Global Positioning System Reference
In-Depth Information
Let us now come back to real situations where several (many indeed) multipath are present.
In order to give an idea of such configurations, we consider the environment described in
figure 11 which is a large car park. The structure is made of metallic beams and concrete
walls. Cars are also modelled as red parallelepipeds. In figure 11 are also shown the path
from transmitters to a receiver that is located in the centre of the building. The black paths
are direct ones, while the green ones are reflected path: the conclusion is quite clear! In such
cases, one can easily imagine that the ACF is even more disturbed than the ones proposed in
figures 8 and 9.
3.6 The performances attainable
The preceding pages have shown that the only multipath problem is enough to disqualify
the pseudolite approach which uses code phase measurements. A few other multipath
mitigation techniques are potentially available, such as the Strobe Correlator or the
Double Delta correlator, but both require rather a high signal to noise ratio (SNR) in order
to properly function
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. This is not so easy to obtain indoors since reflected paths are
bound to reduce significantly the signal to noise ratio (by destructively combining the
electric fields). Thus, this kind of systems is not yet available with acceptable
performance.
The other possibility is to use carrier phase measurements that we know are less sensitive to
multipath (because the ambiguity is reduced to nineteen centimetres instead of three
hundred metres for code phase). Unfortunately, carrier phase based systems are more
complex to use in practice because they require both an initial location which is accurate to a
few decimetres and the carrier phase to be followed continuously, which is much more
difficult than to follow code phase. Such systems exist but are not widely deployed for these
additional reasons (in conjunction with the need for infrastructure).
3.7 Short synthesis
Pseudolite systems require an infrastructure deployment and synchronisation, and have to
cope with near-far and multipath but provide full continuity with the technical approach for
outdoors, GNSS, with only minor modifications to the receiver. This is a good candidate if
no solution without infrastructure can be found but the community of service and
application providers is not yet ready to accept such a solution, except in situations where
an installation cost is counterbalanced by already well identified revenues.
4. The first step in overcoming some pseudolite linked problems: The
repeaters
Following pseudolites, one tries to propose ameliorations to the main drawbacks (Im et al.
2006, Jee et al. 2004). Since it is based on transmitters, the infrastructure is still present, but
some approaches reduce its complexity by the introduction of the concept of a “common
signal” to all the transmitters (Caratori et al. 2002).
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The Narrow Correlator is the only one that does not degrade the SNR while improving the multipath
behaviour.
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