This image shows an AN/APN-4 LORAN display unit mounted in a Consolidated Canso (PBY) aircraft. LORAN systems worked by comparing the time-of-arrival of signals sent from two widely-separated radio stations on an oscilloscope display like the APN-4 shown here. The operator would use the controls at the bottom to align the signals on the screen, and then measure the time difference between them. These delays were printed on maps, which were then used to determine the location of the aircraft.