Radio Navigation

Here is a remnant of history back to the early days of aviation and radio. This image is from Gunnison Colorado.

Back in the early days of commercial and military aviation, flying your airplane and not crashing was a lot more tricky. The workhorses were the Ford Trimotor and DC3. There was no GPS, no weather satellites, no ILS, no radar. Navigation was only possible during daylight hours. Typically the flight path followed roads or railroads or rivers that could be seen from the air and matched to navigation charts. But if there are clouds or storms or strong winds, following landmarks using only “dead reckoning” (flying using a compass and air speed) may not work.

AM radio signals are inherently directional. If the ferrous (iron) core of the receiver is rotated, it is strongest when perpendicular to the radio waves (or creates a null with a two piece antenna that cancels out the other half.. That means by pointing the antenna, you can know exactly which direction that radio tower is. If you have a second radio signal, you can triangulate to find your exact location.

This was useful enough that some important AM stations were used in the airplanes. The simplest and shortest way to fly to an airport was to tune to a radio station and just fly straight toward the signal. These planes were not pressurized and planes were so rare that midair collisions were unlikely. If you were flying to Cincinnati, you tune to WLW-AM and just fly toward the tower. When you get close to the city, the pilot can go to a lower altitude and spot landmarks like the rivers, roads, bridges, tall buildings to find the airport.

Airports are identified by 4 character codes called the ICAO code. The same first letter scheme was used for radio and airports. The international codes for the United States begin in K (W is not used in the US). It was common in small towns west of the Mississippi River to use the same code for the airport and the town’s radio station and the code made visible at the radio tower in the event a pilot was lost.

Navigation was all the more critical around mountains. The Ford trimotor could not fly above 18,500 feet, so getting lost could be fatal. The Gunnison airport is around 7500 feet, where propellor planes start having lift issues from the thinner atmosphere.

The really faded KGUC on the transmitter building is the ICAO code for the Gunnison Airport, and was the Call Sign of the radio station. The current airport was built on the former rail yard of the Rio Grande railroad and taken over by the County in 1975. While I don’t see any evidence the airport or radio station went back to the 1920s, it became a custom when AM call signs were issued.

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5 Responses to Radio Navigation

  1. Parrott says:

    So that is why when you would turn you AM a certain way , it would play. Killer.
    I had read that the engines of the Ford Trimotors just didn’t turn the high RPM some of the later piston engines motors could. Have to remember the Trimotor came out in the 20’s.
    It and the DC-3 really kicked off commercial aviation. That was good reading Fred !
    Oh and this morning pic was pretty cool
    WGBN McKeesport PA.
    Don’t know how you do it but I really like plethora of pics of radio station transmitter sites. Cool.

    • Fred Stiening says:

      On the station web page (here) there is an icon that says GE that launches Google Earth pointing at the transmitter. If you don’t have google Earth, the tower icon will load Google Maps.

      Because the FCC uses a less accurate Lattitude and Longitude system than current GPS, the tower can actually be maybe 500 feet away from where it should be – Google uses GPS and there is not an easy way to fix the data. Because all the FCC paperwork traces back to the 1930s, they are reluctant to “fix” the data.

    • Fred Stiening says:

      Here is the complete manual for the Ford Trimotor

      Because it wasn’t safe to fly at night, Lindbergh and another company that would become TWA offered Coast to coast service where the passengers traveled by air during the day and would land and ride on trains overnight. Getting from New York to California in only 51 hours was a really big deal. Telegraphs and teletypes were in wide use and AT&T was building out the coax network, so news didn’t take 51 hours, but if you needed a physical thing moved Coast to Coast, that was the fastest way. Going completely by rail was the alternatives. One of the niche businesses of the railroads was moving services. Local movers would handle the loading and unloading at each end, then the railroad would transport the household goods from city to city. Up until about the 1930s, roads and trucks were not very well equipped to do that heavy hauling unless it was big city to big city

  2. JayMar says:

    Very interesting. I used to fly TX ANG back in the early 60s and didn’t know that. Thanx.

  3. Linda S. says:

    Very interesting, Fred!

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