The Four-Course Radio Range: Birth of the Modern Federal Airway System

This DC-6 captain uses headphones for navigation as well as communication

In the early 1920s, flying long distances was, for the most part, a fair weather enterprise. Even though airway beacon lights were being established for night flying across the United States, you still needed good visibility to fly at night. The explosive growth of the radio industry during this time frame facilitated the development of radio navigation. The origins of the four-course radio range lay just before the First World War, when engineers at the 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 one or the 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. This way, radio beams could define navigational courses. A variation of this system was used by German zeppelins for navigation during their bombing missions against London during the First World War. With an environment of extreme fiscal austerity in Germany during the war, continued development of the Lorentz radio range system moved to the United States. 

Diagram showing a Four-Course Radio Range

On 1 July 1925, the US Post Office inaugurated the first regular night air mail service which connected Newark, New Jersey and Chicago, Illinois. Beacon lights marked out the route between the two metropolitan areas, but it was quite obvious that what was needed was a system that operated day and night regardless of visibility conditions. Considering that air mail flights and the developing airline industry rarely flew over 10,000 feet, it didn't take much cloud cover make the airway beacons as well as daytime visual navigation useless. Despite the obvious benefits to the US Post Office and the movement of air mail, 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 was used and utilized well-known technology already used by the radio broadcast industry. There were four quadrants, each opposing quadrant broadcasting a Morse code A or N, this way the there were four overlap regions and these four overlap regions 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 aircraft needed was a receiver and the pilot would tune into each successive station and listen for the tones from the ground station. A four-course radio range (or AN range) had a third identifier which was the station's call sign which was transmitted every 24 seconds as a verification you were 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. Three different terms are interchangeably used to refer to this radio navigation system- four-course radio range, AN range, or LFR, for low-frequency range. I'll be using primarily AN range in this article. 

Typical layout of an AN radio range station

Ground stations would be built at certain 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 the third one in Cleveland, Ohio. The Bellefonte AN range station was transferred from the National Bureau of Standards to the Aeronautics Branch of the Department of Commerce (this branch ultimately years later would be come 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 the budget constraints of the day, 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, enough AN range stations were built to permit radio navigation flight as far west as Omaha, Nebraska, just in the first year of the AN range system operation. 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 reached a point where it was 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 that marked out over 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 have some semblance of order and a reasonably navigable airway, some AN range stations had their four courses deviate from exactly 90 degrees to 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 the US military operated some of its own AN range stations that 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 adaptation by the aviation interests in the United States was rapid.

The AN range stations of the United States in 1950- you can see the origins of the current federal airway system
(click for larger image)

Despite the relative low costs and simplicity of the AN range system for airways navigation, there were several issues that were constant challenges to air crews. The first one was the layout of the AN range broadcast area- there were only four possible courses since 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 of the United States AN stations in 1950 above, there was some deviation from 90 degrees, but the practical limit was that the courses had to be separated by more than 20 degrees or the overlap area was simply too big to be of any navigational use. The second draw back was that 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 that beam- it involved a series of maneuvers while listening to the relative strengths of both the A and N signals. Changes in the signal strength while performing a series of maneuvers determined which quadrant you were in as well as which on-course beam you intercepted. Now imagine having to do this in a very noisy prop liner flight deck in inclement weather!

Typical AN range on a chart, this is in the Mojave Desert SW of Las Vegas

The primary drawback of the AN range system was a matter of physics. The longer radio wavelengths used were prone to static interference from thunderstorms and heavy precipitation and at night, (what's called "night effect") propagation of the radio signals went farther due to the ionosphere which meant it was possible for two AN stations normal out of range of each other 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" where the radio waves get 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 valleys versus mountains on each side of a valley could create false beams.

Technical improvements to the transmitting antennas made during the late 1930s and into the Second World War improved signal integrity to some degree, but the signals were still a longer wavelength prone to interference by natural sources- but more importantly, a better AN signal still left users with the limitations of a four-course layout. The AN range system was trickier for landing and was nowhere near the level of precision needed for a true low-visibility approach into an airport. The four-course layout meant that not every airport let alone their runways were aligned along one of the on-course beams and at a distance of 30 miles from the AN range station, the on-course beam was 2 miles wide. "Instrument landings" to use the term loosely were still possible at select airports, but not widespread enough to be of significant utility to the growing number of users in the federal airway system of the day. A variety of programs started in the late 1930s focused on using shorter radio wavelengths as they were much less prone to natural interference and were not limited to just a four course layout. That 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 just 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 that grew out the work to find something better than the AN range system was the ILS, instrument landing system- the development of VOR navigation and ILS will be the subject of a future blog article and is beyond the scope of this feature.

Following the Second World War, more VOR stations were established, displacing many of the AN range stations. Quite a few 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 the simplicity and low cost of AN range stations meant that they persisted well into the 1970s and early 1980s. The last American AN range station was in Alaska and one source on a radio hobbyists' forum indicated that this occurred around 1971 with the the last AN range station in Canada located in British Columbia decommissioning in the early 1980s. One example of an AN range station converted to an NDB was the Spokane, Washington, AN range station. In 1936 it operated at Felts Field at 365 kHz which was Spokane's original commercial airport until replaced by the former Geiger AFB in 1946. In the early 1950s the AN range station was moved to the town of Marshall, southwest of Spokane, to better serve the new airport at Geiger Field (now Spokane International Airport) but it kept the same frequency at 365 kHz. It was one of the AN range stations that was converted to an NDB station when the AN ranges were replaced by VOR stations. In the late 1980s the Marshall NDB, still at 365 kHz, was moved back to Felts Field which had become a general aviation airport. In 2008, the NDB there was moved to Deer Park Airport north of the city, still operating at 365 kHz to this day. That would mean that next year, there has been a radio navigation aid in the Spokane area of one form or another operating at 365 kHz for ninety years!

Sources: Electronics in the Evolution of Flight by Albert Helfrick. Texas A&M University Press, 2004, pp 36-42, 54-57. "Flying the Beam: LF/MF Four-Course Radio Ranges" by Richard Harris, 2013, 2014 at http://home.iwichita.com/rh1/hold/av/stories/avionics/radiorange.htm. "Aviation Low-Frequency Radio Ranges" at ed-thelen.org. Photos: Wikipedia, FAA, Houston Municipal Airport Museum at 1940airterminal.org.