Revised and updated for 2016!
DIRECTV holds leases for several satellite locations. There is so much programming available that doing with one satellite, or even a cluster of satellites in one location, wouldn't work. So, DIRECTV uses a fleet of satellites in five locations for US English programming, plus several others for global, non-English programming. Each satellite location is identified by its position in the sky. All communications satellites sit in orbit approximately 22, 500 miles above earth. This is due to a the mathematics of orbiting objects.
Orbiting is really just falling. Gravity always pulls things down, but something that already moves in a direction of its own will not fall straight down. instead, its path of movement will bend toward whatever is pulling it down. In order to avoid falling down completely, an object must move more quickly as it gets closer to the source of gravity. In this way it will keep moving away at the same time it is falling, thereby staying at the same distance. It's easy to see this work in a draining tub or toilet; things spin faster as they move closer to the center.
It was Arthur C. Clarke, the great science fiction writer, who first posed this question: If everything in a stable orbit moves at a speed defined by its distance from the earth, where is the point where a satellite will orbit the earth in exactly one day? If you could do this, then you could put a radio transmitter in a permanent orbit over one spot on earth, and a radio antenna wouldn't have to move to find its signal.
Mr. Clarke found his answer and we're glad he did, since it's what makes all satellite communications possible. So, since DIRECTV (and everyone else) can put a satellite exactly where they want, we can use dishes to get signals from those satellites. We refer to the satellites by their position in latitude, rounded up to a whole number. DIRECTV operates satellites at the 99, 101, 103, 110, and 119 degree locations for its US English programming.
That's a lot of sky to cover with one antenna. 101 degrees west longitude is a little bit west of Dallas, while 119 degrees is over California. You need not only a wide dish but a lot of clear sky for one antenna to see all five locations. DIRECTV knows this and while their Slimline-5 series dish does the trick nicely, they have concentrated on moving as much programming as possible to satellites in the 99, 101, and 103 locations so that dishes are easier to aim. The Slimline-3 dish reads only these three locations and is recommended for most markets. If you're in a market where your local channels are on satellites in the 99, 101, or 103 locations only, you can save money with a Slimline-3 dish and save time aiming.
See also:
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DSS922 DSS Dual LNBF for Anik F3 118.7 & 119 Satellite Dish CE (DMS International)
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