In the 1920s, commercial aviation was on the upswing in the United States. The U.S. Postal Service, with the assistance of NACA and the Commerce Department, had begun making coast-to-coast air mail flights. The need to keep aircraft moving at night to move the mail led to the invention and introduction of the lighted airway system. Although that helped, it had one major drawback - it was only useful in clear weather. Both commercial and military interests were eagerly seeking a reliable way to safely navigate and land airplanes during inclement weather.
In 1929, Col. James Doolittle (who would later go on to lead a famous bombing raid on Japan in World War II) made the first instrument flight using custom instruments - a gyroscopic attitude indicator and gyrocompass, along with relatively crude radio direction finding gear and a transmitter on the ground allowed him to take off, make a circuit of the airport, return and land with no visual cues outside the airplane. The concept of instrument flight had been proved, and what was needed then was a means to make it standard and available.
It swiftly became clear that although placing directional antennas on airplanes so that they could determine their relative bearing from beacons was a start, this was mostly useful for static navigational checks. A means for pilots to know, continuously, if they were flying the correct course was needed. In the early 1930s, this led to the introduction of the Adcock Range, also known as the Adcock Radio Range, Low Frequency Radio Range or the A-N Range system.
Early versions of the Low Frequency Radio Range system used closed loop antennas, which had been found in World War I radio direction-finding uses to transmit a narrow, directional beam. Later, however, they were to use the more efficient Adcock antenna, consisting of several vertical dipole antennas, which led to the informal name for the system.
The actual 'Adcock Range' or 'Low Frequency Radio Range' was a set of ground-based transmitters which operated in a particular manner. Usually consisting of five towers - a square with a fifth in the middle - the range produced a distinct set of broadcasts. In general, the range was divided into four quadrants - fan-shaped signal areas which projected out from the station in four directions. Opposing quadrants would broadcast the same Morse code signal, so that any two adjacent quadrants would broadcast the two separate codes. The codes used were always 'A' and 'N.' This was important because at the very edges of the quadrants, the two beams were set to overlap in a three-degree-wide cone. The morse for 'A' is dot-dash, and the morse for 'N' is dash-dot. What this means is that if you were in the overlap zone, the two signals (which were synchronized) would in fact blend to become a continuous tone. As you drifted to the left or right, the A or N would become prevalent, indicating that you were to the left or right of the direct path towards or away from the station.
This was called 'riding the beam.' Pilots were trained to fly along the right-hand edge of the overlap zones - that way, airplanes going in opposite directions along the same beam would be separated by the three-degree difference. These stations were emplaced all over the United States, with over 400 operating at the system's high point in the 1950s. Although the initial system used regular audio signals and relied on the pilot to interpret the result, eventually aircraft instruments which visually displayed the 'beam' and the direction of drift were devised. When tuned to the appropriate frequency, these instruments could tell pilots when they were straying, and in which direction.
The Adcock Range system had limitations. One was that each beacon only had four usable airways leading from it; another that airplanes inside each quadrant were not given any information as to their position until and unless they reached a beam intersection. If an airplane directly within a quadrant wanted to fly towards or away from a beacon, they would need to first fly cross-range until reaching the beam, and only then turn towards or away. Finally, the system did not offer any indication as to whether an airplane was headed towards or away from a beacon; to determine this, pilots learned to compare the volume and signal strength of the beam tones over time and determine whether the signal was gaining or fading.
It worked well enough to make practical instrument flying possible, however, and thousands and thousands of pilots learned to 'ride the beam'. When beacons were placed at appropriate known points relative to airports, approaches to runways could be made, allowing landings in poor weather. As the airplane approached a beacon, the directional signals would fade sharply as the airplane entered a region directly above the beacon known as the cone of silence. This was the pilot's cue that he or she was in fact directly atop the beacon and should make any requisite course changes to intercept the proper outbound airway beam.
Eventually, the Adcock Ranges were replaced by VOR beacons and their military equivalents, TACAN. VOR/TACAN systems could provide a receiving aircraft with its relative bearing from the beacon at any point, which (when paired with a decent compass) allowed the pilot to easily navigate directly towards or away from those beacons no matter what their relative position as well as make determining precise location using two beacons much easier. By the late 1960s, the VOR system - or Victor Airway system - had taken over.
(IN 5 14/30)