Even though the theoretical line of sight is 1800 miles, I expect that you'll get much less range. The satellites move very quickly— the satellite that's halfway between you and the ground station is very quickly going to be more than 1800 miles away from one of you. Transmitting signals at such oblique angles is also a lot harder. Part of it is just typical signal strength los due to r^2 law. But if you're transmitting thru that much atmosphere, you're going to experience a lot more interference from things like rain fade.