Just FYI, dome antennas can only receive the KU band broadcasts. DirecTV's HD programming comes from 99 and 103 which is broadcast on the KA band. DirecTV 101 is KU band and all 6 of the Dish satellites broadcast on the KU band allowing SD and HD reception with Dish and a dome. Domes only have 1 LNB so they can only "see" one satellite at a time and switch between satellites as you change channels.
2nd FYI, not all domes can receive programing while driving. There are stationary and in-motion antennas. The in-motion versions are substantially more expensive.