15th February - Users of social network sites like Facebook will soon be sharing their exact whereabouts with their friends in real-time, owing to new technology that uses the mobile phone as a tracking device, experts say.

A new social network site, Gypsii, which launched this week at a phone industry trade show in Barcelona, allows users to search for and identify their contacts, be they in the neighbouring street or across the ocean.

"Look, that's Bill, my friend in New York. He's 3,800 miles away," explained Toon Wee, a director of Gypsii, as he pointed to a coloured pin in the middle of a map of Manhattan while explaining the product at the Mobile World Congress in Spain.

The software, available for the moment on only top-end phones, enables users to find people, places and events in their local area, as well as share photos, videos and audio files with their friends.

"The real time location-based element of Gypsii adds a new dimension to the social networking phenomenon," said the founder and chief executive of the company, Dan Harple.

Gypsii is compatible with other big social networking sites, allowing the core location-specific functionality to be transferred to a user's Facebook page, showing their friends' and their own location.

Dan Winterbottom, an analyst for IT research group Informa, says users of social networking sites are likely to be attracted by the idea of seeing where their friends are.

He accepts however that others will shiver at the thought of being watched or tracked by anyone, even (or perhaps above all) by their loved ones.

"When location-based services become most interesting is for social networking, the idea that you are able to add content and context of where you've been and where you are," he said.

"Only a small slice of the market would be happy to share information of this kind, but then, when you look at Facebook, you've already got people putting their address, telephone number, photos and other highly personal information there."

The location of a mobile phone can be identified in two ways. The first is via a GPS chip, which allows a device to recognise its position based on communication with a constellation of satellites around the planet.

The second is by triangulation. A phone sends signals to communication towers located around it and by measuring the speed with which signals travel to these different base stations a position can be determined.

The ease with which phones can be located -- to within 25-50 metres of their position, say experts -- has sparked a wave of innovation in the mobile phone industry.

Operators and application designers are touting services that range from personal navigation, to locating children and fighting terrorism at this year's event in Barcelona, one of the mobile industry's biggest trade shows.

Nokia said this week that its next generation of digital maps would give real-time walking directions on the mobile phone screen, just like sat-nav systems which guide drivers.

Location technology could also be a powerful tool for advertisers, allowing them to send publicity to a potential customer when they are in a specific location.

Singapore's mobile network operator M1 is experimenting with sending text messages to its more than one million subscribers when they come within 100 metres of a shopping district in the centre of the city.

The company says it sends 200,000-300,000 messages a day with what it claims are valuable promotions from more than 40 shops, restaurants and bars in and around the Orchard Street commercial centre.

"The key is that the message has to be for a tangible benefit," said chief executive Neil Montefiore. "We did a two-month trial (before rolling out the service) and we got absolutely no complaints."

"One of the most successful advertisers has been an ice-cream stand," he added.

Users can put a block on the adverts by replying to an SMS. M1 says it never serves more than two messages to the same user in a single day.

Brian Varano, a market analyst at TruePosition, a company that provides location technology, said that location-based advertising also offered opportunities for search engines such as Google.

"If I do a local search and want to find the closest book store, the search might give me five book stores in the area, but it might be Barnes and Nobles at the top because they've paid for the top spot," he said.

His company also provides solutions to national spy agencies to help track suspected terrorists and also does "forensic work" in which it conducts investigations after a mobile phone has been used to detonate explosives remotely.

Underlining the interest from social networking sites, he said TruePosition was in talks with several operators, which he declined to name.

Location technology is likely to be seen as a dangerous big brother-style surveillance tool by some and an ingenious way to find friends by others.

Many here think it is set to grow and that applications for it, whether for businesses or consumers, are only beginning to be discovered.