Inertial Navigation System

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The Inertial Navigation System (INS) was the most common navigational system in use for international passenger flights prior to the commercial use of the GPS (Global Positioning System). Whereas the LORAN (Long Range Navigation) system used navigational aids external to the aircraft ("master" land station and two or more "slave" stations transmitting low or medium intersecting signals), INS was entirely internally based using gyroscopes and accelerometers, which detect changes in an aircraft's motion in any direction in reference to the gyroscopic lines, that minutely and continuously (seven times a second) adjusted through the automatic pilot the aircraft in flight, taking into account the changing wind, velocity, weight, and other conditions. An aircraft using INS was guided by preflight insertion of up to nine navigational "waypoints". If more were needed, number 10 and subsequent were inserted during the flight replacing entries overflown and "vacated". Three independent but electrically linked INS units were generally installed in a passenger plane - two active and "'redundant" with displays for pilot and co-pilot and one "hot spare". Each unit is comprised of three sub-units:

  1. An Inertial Navigation Unit which senses both the horizon and the various movements of the aircraft as it performs the necessary computations to guide the plane along its desired track. The INU is housed in the electronics bay of the aircraft.
  2. A Control Display Unit, containing digital readout windows for navigational data as well as pilot data entry options. The CDU is located in the flightdeck.
  3. A Mode Selection Unit, used for navigational mode engagement. The MSU is located on the flight deck.

A famous incident involving a passenger plane that strayed from its INS guided flight path was the shootdown by the Soviets of Korean Airlines Flight 007, shot down west of Sakhalin Island in 1983. Either the pilots had failed to switch from Magnetic Heading to INS or did so out of the maximum range of 7 1/2 Nautical Miles from the "desired track". This mistake may have lead to the shootdown itself. It was due to KAL 007's deviation from its route while using INS that President Reagan authorized GPS to be used for non-military navigational purposes

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