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detectors. The capacitive field sensing is used for the implementation of the
Touch Mouse [10] and for Contact which is a system for exchanging data
between objects in direct contact with the human skin [11].
Monitoring of access points in a cellular network . This method takes into
account the presence of a mobile device within the coverage of one or more
AP in a cellular network. Examples of implementation of this technique are
the Active Badge [12] and Xerox ParcTAB [13], which use infrared cells in
an office environment, and the Carnegie Wireless Andrew system [14],
which uses a wireless radio cellular network.
Observation of automatic identification systems . This detection method
is based on the proximity to automatic identification systems such as
credit cards readers, telephone records, access records to computers and
RFID tags [15]. If the scan, query or monitor device is at a known
position, then the position of the mobile object can be inferred.
Properties and features of positioning systems
In this section, we will present some properties and features that are often used
to characterize and evaluate the positioning systems' performance and usefulness.
3.1 Physical vs. symbolic position
A positioning system can provide people with two types of information about
their position: physical and symbolic. The GPS system, for example, provides
physical information (or geometric) by means of latitude, longitude and altitude
of a given place. On the other hand, a symbolic positioning system provides an
abstract information about the position of a mobile object: 'in the living room',
'in Kuala Lumpur', 'at the Central Station', etc.
A system that provides physical information about a place can be
enriched to provide information about a symbolic position too. For example,
a system equipped with GPS can access a database which contains symbolic
information for each physical location, as for instance in the automotive
navigation systems. For this reason single abstract models for both
representations have been studied, such as the one in [16]. The result of the
integration is a hybrid, semi-symbolic model that decouples application
representation from the sensor representation of location information. The
hybrid model can accept location data in both forms and all data in the model
can be viewed from both perspectives (Figure 4).
The accuracy of a physical positioning system may have implications
over a symbolic information provisioning system. In fact, if a physical
positioning system estimates a position with an error of 20 m, the symbolic
position within a building can be wrong if rooms are smaller than 20 m.
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