From: John Tserkezis 
Reply-To: Fidonet AVtech Echo 

Bob Lawrence wrote:

> JT> The spec is +/-15 metres RMS 95%. That means it'll be within a
> JT> 15 metre radius, 95% of the time.
> JT> Typically, it's within about 5 or so.
> 
>  When it first became available, kit was 100m, and the military
> version that called in another satellite to get 1m, but I think they
> all do that now...

  Not quite.  You're thinking of Selective Availability.  It was a system that 
effectively scrambled the lower order accuracy of the signal to increase the 
error to 100m.
  They used a psudeo random encryption system that cycled every two days or so, 
so, if you averaged a stationary reading over that time period you would get 
the 15m accuracy they originally quote.
  Military equipment decrypts that, so they get the 15m accuracy.

  Selective Availability was turned off about May 2000? (can't remember exactly 
when) by the president at that time making available the full accuracy of the 
system to civilians.

  Since then, they have introduced WAAS (Wide Area Augmentation Service) which 
is loosely based on DGPS (corrected GPS) but only works in the US. (And only 
will ever work in the US due to it's control system).

>  Yair. If you look at Africa on a Mercator flat projection, and
> Siberia, Africa is *much* larger on a globe, and stretched out so the
> bottom half ismuch bigger, and Siberia (and Canada) is *much* smaller.

  Yeah, locations close to the poles don't lend themselves to UTM very well due 
to the severe distortion.

>  Anywhere you're likely to drive in a car, is as good as flat.

  Yes, it only matters if you're flying, or sailing, where you're likely to 
cover significant distances in pretty much "straight" lines.

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