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Hi Lee, the 3 IRSs work independently but check each other for a continous
averaging. If one gives a position out of a given margin of error, based on
this averaging, is immediately excluded and a warning message will appear on
instruments. Obviously the most important moment is when you enter your
initial position, because even large input errors will give no warning.
During the flight, primary update system is based on radio distances
automathically received and compared from radiobeacon tuned independently by
one of the flight computers. When you get out of range from the beacons,
then the nav system reverts on IRSs only, but, on aircrafts equipped with
GPSs, they become the last source. In other hand is a very precise dead
reckoning, when you don't have GPSs. As soon as you are in the range of a
radiobeacon, the system starts again to update continously with ground
stations.
Probably celestial has been abandoned due to the incresing in speed of
aicraft and the more and more reliability of nav systems.
Anyway, on my boat, I have 2 GPSs, but enjoy celnav when I can, along with
traditional plotting and navigation.

Gennaro Sammarco
----- Original Message -----
From: "Lee Martin" 
To: 
Sent: Sunday, November 28, 2004 2:51 AM
Subject: Re: Long-range airplane navigation


> ----- Original Message -----
> From: "Gennaro Sammarco" 
> To: 
> Sent: Saturday, November 27, 2004 4:37 AM
> Subject: Re: Long-range airplane navigation
>
>
> > Hi everybody, I fly for Alitalia on B767, long haul flights only, and we
> > don't use sextants and celnav anymore (unluckily). Navigation is taken
> care
> > of by 3 IRS (inertial navigation system based on laser gyro) and 2 GPS
> when
> > avilable, because it must not be the only device for navigation but
needs
> a
> > reliable back up system.
> > With twin jets, anyway, it is still required to plot the route on a
> special
> > nav chart and cross check the position  10 minutes after every meridian
on
> > the track.
> > Some more specific navigation knowledge, celnav included, was required
as
> > professional licence to upgrade to long haul flights till 1992, when its
> > necessity by law was cancelled.
> > Every night crossing, anyway, I have a star finder and a handy program
on
> my
> > palm (planetarium), and try to spot all the useful stars that I can,
> > training for celnav on my sailboat.
> >
> > Gennaro Sammarco
>
> Hi Gennaro
> Thanks for your note, which I found quite interesting.
>
> You mention 3 IRS  . Are these run simultaneously and averaged in any way,
> or are the multiple sets of IRS's, and GPS's for backup in case of
failure?
> I presume the GPS feeds back into the IRS system periodically to re-
> callibrate it, or whatever the correct expression is. Is radio direction
> also a component of navigation?
>
> I guess celestial has become more and more impractical , and other methods
> embraced as soon as possible, as aircraft speeds have risen. Hence the
> relatively  early (to my mind) abandonment of celestial in 1992.
>
> Lee Martin

   

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