The Garmin Astro is well worth looking into. This one is a portable handheld GPS receiver that links to another GPS receiver on your dog. It can be used solo as a personal navigation system or with the dog collar and tracks up to 10 dogs at a time. At all times you have a compass arrow pointing to the dog and a window that tells you exactly how far away the dog is. Anyone familiar with a GPS can operate this one and if you're not familiar, it's pretty easy to learn. My dad bought one for his squirrel dog and I am thoroughly impressed. Ok enough sounding like a commercial. Just check it out. Cabela's sells it for about $600 with one collar and each additonal collar is $200.
https://buy.garmin.com/shop/shop.do?cID=&pID=8576