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    Re: GPS Accuracy Now.
    From: Richard B. Emerson
    Date: 2000 May 03, 11:07 EDT

    Roger M. Derby writes:
     > Rick,
     > 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.
    This isn't my scheme.  I read about it in GPS World magazine a couple
    of years back.  I don't know where the project stands now but the
    article was quite specific about how the system works and about its
    performance.  I'm trying to recall where the work was done and all I
    can come up with is "Univ. of Ohio" or something like that.
    I'm well aware of wingtip flexion, for example, and all the rest of
    the issues you mention.  All I can suggest is to chase down the
    article and see what's what.
     > 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.
    Scuse me?  It's been a long time since I was a pilot in command but I
    learned very early on that he who doesn't keep his altimeter set for
    local pressure is begging for a "controlled descent into terrain".
    Anyway, I leave it to all the "alphabets" to come up with a scheme for
    resolving GPS and pressure altitude compatibility and integration
    issues.  My guess is, however, pressure and radar altimeters will
    remain the standard with GPS altitude being applied only in approaches
    and departures.
    As to the geoid definitions, I beg to differ there, too.  There are a
    number of geoid description systems in use around the world because
    GPS is capable of resolving the differences in the Earth's shape
    (i.e., it's not regular in any direction) compared to a regular,
    mathematically defined shape.  WGS84, for example, works well for the
    continental US.
     > 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.
    No question about that!
     > 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
     > approach.
    I have never met any part of WAAS and therefore can't comment on its
    features.  The DGPS system I referred to, however, was not the one
    meant for broad areas of service (e.g., using USCG beacons) but for a
    very localized (IFR pun) area on approach and departure paths.  This
    technology is alread in use for survery work both commercially and for
    geologic research and yields millimeter accuracy and precision.
     > 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.
    "There are old pilots and there are bold pilots but there are no old,
    bold pilots."
    S/V One With The Wind, Baba 35

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