I am not too well informed about the problems of GPS tracking but if it is economical and technically feasible enough for lost dogs it should be very useful for exploration teams, all commercial aircraft, all emergency equipment on lifeboats. young children etc. Although it might be useful in military groups it might also inform a technically proficient enemy too well to be useful.

GPS is easy, and cheap, with the problem of lost signal in places.

The real work/expense is relaying the GPS data to the searcher, in either licensing radio spectrum and transmitters yourself, or purchasing an existing communication service and equipment (EG Cell provider).

There is a GPS Pet Tracking collar (actually it will fit on any collar). See http://www.gpstrackingsystem.net it is the 3rd item on the page. It is really nice and affordable. works anywhere in the US, Canada and Mexico.slater5

There are two recent products, Pocketfinder and Zoombak, that do this job. They cost $130 and $200 respectively. They were reviewed in the Seattle Times in May 2008 here:

http://seattletimes.nwsource.com/html/businesstechnology/2004435636_ptpetgps24.html