In the early 1920s, flying long distances was, for the most part, a fair weather enterprise. Even though airway beacon lights were being established to mark out routes for night flying across the United States, you still needed good visibility to fly at night. Air mail flights and the developing passenger industry rarely flew above 10,000 feet, so it didn’t take much cloud cover make the airway beacons as well as daytime visual navigation useless. It was quickly becoming clear that a system that operated day and night regardless of visibility conditions was needed.
The explosive growth of the radio industry in these years facilitated the development of radio navigation. Just before the World War I, German electronics firm Lorentz proposed using radio signals in an overlapping pattern – one station broadcast the Morse Code for A which was dot-dash or beep-beeeeeee and the other station broadcast the Morse code for N (since it was the inverse of A) which was dash-dot or beeeeeeee-beep.
If you were in the overlap area, the broadcast of the A and the broadcast of the N would sound like a steady tone and then, depending upon how far left or right you were of the overlap, you heard a stronger A or a stronger N. If you were completely in the broadcast area of only one or the other, then you only heard the A or the N. The overlap area defined a straight line course either away or towards the broadcast station. In this way, radio beams could define navigational courses. A variation of this system was used by German zeppelins for navigation during their bombing missions of London during the First World War. With an environment of extreme fiscal austerity in Germany during and after the war, continued development of the Lorentz radio range system moved to the United States.
It was the US Army that took the lead, partnering with the National Bureau of Standards to develop the Lorentz system further. A four-course layout utilized technology already in use by the radio broadcast industry. There were four quadrants, each opposing quadrant broadcasting a Morse code A or N, creating four overlapping regions and which defined the four courses. As long as a pilot heard a steady tone, he was on course flying within one of the overlap areas. The only equipment an aircraft needed was a receiver. The pilot could tune into each successive station and listen for the tones from the ground station.
Three different terms are interchangeably used to refer to this radio navigation system – AN range, four-course radio range, and LFR, for low-frequency range. The system also had a third identifier – the station’s call sign – which was transmitted every 24 seconds as a verification that a pilot was using the right AN range station. The station’s call sign was a three-letter code that was typically that of the nearest airport- like “DEN” for the AN range station in Denver. Every 15 minutes the broadcast of the A or N was interrupted for a voice weather report for the area. Special weather bulletins would interrupt the AN broadcast as needed.
Ground stations were built at intervals to mark out airways. The first AN range stations had four antennas linked by wires to a central antenna and “radio shack”. As the technology and transmitting requirements increased, the four antennas were towers of their own with the central tower responsible for the special broadcasts. The development and testing of the AN range system was completed in February 1928 with a demonstration of radio navigation flight from Newark/New York to Cleveland, Ohio, using three AN range ground stations – one in New Brunswick, New Jersey, one in Bellefonte, Pennsylvania, and a third in Cleveland, Ohio. Responsibility for the Bellefonte AN range station was transferred from the National Bureau of Standards to the Aeronautics Branch of the Department of Commerce (this branch would ultimately become the FAA), with the other two stations soon to follow.
Revenue flights on the Newark/New York to Cleveland airway by AN radio range commenced in November 1928. Because of budget constraints, airway beacon lights were still being installed as they were considerably cheaper than AN range ground stations. It wasn’t until 1933 that the construction and activation of AN range ground stations took precedence over the airway beacon lights. Despite the Depression-era fiscal austerity, within the first year of operation, enough AN range stations were built to permit radio navigation flight as far west as Omaha, Nebraska. Chicago and Boston were added by 1930 and that same year, an AN range station was built in Key West, Florida, to allow radio navigation to Havana, Cuba.
By 1931, the pace of AN range station construction made it possible to fly from New York to San Francisco by radio navigation only. At the outbreak of the Second World War, there were 90 AN range stations in the United States which marked out more than 18,000 miles of airways.
Marker beacons were also added to increase navigational accuracy along any of the four courses of a given AN range station. In order to create a reasonably navigable airway, some AN range stations had their four courses deviate exactly 90 degrees from each other. Looking at the map above, you might even make out the roots of the current airways on modern aeronautical charts.
Each station operated in the low to medium frequency range from 200 kHz to 410 kHz, but some US military-operated AN range stations went up as high as 536 kHz. Since the technology used in the aircraft receiver and in the broadcast equipment in the AN range stations was based on that used in consumer radio sets and the radio broadcast industry, it was relatively inexpensive and adoption by aviation interests in the United States was rapid.
Despite relatively low costs and the simplicity of the AN range system for airways navigation, several issues were constant challenges to air crews. The first was the layout of the AN range broadcast area: there were only four possible courses as there were only four overlap areas where a pilot could hear a steady tone instead of the A or the N. As can be seen from the map above, there was some deviation from 90 degrees, but the practical limit was that courses had to be separated by more than 20 degrees or the overlap area was simply too big to be of any navigational use.
Second, there was no way of determining position location with the AN range system except if you were directly above the AN station in its “cone of silence.” There were a series of complex procedures that had to be learned to intercept an on-course beam and to identify it, which involved a series of maneuvers while listening to the relative strengths of both the A and N signals. Imagine having to do it in a very noisy prop liner flight deck in inclement weather!
The third drawback was a matter of physics. The long radio wavelengths used were prone to static interference from thunderstorms and heavy precipitation. At night, propagation of the radio signals went farther due to the ionosphere – what’s called “night effect.” This meant it was possible for two AN stations normally out of range of one another to interfere with each other’s signal at night. This distance deviation can range anywhere from 30 to 60 miles and was more pronounced on AN range frequencies above 350 kHz.
The ground conductivity around the AN station could also affect the signals and any location where there was an abrupt transition from land to water could experience what’s called “shore effect,” radio waves being bent off course. This was most pronounced on on-course beams that ran parallel to a nearby shoreline. Terrain also had a deleterious effect on AN signals, particularly in mountainous areas. The difference in ground conductivity of a valleys versus the mountains on each side of it could create false beams.
Technical improvements to transmitting antennas made during the late 1930s and into the Second World War improved signal integrity to some degree, but signals were still a longer wavelength prone to interference by natural sources. The AN range system was tricky for landings and provided nowhere near the level of precision needed for a true, low-visibility approach into an airport. Not every airport was aligned along an on-course beam. At a distance of 30 miles from the AN range station, the on-course beam was 2 miles wide.
“Instrument landings” were possible at select airports, but not widespread enough to be of significant utility to the growing number of users in the federal airway system. By the late 1930s, a variety of programs focused on using shorter radio wavelengths, which were less prone to natural interference and weren’t limited to a four course layout. One was the radio wavelength today we know as VHF. RCA, the Radio Corporation of America, led the development of VHF transmitting equipment for navigational use. Because a VHF station wasn’t limited to four courses like the AN range, it was called VHF Omnirange, or VOR. The first test VOR installation went live in 1940 at Weir-Cook Airport in Indianapolis, Indiana (today’s Indianapolis International Airport). The other VHF navigational initiative was the ILS, instrument landing system.
Following the Second World War, more VOR stations were established, displacing many AN range stations. Some AN range stations were converted to NDB stations; the central tower was all that was needed, so the four outer AN towers were removed. But due to their simplicity and low cost, some AN range stations were operational well into the 1970s and early 1980s. The last American AN range station is said to have been in use in Alaska until 1971. The last AN range station in British Columbia was decommissioned in the early 1980s.