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    Re: GPS accuracy
    From: E.R.Kooi
    Date: 2000 Jul 05, 10:37 AM

    Q: Are you living in the neighborhood of an airfield? Maybe an airplane
    intercepts one of the satellite signals, causing a temporary decrease in
    Greetings, Eppo R. Kooi.
    > -----Original Message-----
    > From- Navigation Mailing List
    > [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of geoffrey butt
    ...... snip ......
    > In the days of Selective Availability I observed the anticipated
    > dance of the
    > instantaneous position around the long-term average position;  routinely
    > observing displacements of 100-200 metres.  What I also recorded however
    > were major 'excursions' where the position readings would track in a fixed
    > direction away from the average position - steadily moving up to
    > 2-3 NM away
    > - and then tracking steadily back along the same radial plot to
    > the vicinity
    > of the average position.
    > These 'excursions' would take between 10 and 30 seconds to complete.
    > They occurred at random times, but typically there might be up to
    > 2 or 3 per
    > hour.
    > They were directed randomly around the compass;  the long-term plot would
    > show them apparantly uniformly distributed.
    > Some days - when SA seemed to be particularly 'quiet' - there would be no
    > major excursion for many hours, but sooner or later they would restart.
    > The same phenomena were recorded in different locations with unobstructed
    > sky aspects.
    > On passage at sea I have observed similar spikes recorded on the track
    > display of the unit.
    > ...... snip ......
    > Since May I have again tried the set-up out at different locations - again
    > observing similar results - and have again observed erratic spikes on the
    > set's track screen when at sea.
    > The frequency of excursions and their uniform distribution in direction
    > remain the same, but I have noted a reduction in the magnitude of the
    > maximum displacement.  (Statistics at the end of this note)
    > Has anyone observed similar phenomena?
    >         - plots of readings I have found now on the web show beautifully
    >         compact areas;  just as predicted!
    > Is there a well-understood explanation?

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