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position, the distance from four known satellites is needed. The larger the num-
ber of satellites in sight, the higher the accuracy of the computed GPS position.
The nominal accuracy of GPS position is 20 meters; however, by using more
advanced techniques, it is possible to obtain higher accuracy: under one meter
with ordinary differential GPS devices, and down to a few millimetres using
specially equipped receivers to detect the phase differences between distinct
satellite signals.
GSM
There are many ways of tracking GSM phones. The most basic one is to use the
call record data containing the IDs of the starting and ending cells associated
with the calls. In this case the uncertainty of the position is essentially due to
the nonspecificity of the spatial information: depending on the density of the
cellular network, the size of a cell could range from a hundred meters to some
kilometers. More advanced alternatives are common to other wireless networks
and are discussed later.
5.2.2 Generic Methods for Wireless Communication
RFID, Bluetooth, and WiFi, as well as GSM, are discussed together, based on
the assumption that the mobile device signal can be identified by some fixed
reference points, called anchors .
Range-Based Methods
This is the case of Bluetooth fixed receivers continuously querying nearby
objects, RFID readers, and WiFi access points, but also of GSM cells, in case
devices entering and exiting cells are logged. Once the coverage of an antenna
is known, it is possible to restrict the position of the object to an area. If there
is more than one anchor, it is possible to intersect the ranges to obtain a more
accurate position. In this case the uncertainty is determined by both the size and
the overlap of the antenna ranges. The denser the cells (as in metropolitan GSM
areas), the more accurate is the determined position. Obstacles, at worst, may
rule out some anchor even if it is in the proximity, thus losing an opportunity
for position refinement.
Range-Free Methods
These methods rely on information about the radio signal strength (RSSI)
received by the different anchors/antennas. The absolute value is not relevant,
since they are conceived to work with the ratio between RSSIs. One of these
methods, for example, is based on the computation of the centroid of the anchor
points weighted with the respective RSSIs, which is invariant if the proportion
of weights is preserved. The uncertainty is due to RSSI error propagation, and
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