Global Positioning System Reference
In-Depth Information
1.3 The main radio based approaches
In terms of technologies for indoor positioning 1 , numerous candidates are almost available,
some of them being proposed as commercial products and solutions. A fundamental point
to understand is that one is always looking for a positioning system that is globally the
continuation of GPS in all environments, i.e. a few meters of accuracy, free for the users and
with no specific infrastructure to be deployed by any commercial operator. Hence the
various directions of works carried out in recent years: indoor GNSS through Assisted-
GNSS, although this is not a solution to the problem (see the first lines of this sections), WiFi
because one considers that the required infrastructure will be deployed anyway for
telecommunication purposes 2 and inertial approaches that really don't need any specific
infrastructure. The accuracy being sought eliminates candidates such as the GSM (Global
System for Mobile) or UMTS (Universal Mobile Telecommunications System), whatever the
technique envisaged.
Among a few others, it is possible to list the following global categories:
Wireless Local Area Networks (WLANs, such as WiFi) or Wireless Personal Area
Networks (WPANs, such as UWB or Bluetooth) based: the main idea is to use these
telecommunication networks for positioning purposes. The main problems for
translating the GNSS time of flight measurements lie in the non-synchronised nature of
these networks and the complexity of the indoor propagation environment. Thus, the
usually implemented technique is based on so-called fingerprinting, described in the
next section. An exception to this rule is the Ultra Wide Band that fundamentally works
in the time domain, thus could potentially allow us to carry out time measurements.
Technological developments are still on-going and initial promises have not yet been
met.
Wireless Mobile Networks (such as GSM or UMTS). The use of mobile networks leads
to the same basic difficulty as WLAN or WPAN. Although non-synchronisation is a
problem, propagation characteristics are probably the largest difficulty. Performances
are not at a sufficient level in order to allow a real continuity with outdoor GNSS.
Nevertheless, some services are available which implement the so-called Cell-Id
(Identification of the telecom Cell the mobile is associated with). This technique allows a
mobile terminal to know the area it is in by analysing the base station it is associated
with. The accuracy is rather poor, ranging from a few hundreds of meters in densely
populated areas to several kilometres.
Inertial systems have typically three problems: time related shift of the accuracy,
distance related shift of the accuracy and the cost of the terminal. Recent smart phones
have embedded inertial sensors but positioning remains a challenge. Nowadays,
techniques are mainly oriented in two directions: integration of the measurements
provided by the sensors (accelerometers, gyrometers and magnetometers) or modelling
1 Note that indoor positioning is seen as the ultimate difficulty in order to cope with ubiquity since this
seems to include all the most difficult phenomena. This is of course not the only environment where
GNSS are not very efficient: so-called urban canyons are also important to be dealt with. Nevertheless,
the topic of this chapter is clearly limited to indoor techniques.
2 This assertion is not 100% right with current proposed solutions since it is almost always necessary to
distribute additional access points to existing networks in order to create the required redundancy.
Search WWH ::




Custom Search