Global Positioning System Reference
In-Depth Information
Technique
Angle
measurements
Time of flight
measurements
System
Fingerprinting
Cell-Id
GNSS
9
WiFi
9
9
i
Bluetooth
9
UWB
9
GSM/UMTS
9
ii
9
iii
9
iv
RFID
9
v
TV
9
vi
Table 1. Summary of a few radio based positioning systems
i.
Since wireless local area networks are not synchronised, distance (and not time)
measurements are used. The distance is estimated through a power level measurement
and a model of propagation (typically modified Friis formulae where the power of the
distance is between 2.5 and 4 depending on the environment). This is not really accurate
and too dependent on the fluctuations of the environment.
ii.
Angle measurements are already carried out at base stations in order to allow the use of
the same frequency channel for transmissions in different directions. Thus, those
measurements are available, but the limitations discussed in previous sections still
apply.
iii.
These networks are not synchronised and mainly differences of time of flight have been
proposed (but direct times of flight have also been proposed). Unfortunately, the
propagation models are not well suitable and the best reported performance is around
one hundred metres outdoors, and can rise to a few hundreds of metres indoors.
iv.
Cell-Id is used by networks in order to route communications: once again, this is
already implemented in mobile networks because it is needed. The accuracy is typically
a few hundreds of metres but it is completely free and available. Many telecom
operators propose services based on GSM/UMTS cell-id positioning.
v.
Many definitions of RFID (Radio Frequency IDentification) are proposed: let us
consider this is a short range technology that allows two radio transmitters to exchange
data, and identification, for instance. A simple way to carry out positioning (but not the
only one) is to consider the cell-id model. The coverage area (or range) of a given
transmitter is approximately known: when a second transmitter can connect to it, then
it is located in the coverage area. In case of a very short range (say one metre or less),
the accuracy of the positioning is thus better than one metre. The consequence is that
the positioning is no longer a continuous process in space and time (as for GNSS for
example), but becomes typically discrete.
vi.
Television signal are available almost everywhere in modern countries: why not use
them in order to position a receiver? This idea was developed a few years ago and an
accuracy of around ten metres has been reported through time of flight measurement,
even indoors.
3. The first GNSS signal approach using pseudolites
Although this chapter is dedicated to infrastructure based GNSS systems, other solutions
have been investigated by the GNSS community. For instance, High Sensitivity GNSS, HS-
Search WWH ::
Custom Search