Global Positioning System Reference
In-Depth Information
3. High sensitivity acquisition problems
3.1 Acquisition in harsh environments
The conventional acquisition stage in Fig. 1 is designed to work in open-sky conditions.
However, in harsh environments, high sensitivity (HS) acquisition strategies are required.
In principle, as a nature of DS-CDMA, the longer the coherent integration time (
T
int
) between
the local and the received signals is, the better the de-spreading gain (i.e. signal-to-noise ratio
improvement) that can be obtained after the correlation process. However, the presence of
unknown data bit transitions limits the value of
T
int
≤
≤
20 ms as for GPS
L1 C/A signal) to avoid the correlation loss. This limitation is only neglected if there is an
external-aiding source, which provides the data transition information.
T
b
(e.g.
T
int
The sensitivity improvement obtained by increasing
T
int
is traded-off with an increased
computational complexity. As pointed out in Section 2.2, the size of the Doppler step (
f
D
)
reduces as
T
int
becomes larger and this fact increases the search-space size. Furthermore, the
instability of the receiver clock causes difficulties for the acquisition stage, especially if
T
int
is large, because of the carrier and code Doppler effects. Therefore, one should consider the
trade-off between the sensitivity improvement and the complexity increase when changing
the value of
T
int
.
Considering
the
availability
of
external-aiding
sources
and
the
trade-off
between
the
sensitivity and the complexity, the HS strategies can be divided into:
•
Stand-alone approach (to deal with light harsh environments, e.g. light indoor)
•
External-aiding approach (to deal with harsh environments, e.g. indoor).
Modern GNSSes broadcast new civil signals on different frequency bands and the new GNSS
signals embed the combination of the data channel and a pilot (data-less) channel, per carrier
frequency. Examples are E1 OS, E5, E6 signals of Galileo and L5, L2C, L1C signals of GPS.
All these facts make possible another approach designed to provide improved acquisition
sensitivity:
•
Channel combining acquisition approach.
These three approaches are presented in details in the following.
3.2 Stand-alone approach for light harsh enviroments
Without the availability of external aiding sources,
the strategies of this approach use
≤
T
int
T
b
. The sensitivity obtained at a specific value of
T
int
is improved by combining
the correlator outputs in different ways: coherent, non-coherent and differential combining.
These techniques are referred as post-correlation combination techniques.
3.2.1 Coherent combination
For each cell (
c
,
ˆ
,
f
D
) of the search-space,
M
correlator outputs
R
1
,
R
2
, ...,
R
m
, ...,
R
M
}
obtained by correlating the incoming and the local signals at length
T
int
,
θ
{
see (5),
are
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