The low Larracos ring holding the Piccadilly Theater and the hidden signal generator.
Donut identifies the antenna's location precisely: inside the Piccadilly Theater, the one on the 9th level of the city. The enemy chose the spot because she couldn't see it from up on The Nest tower.
The advancing The Lava Flow is about to land right in front of it, blocking the sightline even further and forcing Carl to bomb it from the railing on the fiftieth level instead.
A low tier of the funnel city, hidden under smoke and about to be flanked by falling lava.
The objective's location, deliberately chosen to be out of Donut's view.