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    Re: GPS Accuracy Now.
    From: Roger M. Derby
    Date: 2000 May 03, 8:36 AM

    What a fascinating speculation.  While your scheme of deriving attitude is
    certainly feasible, you can, with many nines probability, bet that any
    aircraft you see will be deriving its attitude information with gyroscopes
    or Mark I eyeball.  The gyros are laser based for the exotics and mechanical
    for we, the economically challenged.  (Wouldn't your system need an
    initialization and integration to detect inverted flight?)  Actually, I
    believe the use of lasers is limited to the navigational systems and that
    the cockpit displays and autopilot systems are driven by mechanical gyros in
    most of the "exotics".  Under mechanical I'm lumping the spinning weight,
    the vibrating reed, and the air puff systems.  Incidently, aircraft bend
    more than a few millimeters, so you'd need a rather exotic algorithm to
    compare nose/tail measurements and or wing tip inputs.  In some aircraft the
    deflection is measured in feet.
    The problem with absolute altitude information from GPS is that the geoid
    used isn't sufficiently well correlated to the terrain and/or charts.
    Except for final approach, altitude is based on pressure measurements and is
    used to separate aircraft.  Thus when aircraft travel into a low pressure
    area, all of them descend.  When leaving the low pressure area, they all
    climb.  A similar deviation occurs flying into an area of cold air.  One
    wouldn't want to have a mixed system in use.
    One main guiding principle in aircraft design is to avoid any and all single
    point failures.  In my Cessna, for position, I use ADF, dual VORs, DME,
    ground based radar (as a last resort), and GPS.  You can tell, if you take
    the time to analyse, which one is lying to you, and you should assume that
    one will be.
    The DGPS systems in use today are very localized as you say.  WAAS is (was?)
    supposed to provide a more wide spread solution.  Its main problem, as I
    understand it, is detecting out-of-service components and providing flags to
    the pilot when he needs to switch to an alternate scheme or abandon the
    My point was that, except for schedule, there is no reason to fly into an
    area with a ceiling below 200' AGL.  A few hours delay is not worth risking
    your life for.  Even if you had a guaranteed system for knowing your
    position and attitude, there will still be "no fly" areas; e.g., freezing
    rain, hail, tornados, earthquakes, presidential haircuts, lawn mowers on the
    runway, etc.
    ----- Original Message -----
    From  "Richard B. Emerson" 
    Sent: Wednesday, May 03, 2000 12:55 AM
    Subject: Re: GPS Accuracy Now.
    >   For aircraft, aside from positioning on a flight path,
    > attitude information is obtained by measuring the phase differences at
    > the nose, wing tips, tail and so on.  In that case, it's not where in
    > space the parts of the airplane are as where they are relative to each
    > other.  In this setting, millimeter resolution is possible.  Anyway,
    > it's my understanding that GPS-based landing systems are a very
    > localized version of DGPS.  In fact, one of the reasons US GPS landing
    > systems haven't been embraced by world commercial aviation is who
    > wants to think their air fleet is tied to the Yankee air pirate
    > navigation system?  By dropping SA, there's some hope that US makers
    > have a better chance to compete.
    > Rick
    > S/V One With The Wind, Baba 35

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