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Radios have been around for roughly as long as aircraft, and their use has spread drastically in every part of life, from transport, to war, to home entertainment. It didn't take long for the peculiar properties of radio waves to be discovered and promptly applied in clever ways. Even before aircraft had begun to experiment with radios, naval systems had received attention and were even in use for some time!
One of the first applications of radio for navigation in ships was to fit the vessels with a radio beacon, and upon the shore, a series of radio direction finding stations. These stations work by rotating a loop antenna and measuring the strength of the signal relative to the rotation. Using that information, two stations could relay their findings to a ship, from where the vessel's crew could determine their location. The basis of this system, that is, finding two lines from two known positions in order to determine your own, is the foundation of most radionavigation systems. The first of the major systems to be adopted for navigating aircraft was the four-course radio range. This system, put in place in the late 1920's, offered aircraft just four beams from the station to navigate by. This allowed aircraft to fly towards the station along one of the beams, and then to fly away from the station on another beam. This station is no longer in use today - with the last being decommissioned several decades ago in favor of a similar but more sophisticated system called VOR's. A similar system present in the 1900's, and still in use in some places (such as Alaska!) today is called the NDB/ADF, or Nondirectional Beacon/Automatic Direction Finder system. Very similar to the systems used in maritime navigation of the 1920's (as mentioned previously), these two systems work together to allow an aircraft to determine a bearing from their aircraft, to a beacon. While not used very often in the continental US, they are still utilized in many other countries - many of whom cannot afford the more advanced systems like VOR/DME's in such widespread fashion as the United States - or in areas where the benefits are specially desired. Over VOR/DME's, NDB's offer drastically increased range, at the cost of less functionality, and a greater propensity to error. VOR/DME's, or rather, a combination of Very High Frequency Omnidirectional Range and Distance Measuring Equipment, is the modern method of aircraft radionavigation. It functions very similarly to the original four-course range, but instead of offering only merely four 'ranges', it offers 360 - one for each degree of a compass, through the use of a clever transmission technique. These allow pilots to navigate with far more freedom in regards to course selection, and - with the additional Distance Measuring Equipment - allows a pilot to determine their position with only a single reading from a VOR. These are still in widespread use across the world, especially the US, and are second only to GPS systems. |
The first radio navigational aid, 1928.
https://www.centennialofflight.net/essay/Government_Role/navigation/POL13G4.jpg 
Direction finding on Ships.
https://www.britannica.com/technology/navigation-technology/Radio-navigation 
A Non-Directional Beacon
https://www.systemsinterface.com/products/navaids/ndb/ 
A VOR station.
https://www.thebalancecareers.com/vor-navigation-system-282557 |