Global Positioning System Reference
In-Depth Information
User #1
Transmitters
User #2
Fig. 4. The cell-id approach
2.2 Main differences with outdoor techniques
Let us come back to the specific case of indoors: some major differences have to be kept in
mind in comparison with outdoors. Let us also discuss the case of GNSS since this chapter is
dedicated to indoor GNSS-based solutions. First of all, the various techniques are based on
time of fight measurements, the same as outdoors, but consider the following parameters for
discussion.
•
Propagation environments
: indoors is a very difficult environment and acceptable models
are not available. This means that signal processing must solve problems that are either
not present, or less difficult to solve, outdoors, the most challenging being multipath.
Another problem is related to the possibility of Non Line of Sight (NLOS) path from
transmitters to receiver, which happens more often than outdoors. The same kind of
techniques could be envisaged but outdoors they are usually based on a certain
redundancy of available signals, which is not the usual case indoors.
•
Dilution Of Precision (DOP)
: the geometrical distribution
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of the transmitters is a very
important point to consider when dealing with positioning systems that use distances
in order to carry out the calculation. Outdoors, for a location on earth, with GNSS for
instance, there is disequilibrium between the horizontal DOP (HDOP), calculated in
the horizontal plane, and the vertical DOP (VDOP), calculated in the vertical plane.
This discrepancy is due to the fact that when the distribution can be really uniform
horizontally (all the satellites being uniformly distributed around the receiver), leading
to a good HDOP, this distribution cannot be so good vertically since only satellites
above the radio horizon (which is quite similar to the geometrical horizon in the
present case) are visible. Thus, the HDOP is usually better than the VDOP. Indoors,
things are quite different since one can decide the location of the transmitters: it is very
important to locate at least one transmitter below the receiver in order to reduce the
VDOP (Vervisch-Picois and Samama 2006). Evaluations have shown a dramatic
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This DOP allows the receiver to give a real-time estimation of the accuracy provided to the user (the
User Estimated Range Error in GPS for example): it is of uppermost importance for any application or
service.
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