Global Positioning System Reference
In-Depth Information
Operators stand to lose revenue if the system is prone to failure, cheating, and
poor enforcement.
Very accurate and exacting point positioning in real time is needed (<1m at
barriers but otherwise <10m is adequate). Low latency is required. Multiple
technologies can be converged, including existing RFID technology, smart card
(and virtual smart card) solutions, cellular positioning, and GNSS. There is a need
for a GIS and integrated timetable and fare schedules. Intelligence software is
needed to predict problems for individual travelers in real time, to alert them using
various messaging methods, and to offer solutions and guidance services [6].
4.5 Emergency Calling
Before the advent of mobile networks, the plain old telephone service (POTS)
offered a very basic location service to emergency control centers. Although a
driver may have had to walk for some time to find a fixed telephone, at least when
the emergency call was made to the network operator's emergency call center, the
operator could see where the call was coming from geographically. It is worth
noting that (some) fixed communications systems can offer a very high level of
positional accuracy since the terminal positions are fixed and known.
Unfortunately, with the advent of Voice over IP (VoIP) telephony, the association
of line with terminal has been abandoned. VoIP phones (or PC applications) now
require an external Whereness service to restore the positioning functionality.
Digital mobile phone networks provide various levels of positional
information that can be associated with voice calls, messaging, or data services.
The most basic uses the identity of the base station handling the session. This Cell
ID can then be referenced to a known coverage area (perhaps a set of postal codes
or some other geo-fence). In Chapter 6, more complex arrangements will be
discussed but here is a description of Cell ID.
The operation of the underlying network mobility management system for
the dynamic allocation of radio channel resources requires that a database system
track each terminal's approximate location based on the local fixed station with
the best quality of service. As a terminal moves, the logical channel (handling the
call) is reassigned automatically during a handover process from cell to cell along
the approximate geographical track of the user. In some third-generation systems
this approach is somewhat more complex, whereby several base stations can be
handling the call concurrently and be involved with a “soft” handover process. In
general, however, it can be said that a particular geographic area represents the
coverage area or “cell” that has a unique identity, or ID. The size of the cell will
vary according to environment from hundreds of meters in dense urban areas to
tens of kilometers in very rural situations. Satellite cellular services may be even
more inaccurate, as spot beams illuminate hundreds of square kilometers. The
great advantage of Cell ID is that it is information that is virtually free to the
operator. Network signaling systems sometimes contain the information as a
 
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