by Scott Thompson

In the 1920s, the development of a transcontinental airway that used beacon lights provided the first means of dependable night time navigation and served to establish air mail routes. But the limitations of relying on lighted beacons was clearly evident: navigating during periods of low ceilings and visibility was not practical. Efforts to develop reliable navigation, approach, and landing aids was a top priority for the new airlines, the infant military air services, and the new Bureau of Air Commerce, which was established in 1926 to regulate and advocate for civil air transportation.

Efforts to allow low visibility approaches and landings paralleled the development of radio aids for en route flying; the technical challenges were similar but en route navigation proceeded at a faster pace. The standard for airway navigation became the four course range, whereby a radio range of specialized antennas created radiation patterns that provided for four courses. The radio range consisted of specialized loop antennas that created four quadrants where a Morse code “A” (dit-dash) or “N” (dash-dit) signal predominated. Where the signals merged and became equal, a steady tone would be heard for a pilot using earphones. This method was used to create the four courses, which were basically ninety degrees apart,. However, by adjusting the power output of the transmitters, the orientation of the four courses could be somewhat adjusted to optimize the airways created. It was a complicated system to install, maintain, and to fly. Pilots ‘flew the beam’ by flying a course that maintained the steady tone in the earphones they listened too, but orientation from the station could be confusing and distance information was not available. The transmitters were tuned in the low/medium frequency range, so weather and atmospheric conditions caused interference. It was rudimentary and imprecise, but it was a big improvement on the airway beacons and soon four course ranges were being installed by the Bureau of Air Commerce and its successor, the Civil Aeronautics Authority. The ranges also transmitted weather information and provided radio links to new airway stations.

See the November 2016 EAA Sport Aviation magazine column “Contrails” by Jeff Skiles for more on four course ranges

This view is from a June 1945 Army chart and shows the Sacramento Radio four course range transmitting on a frequency of 236 kilocycles. It created four courses that defined navigable airways named with colors and numbers, (for example, ‘Green 3’ seen here). This radio range was installed in what is now the Pocket area. It appears the Sacramento airport tower used the same transmitting site. These four course ranges were enhanced with radio beacons and marker beacons to make them more useful, but it was not until the advent of the VHF Omni Range and DME before airway navigation became practical and predictable.