Words over the exit door at Horham St. Faith's
Officer Club, 95th BG(H), 8th AF.
The Second World War, like any military conflict through the ages, saw the advancement of not just military weapons and methods but civilian also.
Radio navigation during the 1920s and 1930s was a slow steady improvement. However, since it was run under the Commerce Department and it was not well funded the pace was only equal to the number of pilots killed. The more pilots that were killed in flying the more effort was put into fixing the problems that caused them to be killed. This is also how the system still works today. (As it does in any product really.)
The Low Frequency, LF, navigation system that came about in the 1930s used morse code and the radio spectrum from 190 to 535 kHz at 1500 watts. It broadcast morse code letters A and N — Dot Dash and Dash Dot — in opposite directions along four radials 90 degrees apart.
The stations would be set up in pairs so that a station would broadcast an A toward the station that is sending an N.
The pilot would tune into the appropriate stations and listen on his headphones (well, there were women pilots too but there were very few flying and none were in the commercial or military at the time) and if on course the two opposite morse broadcasts were merge into a steady tone.
So if you were flying "on the beam" the tone was steady, if you flew off the beam the tone would drift so that you would hear either the A or the N. If you were flying west toward the A that meant the N was behind you and if you drifted left then eventually only the A morse code and the station identifier would be heard (the station broadcast it's own morse code ID every 30 seconds).
The drawback was that this method only worked if you were flying along one of the four 90° radials of the station. In between there was only the morse code and you had no idea as to the actual direction to the station. You had to be along the beam to begin with in order to navigate. That is how the name four course radio range came about.
Most of the pilots trained during WWII had to learn this method of navigation.
This navigation method was only valid and useful while in the United States — once overseas these types of stations did not exist.
ADF — Automatic Direction Finder — equipment was then created to allow pilots to know if they are flying toward or away from a radio station.
This method measures the signal delay that two (or the different parts of the same) and figures out which signal arrived first. Thus if the right side of an loop antenna gets the signal first, then it is closer to the transmitter than the other side. This means that with a fixed loop antenna you turn the whole aircraft until both signals arrive at the same time and then you are either flying TOWARD the station or AWAY from the station.
The only way to know that is have situational awareness of where you started out in relation to the station and your compass. During the World War II there are some famous cases where German pilots thought they were flying toward France and actually flying toward England and subsequently landed there and were captured.
After the US entry into the 2nd World War, the U-Boats in the Atlantic were the primary problem that had to be negated before anything meaningful could be done to defeat Germany.
Some bright person then thought of using ADF in reverse to find out locations of the U-Boats by utilizing their daily transmittal to Admiral Donitz. The wolf pack tactics that the Germans used, centralized command and control of U-Boats to attack convoys, meant radio transmissions every day by every u-boat, which allowed the allies to use that to their advantage.
The idea was that when a U-Boat transmitted its report back to HQ it sent out a radio signal for a very long time. Set up stations that just monitored those frequencies and use the ADF technique in reverse to find a bearing to the original transmitter. Now take two such stations working together, but some 500 or so miles apart, and with both tracking the same signal you have two bearing that instantly gives a fix to where the transmitter is at.
These stations would then notify either planes or hunter killer groups to the location of the u-boat which would then be sunk or effectively nullified in its ability to sink allied shipping.\
Thus ADF was used both to navigate to a location and to find the transmitter's location. After the war this ADF finding was used by search and rescue to help aircraft who allowed themselves to become lost in weather or over sparse terrain to be easily located by the CAA/FAA and directed to an airport.
During the Second World War VOR — Very High-Frequency Omnidirectional Radio Range — system was developed. However it was not fielded during the war since the existing HF system was in use and there was no immediate benefit to diverting resources to place thousands of sites in the US when the current system was good enough.