Global Positioning System Reference
In-Depth Information
The mentioned A-GNSS specifications, basically define the procedures for requesting and
sending information on user position and assistance data.
These are typically of two
paradigms:
•
Mobile-based: assistance data are provided to the User Equipment (UE), which measures
the pseudo-ranges and provides the position estimation to the proper network service.
•
Mobile-assisted: the UE measures the pseudoranges and sends them to a location server
which performs the positioning and service-related tasks.
In both these two modes, the position estimation may benefit of the knowledge of additional
information available to the location server gathered from one or more reference receivers (e.g.
differential corrections, precise timing and ephemeris, etc.). In the followings, the challenges
of the external aiding approach are discussed. It should be noted that the chosen signal for
analyses is GPS L1 C/A.
3.3.1 Navigation data wipe-off
The typical effects of both the data wipe-off and non-removed bit transitions are in Fig. 4(a)
and Fig. 4(b) respectively. In the first case, the main correlation peak is easily identified whilst
in the other one no peak can be distinguished over the floor. Under the AWGN assumption, in
(a)
(b)
Fig. 4. CAF along code-phase - simulated GPS L1 C/A signal with code-phase of 500
μ
s,
C
/
N
0
= 24 dB-Hz and
T
int
= 1 s: (a) with data wipe-off; (b) without data wipe-off
the correct Doppler and code-phase bins
α
mean
is theoretically proportional to post-correlation
Signal-to-Noise Ratio (SNR) and it is expected to increase by 3dB when
T
int
doubles. This can
be seen in Fig. 5, where we show the effect on
α
mean
and
α
max
of a coherent correlation with
=
24 dB-Hz and
T
int
=
{
}
C
/
N
0
ms performed on simulated GPS signals, both
with and without data wipe-off. In Fig. 5(a), we observe that at the highest values of
C
/
N
0
the
peak-to-floor ratios change linearly, i.e.
100, 500, 1000
α
mean
increases by 3 dB when
T
int
doubles (e.g. from
500 ms to 1000 ms). In this case,
R
peak
is the correct correlation peak. At the lowest
C
/
N
0
,
α
max
2
2
is practically 0 dB and the detected peak is likely a noise peak, thus
|
R
peak
|
≈
max
|
R
floor
|
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