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Q: Are you living in the neighborhood of an airfield? Maybe an airplane
intercepts one of the satellite signals, causing a temporary decrease in
accuracy.
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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