During one of the earlier missions in the game, Yuri will request assistance in bringing back the police departments surveillance towers, which have gone offline. After completing the mission, the rest of the towers will be marked on your map by small towers emitting signals, but nearby towers will also be visible on your screen as you won't be able to see the rest of the map until you activate them.

Each of the 20 towers corresponds to a certain district, which will become visible on the map upon the tower's activation. Every tower is the same in that you'll need to change the frequency to activate it, but the frequencies do vary.

Amazing Coverage Video Guide

Credit to S1R CARNAG3 for the video.