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Old 25th Sep 2008, 15:03
  #28 (permalink)  
FE Hoppy
Join Date: Sep 1998
Location: wherever
Age: 50
Posts: 1,609
I concur with BOAC to a certain degree.

IR(S) System
IR(U) Unit
I(NS) Navigation System

All different.

Think of the Unit as outputting accelleration values

System putting those values to some use.(perhaps using them to calculate a position relative to a given starting position)

Navigation System using a calculated postion to output stearing comands between given waypoints.

The Navigation function is usually done by FMS these days, hence the use of IRS rather than INS to describe the unit.

So the best description of an IRS is a navigation (position) sensor.

The IRU has to align itself in-order to accurately diferrentiate between aircraft accelerations and earth accelerations.

Some newer IRS can do this inflight. Honeywell do a nice micro IRS system with this capability so after a power interupt the unit will automatically re-align itself using GPS position to calculate aircraft track values and subtracting this from its total acceleration to calculate earth related accelerations then use these to re align to true north.

Way too complicated for my small brain but very very nice. Align In Motion will take between 15 and 30 minutes.
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