No one here has suggested the findster. I use a pair on my working terriers and they're small, lightweight, and perfect.
GPS Pet Tracker for Dogs and Cats - Free of Monthly Fees | Findster
They're much cheaper than the garmin alternative, the only problem is that you need a smart phone to pair the module with. These or your phone don't require mobile phone signal. The collars have GPS in and transmit this to a small guardian module you keep on you, this is turn uses bluetooth to tell your phone where the dogs are on a map. It's perfect for me and gives complete peace of mind. No subscription fee, no need to rely on being somewhere with network coverage and money left over to spend on other shooting kit.