Originally released as pre-order DLC. If you have it installed:
After the intro level, you'll arrive at XCOM HQ. Some cutscenes and dialogue later, you can move around, and will notice the radio nearby broadcasting a message. Inspect the radio, then the note, then talk to Chulsky across the hall. The options are 250, North, and 875. That's all for now, so go on with the main storyline. When you get back from the Doctor Wier mission, you'll be able to select this mission along with the other two standard ones.
You're dropped off at the security gate of a tunnel, just head through. You'll come up on a few grunts on the broken roadway, take 'em out. Head down the dirt path, where a few silacoids will pester you. Continue, then drop down into an area with Grunts, and Elite, and a Laser Turret. Eventually, a Drop Pod will appear on the corner of this area counter-clockwise from where you dropped in (so you may want to move, or risk of being immediately flanked by this drop). You can pick up the Laser Pistol in the small outsider hut opposite where the pod drops. Once done, head through the Outsider structure and on to the radio dish's control building.
The Armor Piercer pack can be found down here in the offices, in the Director's in the back. There's a silacoid in the staircase. In the room in the second floor, you'll see a wall of calculus, and a lone sleepwalker in front of a projector screen. You have the option of either euthanizing (shooting) him, leaving him be (both of which will get a 'no civilian survivors' note in mission results), or pressing the prompt on him to call an evac when you clear the area (and a note of the rescue in the results-- but either way, this doesn't affect gameplay beyond that). Head down and out to the area right outside the dish itself.
Elites in the courtyard in front of the converted dish are the biggest threat here, plus a Rocket Sniper in the high structure. There'll be another drop pod right in the middle, so take care of that wave (near the end, so dunk your grenades on this one. Lastly, two drones will appear. A Laser SMG is available in the corner front and left from the entrance. Go up to the dish's console, hold the prompt to activate your bomb, then head to the Skyranger to finish the mission.