The Stargate satellite was a device used by the Asurans as a mobile weapon platform.
The satellite is comprised of many of the same basic systems found on a spacecraft. It possesses a hyperdrive engine to travel to its destination, an attitude control system to aim at a target, a navigation system, a defensive energy shield, and a small power source. This power source is designed to power the engines to take the satellite to its intended destination and power the energy shield until the Stargate mounted at the satellite's center can be dialed.
The satellite's weapon isn't located on the satellite itself, but rather is sent through the Stargate. Once a wormhole has been established, a continuous energy beam is fired into the dialing gate and emerges out of the dialed gate. Once this beam is active, the satellite ceases to be powered from its on-board power source, but is rather powered by the energy of the beam itself. Thus, as long as power is provided to the beam from the dialing gate, the Stargate and satellite will remain active. If powered by a black hole or multiple Zero Point Module, it is possible for the energy beam (and thus also the Stargate) to remain active indefinitely.
If a Stargate is already present at the satellite's destination, the satellite's Stargate will prevent the native Stargate from achieving a wormhole due to its proximity, if active. (SGA: "First Strike")
In response to the destruction of much of their fleet, the Asurans sent the satellite to Lantea as a means of ending the efforts of the Atlantis expedition once and for all. When the satellite arrived in Lantea's orbit the Apollo was sent to investigate. Once close enough to the satellite the Apollo was able to discern a Stargate, which activated and directed its beam at the ship. While the ship's Asgard shield struggled to hold back the beam, the satellite swung towards the planet and directed the beam to Atlantis. The city managed to raise its shield in time to block the attack, and deduced that the beam would continue to fire as long as power was available. In order to relieve the pressure off of the shield the Atlantis expedition submerged the city, only to discover that the satellite's beam was much more efficient at penetrating the water than was hoped.
Realizing that there was nowhere on the planet safe from the satellite, the Atlantis expedition decided to activate the city's Stardrive for the first time in millions of years. The city's sole Zero Point Module did not have enough power to fire the Stardrive on its own, however, so they utilized the recently discovered Mobile drilling platform to provide more power. Still lacking the necessary energy due to the strain on the shield, a squadron of F-302 fighter-interceptors hurled an asteroid in front of the satellite, which was dense enough to intercept the beam; the city's shield was lowered, which provided just enough power for the stardrive to achieve escape velocity. Unfortunately, the beam then cut through the asteroid and grazed the city as its shield was being raised again, causing Atlantis to drop out of hyperspace prematurely due to the damage caused by the beam. Whether the satellite is still in Lantea's orbit or was recalled to the Asuran homeworld is currently unknown. (SGA: "First Strike")