Can anyone answer this...
Can the ring transporters work without a ground platform to ring to? I do recall the Ori once dropping a platform on the ground and then soldiers were ringed down on it, with the rings emanating from the base...but I'm having faint memories of them being used without a platform too.
If they work that way as well, then why would the LA need to land a cargo ship and leave it there? You can plant bombs simply by ringing them down from orbit, or at least while hovering over the target location..
It appears there are two ways to use the rings: sending the rings directly, or sending the transport beam that they use. In the latter case, it has to go to a receiving platform.
We've seen ships dropping rings directly, with no ground platform, on quite a few occasions. In "Tangent", Jacob sent the rings into outer space to retrieve Jack and Teal'c after they ejected from their death glider. Similarly, they once ringed a Kull warrior right out of a cargo ship at high altitude, effectively dropping him to his death. I also remember seeing an alkesh drop rings several hundred feet from ship to ground to retrieve someone. It was fairly obvious there was no platform there. I forget the episode; possibly "The Other Guys"?
I would bet that having a receiving platform makes it easier to operate. You just say, "Send to that platform over there" and it works automatically. Without one, you probably have to program the distance you're sending the rings, to make sure you're not materializing someone ten feet in the air, or pounding them into the ground, or some such thing.
But it appears you only
need a receiving platform if you're ringing inside a structure. When you send the rings directly, they have to physically move back and forth, which means they would crash through the ceiling of a building every time they were used directly. (We saw that once.) Or more likely, an intervening structure would block them from being used.
But it appears the transport beam can pass through solid matter. So all that's needed is a receiving platform inside the structure that the beam can travel to. (The rings that you see there aren't the ones from the sending platform; they just pop up from the receiving platform for reintegration.)