The Satellite is a weapon unlocked by completing the "Showdown" campaign. Since Showdown can only be unlocked by beating every previous campaign missions, Satellite is effectively the last weapon unlocked through the Single-Player Campaign missions.
The Satellite is very easy to use, just aim it and make sure it will land on the enemy. The beam have a slight wide hitbox, ensuring that it will hit the target(s) even if it lands a little away from the foe.
In order to first get the Satellite, you must complete the all missions in the final campaign "Showdown", wich is infamously know as having two of the most frustrating challenges: Fiesta Forever and Final Boss.
The UFO is the sole upgrade of Satellite. It have the same function as it's parent weapon, but the laser have a wider radius, so they can hit more targets that are close to each other, but with a gap over them that would made the Satellite impossible to catch them all. The damage remains the same.