Successive generations of mobile telephony networks (GSM, GPRS and the forthcoming UMTS) do not just provide higher and higher bandwidths. They also offer a precise determination of a subscriber's geographical location.

This capability, known as geo-location, opens the way for personalised applications such as route planning, comparative journey-time calculations, local entertainment, information and services such as weather forecasts, and multimodal guidance (pedestrian, public transport or private car).

It also enables advertisers to target users through local promotions. City guides have already begun to offer this kind of "right-time, right-place" service to mobile internet users. "Geocentric" (sometimes called "contextual") information is also highly appealing to direct marketing operators.

Interactive content providers (for the internet, Wap, SMS and the recent Openwave browser) are also excited by the potential of geo-location. There are four automatic geo-location technologies, which vary in precision and cost. With the less intrusive, manual alternative, mobile subscribers enter the postcode for their location and then receive local information and services.

Cell-ID: precision from a few kilometres to a few hundred metres
Cell-based location is already used by some mobile operators. A mobile network is formed from a vast mesh of thousands of radio cells (a little like a fishing net cast over the geographical coverage area) and a mobile operator can identify which cell a mobile phone is in at any given time. This is the least costly geo-location system for an operator, even if precision is at best a few hundred metres. Precision also varies with cell size, which tends to be smaller in cities and larger in rural areas.

Triangulation: precision from hundreds to tens of metres
This technique locates the mobile phone by means of data from three radio relays, known as base transceiver stations (BTS), which change as the subscriber changes location. Triangulation is already used for "handover" - ie the constant switching of a subscriber phone to the BTS offering the best signal conditions at any given time. It is capable of tracking the mobile subscriber permanently with a precision of a few tens of metres.

GPS (Global Positioning System): precision from tens of metres to a few metres
Satellite-based GPS location is increasingly used on certain types of vehicles (taxis, delivery lorries, boats, luxury cars) and could well make an appearance on the mobile internet devices of the future, provided they are fitted with the required chip. GPS works by picking up signals from orbiting satellites, then defining location by measuring the signal transit time.

The range of precision can reach 10 or 20 metres, which can be further improved by the mobile network itself (assisted GPS). This technology is costly, and performance can suffer considerably from the presence of buildings, especially in a dense urban environment. But with the emergence of web-derived geographical descriptions such as GML (Geographical Markup Language), GPS could become an attractive proposition.

EOTD (Enhanced Observed Time Difference): precision from several hundred metres to tens of metres
With EOTD technology, a mobile device determines its position by sending signals to the closest radio stations and calculating the time taken for a return signal. Precision ranges from hundreds down to tens of metres. A more costly technique involves a similar calculation but with the signal being emitted by an intelligent cell itself.

Glossary

GSM - Global System for Mobile Communications

GPRS - General Packet Radio Service

UMTS - Universal Mobile Telephone Service

Wap - Wireless Application Protocol

SMS - Short Messaging Service

Email Charles de Laubier at claubier@lesechos.fr