Global Positioning System Reference
In-Depth Information
the walking of an individual based on the detection of some very specific instances,
such as the precise time the foot touches the ground. Then, the method consists in
counting the number of footsteps. These approaches are not yet mature for mass-market
applications but research is still being carried out.
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GNSS based systems. In addition to Assisted-GNSS, which is once again not a solution
for ubiquitous positioning, the following sections will deal specifically with this
problem. Various approaches have been proposed with rather good accuracy results:
the remaining problem is clearly the need for an additional infrastructure that needs to
be deployed locally. Operators are not ready for this and although very good results are
reported, very few systems are really available.
The last category is related to sensor networks. Many systems have been proposed in the
last fifteen years, but the lack of standardisation and the high number of sensors that need to
be deployed are currently a real drawback.
1.4 The perceived and real needs
If we take a little break to try to analyze the needs (i.e. requirements) for the continuity of
service definition, it will quickly become apparent that it greatly depends on the targeted
applications and services. But if you ask anybody, the answer will very often be given in
terms of positioning accuracy, availability and latency: it should be accurate to better than
one meter, available everywhere and instantaneously in real-time. Curiously, the fact that it
should be available in three dimensions will almost never be mentioned. Although it really
depends on the application (the requirement is not the same for the guidance of a robot in a
nuclear reactor and for finding the nearest restaurant), one should be able to distinguish
between the positioning “engine” and the resulting services. For instance, GPS does not
provide a one meter positioning everywhere, even outdoors, but car navigation systems are
very accurate for the delivered service, thanks to map matching and Kalman filtering. The
same should apply to ubiquitous positioning. Nevertheless, a good rule of thumb could be
to consider that the major difference, in terms of environments, between outdoors and
indoors is that indoors is typically a 3D environment, thus requires full 3D positioning
capabilities. In that sense, the accuracy should probably be enough to allow the floor level to
be determined, i.e. an accuracy of typically half the height of a given floor. In most buildings
this means roughly one meter.
Following this general presentation of the indoor field, this chapter is going to focus on
radio positioning solutions, and more specifically on GNSS-based radio approaches. The
second paragraph is dedicated to an introduction to radio positioning. It is followed by
three paragraphs dedicated to GNSS-based architectures: pseudolites, repeaters and
repealites. The chapter ends with a synthesis and some hints for the possible future, as seen
by the author.
2. The concepts of indoor positioning using radio transmitters
Not all the techniques proposed have, of course, been based on radio techniques, but they
are the most important ones for two main reasons: their level of development and maturity
on the one hand and their ability to “cross” or to “get around” obstacles such as walls,
furniture or people on the other hand. Optical based techniques, like laser based distance
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