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When Selective Availability was in force, we knew the error was there because
1) The navigation message said so (as reflected in the URA).
2) The scatter in positions made it obvious.
3) Receivers with RAIM capability can see it (and other errors).
More info on SA is in "The issue of selective availability" by Y. Georgiadou and K.D.
Doucet in GPS World,
September/October 1990, Vol. 1, No. 5, pp. 53-56
.
-- Richard Langley

Quoting Bruce Hamilton :

> The Technology it just fine. It's the politics that add the error.
>
> Celestial has never had error introduced into it at someone's whim. This
> makes me cautious about relying on it as an infallible source of
> information.Besides, I think I could tell if someone was at my almanac with
> the whiteout.
>
>  If the US thinks it is in their interest to put in error, they will,
> especially if it is a defense issue.
>
>  That is why other other GPS systems are being planned. Unless you have a
> DGPS or another reference, you will not know the error is there.
>
> --
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> Or post by email to: NavList@fer3.com
> To , email NavList+@fer3.com


===============================================================================
 Richard B. Langley                            E-mail: lang@unb.ca
 Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
 Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
 University of New Brunswick                   Fax:      +1 506 453-4943
 Fredericton, N.B., Canada  E3B 5A3
     Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
===============================================================================


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