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    Re: Long-range airplane navigation
    From: Ken Gebhart
    Date: 2004 Nov 28, 22:26 -0600

    on 11/28/04 12:20 PM, Gennaro Sammarco at skipper767{at}YAHOO.IT wrote:
    
    > 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
    >
    Hi Guys,
    
     I feel like I should not let this moment pass without saying something
    about aircraft navigation, since that has been my background.  It was
    suggested that high speeds have contributed to the demise of CN in
    airplanes.  I believe it has been the proliferation of electronic methods
    rather than high speeds that caused this.  High speeds are themselves no
    problem, except for neophyte navigators (and for that matter pilots too).
    It merely requires ?staying ahead of the airplane?.
      When I was with Boeing, we routinely ferried ?green? 707s across the north
    Atlantic.  Green meant that the planes had only the bare rudiments of nav
    equipment, effective within 150 miles from land.  Oceanic navigation was
    purely celestial (usually one sextant, one man, and a box lunch).  Apre?
    flight analysis usually revealed that the plane could be kept to within 4 nm
    of intended track  by taking a series of sextant sights every 40 minutes.
      An interesting aspect for this group might be that coriolis and rhumb line
    corrections were extremely important aspects of bubble sextant operation,
    and could reach as high as 28 nm at 450 kts at high latitudes.
    
    Ken Gebhart
    
    
    

       
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