϕ
)+
n
W
[
n
]
(1)
where
C
is the carrier power (W);
d
is the spreading code,
f
IF
,
f
D
denote the Intermediate Frequency (IF) and Doppler shift (Hz) respectively;
T
S
[
n
]
is the navigation data;
c
[
n
]
=
1/
F
S
stands
for the sampling period (s) (
F
S
is the sampling frequency (Hz));
ϕ
is the initial carrier phase
(rad);
is the initial code delay (samples) ; and
n
W
is the Additive White Gaussian Noise
(AWGN) with zero mean (
τ
n
(
n
W
n
μ
=
0) and variance
σ
∼N
(
0,
σ
)
).
In fact, most of the current and foreseen signals of GNSSes use either BPSK or BOC
modulations (Ta, 2010). For these modulations,
c
[
]
n
has the representation as follows:
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