How Pilots Navigate: Dead Reckoning, VOR and GPS
Pilots combine arithmetic, ground radio beacons and satellites to know where they are. A look at dead reckoning, VOR and DME, and GPS, with dates and figures.
In this article
A pilot can work out position in three main ways: calculate it from speed, heading and time (dead reckoning), take bearings from ground radio stations such as VORs, or use satellites such as GPS. Modern aircraft combine them. This article explains each, using the descriptions in Wikipedia and NASA.
Dead reckoning
Dead reckoning estimates where you are now from where you were, by applying your estimated speed and heading over the time that has passed. Distance is speed multiplied by time. Pilots use the wind triangle to correct for the way wind pushes the aircraft sideways, plot the course line with estimated positions, and adjust when something gives a new fix.
The method is simple, but errors add up. It cannot see wind or drift unless they are accounted for, and each position depends on the previous one, so small mistakes compound over distance. Wikipedia says navigators reduce this by taking a new fix part-way, such as spotting a known landmark. It adds that dead reckoning is taught in visual flight rules (VFR) pilot training worldwide, even on aircraft with GPS.
History is on the same list: Wikipedia says Alcock and Brown navigated across the Atlantic this way in 1919, and Lindbergh used it on his 1927 flight to Paris. It also says the popular claim that the term abbreviates "deduced reckoning" is a myth, since that usage did not appear until 1931, while the term was in print by 1613.
VOR: bearings from a ground beacon
A VHF omnidirectional range (VOR) is a ground station. It sends two 30 Hz signals, one fixed and one that changes with direction. An aircraft's receiver measures the phase difference between them to find its bearing from the station. Each station also broadcasts a three-letter Morse identifier.
A radial is a line running outward from the station at a given bearing, measured from magnetic north. A pilot selects one with the omni bearing selector and follows a course deviation indicator needle to stay on it. Two radials from different stations cross at the aircraft's position.
Adding distance with DME
Distance measuring equipment (DME) gives the slant-range distance to a station. A VOR with DME is called a VOR-DME, and one radial plus one distance gives a position from a single station. A VOR sharing a site with a military TACAN beacon is a VORTAC.
Range and limits
VOR is line-of-sight, so the horizon and terrain limit its reach. Wikipedia says coverage can reach about 200 nautical miles depending on site elevation and aircraft altitude.
VOR history and the move to satellites
- 1937: David G. C. Luck filed a patent for the rotating beacon
- 1949: the US Civil Aeronautics Administration began deploying VOR
- 1950: ICAO standardised VOR and DME
- 2013: the United States had 967 stations
- 2016: the FAA issued its final policy statement on decommissioning, planning to retire about half and keep a Minimum Operational Network as a backup for GNSS (satellite navigation)
Wikipedia also says the UK cut its stations from 44 to 19 by 2020, and that in Australia satellite navigation became the primary means for instrument flights in 2016.
GPS: timing signals from space
NASA explains that GPS satellites broadcast their positions and the time. A receiver measures how long each signal took to arrive and calculates its own position from those travel times. NASA says GPS can give a position to meter-level accuracy and time to within 10 nanoseconds. Under the published performance standard, the position is good to about 10 metres horizontally and 20 metres vertically at 95% confidence, and the global average error has recently been about a metre or better. NASA's page does not discuss aviation specifically.
Why pilots keep more than one method
Each method has a weakness: dead reckoning drifts, VOR depends on distance and terrain, and satellite signals can be lost. That is why the VOR network is being thinned rather than removed, and why dead reckoning is still taught. Inertial navigation systems on advanced aircraft also use dead reckoning internally, Wikipedia says.
Sources
Share
Keep reading
Related articles
How GPS Works: Satellites, Clocks and Your Position
GPS finds your position by timing radio signals from satellites about 20,200 km up. Here is how the satellites, atomic clocks and your receiver fit together.
How Contrails Form Behind Aeroplanes at High Altitude
Contrails are lines of ice crystals that form when water vapour from jet exhaust meets very cold air. Learn why some vanish quickly and others spread for hours.
How a Helicopter Flies: Rotors, Pedals and Sticks
A helicopter's spinning blades are wings. Three controls change their pitch to climb, move and turn, and a tail rotor balances the spin. Here is how each part works.