Bernard Foot spoke about how the navigation for a flight is planned to allow for the effect of wind by using a special manual “computer”, or by mental arithmetic to arrive at an approximate plan. However, the navigational plan never works out in reality, largely because of the difficulty of getting accurate forecasts of wind strength and direction. So a major element of navigation is finding where the aircraft actually is so that course corrections can be made. In the early days of flying, pilots would fly low along line features such as roads and railways and try to read things like station names. In the 1920s, the USA created a network of 1,500 arrows on the ground to allow the airmail service to find its way across the continent. In the 1930s the first radio technology was deployed – the Non-Directional Beacon (NDB): this allowed the pilot to see where the beacon was relative to the direction that the aircraft was pointing.
During the Second World War, the RAF had to develop new navigational techniques because it realised its bombing campaign was not effective. Airborne ground radar (“H2S”) helped navigators determine where they were. The Gee system let navigators determine their position by measuring the time difference between signals arriving from several transmitters. The Oboe system allowed ground stations to guide aircraft to their targets by measuring how long a radio signal took to get to the aircraft and a response to be returned. The Gee-H system worked in a similar way to Oboe but here it was the aircraft that sent the signal and measured the delay up to receipt of the response: this technology was commercialized as the DME (Distance Measuring Equipment) system used in civil aircraft after the war to measure their distance from a ground station. The VOR (VHF Omnidirectional Ranging) radio beacon was introduced in the 1950s. This is more sophisticated than the NDB and enables the aircraft to determine its position by measuring the phase difference between a primary omni-directional signal and a secondary rotating signal. When the aircraft is close to its destination it can make a bad-weather landing using ILS (Instrument Landing System) which has two radio beams, each with modulated side-lobes of 90 and 150 Hz, to guide the aircraft onto the runway by providing left-right and up-down guidance to the pilot derived from the relative strength of the received 90/150 Hz signals.
Trans-oceanic navigation has added difficulties because there are no radio beacons (so aircraft used to have INS – Inertial Navigation Systems – to guide them approximately to landfall) and because great arcs have to be flown because they are shorter than compass headings.
However today most commercial aircraft and many private aircraft are mainly using GPS to navigate, and they can couple the autopilot to the GPS so that the aircraft can fly the route automatically (the “glass cockpit”).