There is one ctOS System at every district, when you hack the ctOS System all ctOS Towers are shown on the Map (total of 13) Finding all of them will reveal all collectible locations on your map and earn you the Clear Signals Trophy/Achievement. All Towers are on rooftops, you need to climb and use the cameras to find the Security Boxes witch open the Doors to the ctOS


Note: Pawnee has only one ctOS Tower (but it won't count this this Achievement), this Tower (and the Fast travel Point) will be unlock at Act 3.
Unlock: SMG-11 and the Amargosa Turbo (Car)