INFORMATION:

During the Battle of Britain a the RAF deployed an Identification Friend or Foe (IFF) system code named "pipsqueak". RAF fighters had a clockwork mechanism that regulated the broadcast of a signal over an HF channel for fourteen seconds of every minute. Each Fighter Command sector had Huff-Duff receiving stations that would monitor the "pipsqueak" broadcasts and telephone the bearings back to the sector control rooms where they could be triangulated and the squadron's location plotted.