Near the end of Mission 6: Chapter 3 - The Beacon (main objective: "Get a working module from the dish"), there are some green wall panels that you can shoot with the Arc weapon to activate nearby robots, called Stalkers. There are three wall panels and each one unlocks two robots. They will follow you around and protect you. If you have already completed this mission, select Mission 6: Chapter 3 so you do not have to replay the entire mission.