Re: Inertial Navigation ???

From:         Chris Dahler <no@spam.net>
Date:         Tue, 14 Mar 2000 04:51:06 GMT
Organization: NASA Dryden
References:   1
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> The INS/IRS systems on Boeing Aircraft (assume very similar on others)
> use the DME(radio ground stations) to confirm and update their
> position as they travel.
>
> Now with GPS - the NAV systems can be updated even when out in the
> Atlantic/Pacific where DME may not be available.  Which i suppose is
> ''re-zeroing'' itself.

Hmmm.  Actually, the way navigational systems work in Boeing airplanes
is not by updating the IRS's.  As far as I know, once the IRS's are
aligned on the ground, their position information cannot be updated
until the aircraft is stationary again.

Navigation is actually taking place via the Flight Management Computer
(FMC).  The FMC accepts navigational inputs from several sources (GPS,
DME/DME, VOR/DME, IRS).  The FMC chooses which position to accept
based on a hierarchy.  If GPS is available, that's primary.  Next is
DME/DME, then comes VOR/DME, and lastly comes the IRS position.  In
some types of setups, the FMC can be "told" what the current
positional error of the IRS's is (based upon positional information
from other sources like GPS), and the FMC can from then on apply a
correction based upon that deviation to the lat/long info it receives
from the IRS's, but the IRS's themselves cannot be realigned in
flight.

Now, before some avionics expert from Boeing or Honeywell or someplace
flames all over me, let it be said I'm just a pilot and am just taking
the information they gave me in class.  I don't pretend to know all
the inner workings of IRS's, beyond the fact that they are quite good
at what they do.

Chris