The Garmin Astro Dog GPS sends a radio signal to your Handheld GPS unit with the exact location of your dog. The Handheld GPS unit plots that information on your MAP SCREEN so you can see your location, your dog's location, direction, and whether or not your dog is moving.
The DC 40 has a "Line of Sight" range of 7 miles. This means in completely flat terrain with nothing between you or your dog you should be able to get a range of 7 miles. (There are not many places that we hunt or spend time with our dogs that have this type of terrain.)
"What's the real range? 7 miles sounds fine, but what is it going to get in grouse woods? I hunt some big open country, but I like to hunt ravines and hill country too." -- In thick rolling country we could get between 1 and 1.3 miles with the handheld. Adding the car top antenna more than doubled that range out to 2.8 miles. In flat thick, cover (no changes in elevation) I got 2.5 miles with just the handheld and over 3.5 miles with the additional long range antenna.
Some situations will create more range and some will create less. The thickness of the cover does not seem to ever be a problem. We found the greatest range drop off in rolling terrain. Just like any radio signal (this applies to tracking collars also), the more dirt that you have between you and the dog, the less of a signal you will get. Radio signals do not go through dirt or rocks. They bounce off them and go up. This means that the best way to track in rolling, hilly or mountain terrains is from the highest possible location.