Ancient geo-synchronous satellites
From The Stargate Omnipedia
A series of 100 satellites in a stationary orbit over an area of the planet M4D-058. Set out in a perfect 10 by 10 grid, they were used to relay instructions from the game console housed in Atlantis to other units on the planet that the inhabitants of Geldar and Hallona called The Portal.
They also relay information about the planet and it's people back to Atlantis, updating the console there.
KEY EPISODE
The Game - Major Lorne discovers a series of satellites orbiting above a planet too primitive to have produced them.