There are a number of reasons why it is useful to be able to pinpoint the position of a mobile telephone, some of which are described below. Location-Sensitive billing Different tariff can be provided depending upon the position of the cell phone. This allows the operator without a copper cable based PSTN to offer competitive rates for calls from home or office. Increased subscriber safety.
Description of Cellular Positioning
A significant number of emergency calls like US.911 are coming from cell phones, and in most of the cases the caller can not provide the accurate information about their position. As a real life example let us take the following incident. In February 1997 a person became stranded along a highway during a winter blizzard (Associated press 1997).
She used her cellular phone to call for help but could not provide her location due to white-out conditions. To identify the callers approximate position authorities asked her to tell them when she could hear the search plane flying above. From the time of her first call forty hours elapsed before a ground rescue team reached her. An automatic positioning system would have allowed rescuers to reach her far sooner.
There are a variety of ways in which position can be derived from the measurement of signals and these can be applied to any cellular system including GSM. The important measurements are the Time of Arrival (TOA), the Time Difference of Arrival (TDOA), the Angle of Arrival (AOA) and Carrier phase. All these measurement put the object to be positioned on a particular locus.
Multiple measurements give multiple loci and the point of their intersection gives the position. If the density of the base stations is such that more measurements can be done than required then a least square approach can be used. If the measurements are too few in number the loci will intersect at more than one point result in ambiguous position estimate. In the following discussion we assume that the mobile station and base station are lying in the same plane.
This is approximately true for most networks unless the geography include hilly topology or high rise buildings.