Global Positioning System Reference
In-Depth Information
Calculations are then carried out at the master receiver and synchronisation values are sent
back, through wires, to the pseudolites. Another way consists in transmitting these
synchronisation data through a wireless link, leading this time to latency problems and
potential interference (but this is an interesting approach). In addition to this concept, one
imagined working the other way round by placing the receiver (which listens to the signals)
in the same place as the pseudolites. By considering that one (or several) pseudolites are
“pilot(s)”, the receiver can synchronise its own pseudolite if it knows the distance(s) that
separate(s) it from the pilot(s) pseudolite(s). The difference between received times for two
different pseudolites (indeed the associated receivers) allows the synchronisation of the
pseudolites. This once again requires data links. Of course, these solutions are clearly
adding cost and complexity to the system.
Another simple approach consisted in locating pseudolites in places where the GNSS signals
are available, namely outdoors, and to use the constellation time to synchronise the
transmitters.
Code and carrier phase measurements
are possible. In the first case, code phase measurements
are carried out: the positioning accuracy of the pseudolites needs to be in the range of a few
decimetres. The resulting positioning is intended to reach a few meters, as outdoors. Note
that multipath are bound to largely degrade this very optimistic goal (discussion follows).
The other approach described is based on carrier phase measurements (Kee et al. 2001, Rizos
et al. 2003). We know that this kind of measurement is much more accurate but suffers from
the ambiguity resolution problem. Nevertheless performances reported are in the range of a
few centimetres
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: the requirement in terms of pseudolite location accuracy is also increased
to typically one centimetre (this task is not so easy to carry out).
Ambiguity
is no longer such a difficult problem. In the case of code phase measurements,
ambiguity is totally suppressed since indoor distances are much smaller than three hundred
kilometres. In the case of carrier phase, ambiguity is still present but is not so high: typically
fifty metres for indoor distances, the carrier phase ambiguity for frequency L1 is around 260.
Current works are evaluating the possibility to use classical code phase ambiguity
resolution methods for the carrier phase resolution indoors.
A
potential accuracy of a few centimetres
is achievable with the carrier phase approach, even if
these measurements are probably not the most important ones for the foreseen applications
looking forward to the continuity of the positioning for mobile phones. Nevertheless this is
significant of the capabilities of the principles.
Near-Far effect
is a new propagation concern (Madhani et al. 2003). Since the deployment
complexity of the pseudolites must be reduced, their number should be reduced to a
minimum. As a corollary, the distance between pseudolites should be increased to a
maximum. Unfortunately, the Pseudo Random Noise (PRN) codes used in the case of GPS,
for instance, have auto correlation functions that present some secondary peaks. These
secondary peaks can have amplitudes of about -24 decibels (dB) in comparison to the main
peak. This is very good for outdoors where the difference of distances from various satellites
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Techniques similar to high accuracy methods for outdoors are used together with the associated
problems such as the determination of the initial location.
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