Tracking with GSM
(where GSM is the primary means of location! not just for communications)This type of tracking is possible to a degree, but it must be stressed that it has its limitations compared to using GPS.
GSM tracking is far less accurate than GPS tracking but it is invariably more cost effective if you would just like to know the "general" location of an asset, rather than its exact location. It will work anywhere that you have GSM coverage, and it will penetrate most buildings by as much as your own mobile phone will.
It works by measuring the signal strength, or more importantly the quality of it, from all of the GSM cell towers that it can hear nearby and by comparing this data with their known locations** it can then predict a general area of say between 300m to 2km depending upon the distance between them and the number of towers within range.
Not bad if you just want to know that someone you care about is in the town centre for instance or on their way home.
WARNING:
It will NEVER provide you with a pinpoint position, no matter what other websites may claim and should never be relied upon solely as a means of protection (ie for tracking your loved ones or for lone workers)
However, it is very cost effective as it requires no other equipment other than a mobile phone (or simlar GSM product) and the internet. Of course, if you are using a mobile phone to legitimately and legally track someone, you could always just phone it and simply ask them where they are?
**of course, this service is only available if the local mobile networks allow you access to this Cell tower location database. In the UK most networks will do this but the further afield you go, the more reluctant GSM networks seem to be to hand over this data.