Mobile tracking devices on trial: "Your mobile phone is a beacon - a radio transmitter in a box. Therefore it is possible to trace the signal and work out where it is.
There are now several web companies which will track your friends' and family's phones for you, so you always know where they are.
But just how safe is it to make location details available online?
There are several reasons why you may want to track someone. You may be a company wanting to keep tabs on employees during work hours, or a parent wanting to check up on a child's whereabouts.
These sorts of tracking services, now available in the UK, get information from the network about which cell your phone is currently in, and, for a small fee, display the location on an online map.
As well as checking where a certain phone is right now, you can run scheduled lookups, or snail trails, to record the phone's movements throughout the day, and produce a report for you to peruse at your leisure.
Obviously you cannot just enter any mobile phone number and expect to track someone.
First of all you need to prove your identity, via a credit card, and then, crucially, the owner of the phone in question needs to consent to being tracked.
The owner is sent a text message telling them about the tracking request, to which they must reply.
Experiment
The question is: is it possible to circumvent this security, and track someone without their knowledge?
I attempted to find out, using regular contributor Guy Kewney, an independent technology journalist and, for one day only, human guinea pig.

10:13 - Guy was at Westminster
I sent him on a tour of London. He could go anywhere he wanted, and I planned to meet up with him later and tell him, hopefully, where he had "