Global Positioning System Reference
In-Depth Information
indoor environments, the difficulty comes from the fact that the propagation is characterised
by a large number of reflected path (from walls and all reflective objects), called multipath.
Those multipath are even sometimes more powerful than the direct signal, which can in
turn also be absent. Thus, even if the angle measurement is accurate, the environmental
conditions are likely to mislead the positioning algorithms.
North
Transmitter #1
Transmitter #1
North
Transmitter #2
Transmitter #2
Transmitter #3
Line L1
Line L2
Line L3
Line L1
Line L2
Fig. 1. Angle measurements
The second technique is called
fingerprinting
. The first idea of this method was reported
around the sixteenth century when a solution to the longitude problem was being sought.
Some scientists had the idea to make a complete geographical cartography of the magnetic
field of the earth: if there is a unique link between the location on earth and the value of the
magnetic field, then one can consider that the magnetic field value is a perfect indicator for
finding a location. Unfortunately, the magnetic field is not a good candidate for such a
purpose. This idea came back to engineers with the development of wireless networks: the
complexity of the indoor environment for propagation led to the revival of the
fingerprinting approach: the received power of the radio signal is now the physical value
that is measured. The indoor environment is then cut into squares and the fingerprints (the
received power) measured at each intersection of the grid (see figure 2): the “map”
associated with transmitter #1 (a data base indeed) is created. The problem is now that
many different fingerprints are identical for different locations. The method of multiple
measurements is once again implemented: in this case, a second (and more, if required)
transmitter is added and a second map is filled in. The location is no longer characterised by
a single value but now by a couple of values. In the case of n transmitters, then all calibrated
locations are characterised by a vector of length n.
The way in which positioning is then achieved in real-time is quite simple: the mobile
terminal carries out received power measurements from all the “radio visible” transmitters
in its environment and fills in its own vector. The location is obtained by finding the nearest
neighbour in the complete set of maps (data bases) available. The need for this “calibration”
phase is clearly a drawback of the method because it is time consuming and, moreover,
because it is not a stable operating mode, since the power received is bound to be modified
by any movement of any obstacle (including people for instance). Thus, techniques have
been proposed in order to manage in real-time (or for longer periods of time) the variation
of the maps in comparison with the reference maps. Note also that more measurements
should lead to a more accurate positioning.
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