There are basically 3 services DirecTV DirecHDTV and dish (both HD and NTSC)
IF you have an DOME antenna so that you can record programming as you drive, DirecTV (NOT HD) works 100% with domes, Few if any problems provided the dome is properly operating, however you want to install an A/B switch with A connected to the dome up Abov e and B connected to a connector down Below (Bulkhead or basement, in my case the brake bell crank bracket. This way you can hook up a ground mount, switch to B aim and enjoy,, (Of course if you don't have a dome, Just a ground mount, skip the above)
DirecHDTV requires a special dish type antenna, it has 3 LNB's covering 3 satellites and two frequency bands, This can be roof mounted, automatic, or ground mounted, (If roof mounted see the "A/B Switch" portion of above as it applies to all roof mounts)
DISH SD uses two, and HD used 3 satellites in the same frequency band.
SOME dome antennas, if operated with a SINGLE receiver (NOT DVR) can switch between the three (There are really six but they are two groups of 3, the other 3 are for use on the atlantic coast side of the country) satellites under receiver control. Some can not, Mine can not even see the 3rd without a new program and alas no more customer support for that model.
With more than one receiver, or a dual-(or mroe) channel DVR, the odds are you need to see two birds at one time to get both shows, A dome can not do that.
Again, with the proper 3-LNB (Triplet) DISH type antenna, Be it roof or ground mount, no problem at all, (or multiple domes if you want to go that route.
Now: Why the A/B switch if you use a roof mount.
Simple.. I sometimes park under the evil signal eating tree, Since the dome can not see through the trees, when that happens, switch to ground mount and ... Enjoy.
Of course right now for other reasons I'm running only Over the air.. So far, So good.