Global Positioning System Reference
In-Depth Information
GNSS, had the objective to provide continuity of service with no additional infrastructure.
The simple underlying idea is that the signals are still present indoors, but even lower in the
noise than outdoors. Thus, if one is able to design a very highly sensitive receiver, it should
be possible to locate indoors. A similar, but not identical, idea led to the design of the so-
called Assisted-GNSS (Duffett-Smith and Rowe 2006). The initial goal was also to provide
indoor positioning by “aiding” the receiver to find the signals in difficult environments. In
such situations, one major problem with stand alone receivers is the impossibility of
decoding the navigation message (too long to envisage having good radio conditions for
such a long duration). Thus, a solution could be to send the navigation message through
telecommunication networks that are widely available indoors. Thus, knowing the message,
the receiver is able to use the high-sensitivity in order to acquire the GNSS signals and then
is able to calculate a position since all the parameters needed (from the navigation message)
are available. High sensitivity and assisted approaches are thus quite complementary.
Unfortunately, with a higher sensitivity, the receiver is now jammed with reflected signals
in such a large amount that positioning, although possible, is really bad because there is too
much interference. Thus, even if real improvements have been proposed in environments
where the signals were just at the detection limit, these approaches are clearly not the
ultimate solutions for indoor positioning and continuity of service. One has to move to
infrastructure-based techniques.
3.1 Technical historical introduction
In the early 1980s the first ideas of GPS-like signals transmitters arose from the
considerations of the obvious limitations of the original system. How to use a GPS receiver
when fewer than three or four satellites are available? What kind of approaches could be
imagined to position the Mars rover? How to improve the VDOP of the constellation in case
a good vertical accuracy is needed? Etc.
One answer could be to increase the number of satellites by a factor of two or three but the
associated cost for the relatively reduced increment in performance was judged to be non-
viable. One has to find another way. The idea of implementing GPS-like signal generators
that could be locally deployed came out: the pseudolites were born.
3.2 The concept of pseudo-satellites
A pseudolite (which comes from the contraction of pseudo and satellites) is a generator that
transmits GPS signals but which is not a satellite. Such a generator can easily be deployed
on earth in places where the number of visible satellites is too low to allow standard
positioning (Klein and Parkinson 1986). The first applications were thus naturally oriented
towards open cast mines for optimisation purposes. Indeed, as the mine is dug, the view of
the sky is reduced and the optimal number of satellites reduces. Adding a pseudolite allows
a continuity of the positioning service to the mine to be provided.
A similar idea was developed in the context of so-called Local Area Augmentation Systems
(LAAS) where the problem was to provide a good vertical accuracy to landing planes, for
example. We know that this vertical accuracy is linked to the VDOP and that locating a
satellite below the plane would greatly improve the VDOP. Since it is not possible, the use
of a pseudolite seems once again a good idea (Bartone and Van Graas 2000).
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