Airliners navigate utilising GPS, Radio Aids, and onboard Inertial Reference Systems. The systems vary in precision, with GPS being the most accurate (like a car’s Sat Nav). Still, internal navigation systems don’t utilise external references and radio beacon receivers used for specific phases of flight, such as departure, arrival, and approach.
Before takeoff, pilots load a specified route into the Flight Management System or Computer (FMS) or (FMC). This creates a moving map of the flight route that pilots can monitor on their screens. Pilots can view other airports, aircraft, high terrain, and poor weather on their screens. The aircraft’s navigation systems compare its location to the predetermined itinerary. They include: The GPS, the IRS, RADAR
Global Positioning System (GPS)
The aeroplane monitors its GPS position as one of its key navigation sources. This is the most accurate navigation system on current commercial aircraft, allowing manoeuvres down to 0.1 nautical miles. If the aircraft's receivers fail or the GPS/satellites go offline, it can still navigate reasonably.
Inertial Reference System (IRS)
The IRS can track an aircraft’s position without an external reference. Accelerometers and gyroscopes calculate aircraft movement and acceleration along all the three axes. At the beginning of the trip, when the pilots are setting the aircraft up, readying the flight, the crew tells the IRS, the aircraft’s latitude and longitude. It then senses acceleration along any axis and estimates the aircraft’s position. It needs only to know where the plane is before take-off. It is not as accurate as GPS, but it allows the aircraft to navigate with respectable accuracy if all other navigation references are lost.
Radio beacons on land send out radio beams that signal aircraft range and direction. This lets the plane’s computers calculate its location. More radio signals mean more accurate position estimates.
Source: HowStuffWorks & flightdeckfriend.com
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