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on 2/1/02 11:25 PM, Brian Whatcott at inet@INTELLISYS.NET wrote:

> Instead, bigger airplanes used a flux gate sensor to drive a 'slaved' gyro.
> (This may be implemented as the compass card of a Radio Magnetic
> Indicator ("RMI") or lately as a Horizontal Situation Indicator ("HSI") or
> currently as a compass card surrogate depicted on the
> CRT or LCD or Plasma display of a "glass cockpit".)

Actually on modern "big" airplanes like the747-400 I used to drive the
navigation system has no magnetic compass at all! The system must be
"initialized" before each flight during which time the aircraft must not be
moved, or even jostled by ground equipment. The inertial reference system
(IRS) utilizes three ring laser gyros and accelerometers to establish a
horizontal plane. This plane tilts as the earth rotates in an easterly
direction. Thus I believe the system actually establishes an East West
reference from which even I can figure out the geometry to establish a North
reference. The IRS can then provide direction, course, and speed to the
flight management computer (FMC.) The IRS position is always kept in the
computer, but the FMC can update the displayed position with radio fixes
since it always knows where it is and it's data base includes the
geographical position of all radio aids and their frequencies, it auto-tunes
them and utilizes the best sources for navigational updates to the inertial
position. It also has magnetic variation in it's data base, so it can
display magnetic heading. But it has no compass.

There are of course three FMCs so that a voting program can establish a mean
position during periods over the ocean when there are no radio updates, in
case one of the IRSs decides to "take off."

John LeRoy
M/V Traveller

   

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