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Inertial Navigation System (INS)

An INS is very simple in theory, but complicated in practice. Put simply, it is a totally self-contained dead reckoning system. Given its starting position, INS keeps track of all movements in all directions so it calculates the aircraft's flight position in relation to that point. To detect movement, the INS uses three accelerometers, one each north-south, east-west, and up-down mounted on a stable platform. An accelerometer is an electronic device that provides information similar to a gyroscope. Part of the accelerometer is in a fixed position and the other part is free to move with the aircraft. A magnetic field is produced by electricity between the two parts. Any change in movement by the free part will disturb the magnetic field. This disturbance will be recorded into the onboard computer which reads the data and calculates the amount of movement. The accelerometers use sliding shuttles and can detect accelerations up to a thousandth of a G force. The platform is stabilized using three gyros, one each for pitch, yaw and roll. This way the aircraft's movement is constantly monitored and helps the pilot keep the aircraft on course.

Newer Inertial Navigation Systems use ring laser gyros that are made up of a series of lasers aligned in the same plane and forming a ring. Interference patterns are generated as the aircraft accelerates indicating changes in the airplane's movement. Accuracy is within 1.7 nmph. So after an hour the accuracy is 1.7 nm.

The INS must be initialized on the ramp prior to takeoff. The pilot merely enters the aircraft's coordinates and the system performs the calculations since it has an internal clock calendar. Warm-up time and the time it takes for the INS to "sense" north can take from 2.5 to 45 minutes.

This system computes for the pilot the following flight data:

• track
  
• drift angle
  
• cross track error
  
• distance traveled
  
• distance remaining
  
• flight time remaining