There are basically two different kinds of GPS trackers for dogs. The more popular design uses a GPS tracker, which is attached to your dog’s collar, and requires a monthly subscription plan to actually track your dog’s location. Many of these allow you to place a virtual boundary around your yard, and if your dog leaves this area, a text or email alert is sent to you. If your dog goes missing, you can track their location with texts to your cell phone or maps on a smartphone or computer.
The other design uses a GPS tracker, which is usually built into a specially designed collar, and has a portable receiver. The receiver provides you with either a map (e.g. Garmin Astro) or just an arrow with distance (e.g. RoamEO), and you use this to track down your dog. The receivers have a maximum distance of 7-9 miles and perhaps a minimum distance of 1 mile. No activation fee or subscription is necessary. The shortcomings of this design are the limited range and short battery life, and you always have to carry the receiver on you to find your dog.
There are a few companies in the UK such as PawTrax, Snooper and Loc8tor Pet that have developed a pay per use GPS tracker. These are similar in design to the GPS trackers that require a monthly fee, but instead you prepay for a certain number of locations. PawTrax may now be available for purchase in the US. Email firstname.lastname@example.org for more information.
For a more complete list of all the GPS trackers currently available for dogs and cats, check out this Comparison of GPS Pet Trackers. If you know of any that I have missed, please let me know. In February I purchased Tagg the Pet Tracker for my own dog, and I plan to write a review on this particular tracker. Overall, I am happy with the purchase and it seems to work quite well though I have fortunately never had the chance to try it out for real.