In a message dated 25/12/04 00:00:14 GMT Standard Time,
[log in to unmask] writes:
> Date: Fri, 24 Dec 2004 08:14:17 -0800
> From: Andy Alexis <[log in to unmask]>
> Subject: Re: Veering Off-Topic (GPS)
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> It is true; up until Bill Clinton, GPS units had what is called
> "selective availability"; which is intentional degradation of the
> signal such that 100 meters was as close as you can get. The signal
> is degraded randomly too so you can't even do anything fancy to zero
> in. Bill Clinton , pre-9-11, announced they were turning SA off.
> George has never said they turned it back on, but I wouldn't be
> surprised. In the last week or so, the Bush administration announced
> that in a national emergency, GPS would be turned off alltogether, at
> least to the public.
> But what do I know? I just play banjo
Well the "Selective Availability" COULD be switched on as and when required
but as it names suggest's this was "selective" ie it WASN'T on all the time
Bill did announce it has been turned off and as far as I know would require
an Act of Congress to turn it ON again - basically it has to be made available
to the American people as they paid for it - but we really appreciate your tax
dollars over this side of the pond!
Even when SA was ON the 100m accuracy was the worse case scenario it was
usually between 10 - 30m. A 6 figure grid reference (3 digit Northing + 3 digit
Easting) as used on the UK Ordanace Survey Grid System is regarded as pretty
accurate and this 6 figure grid ref actually defines a 100m square on a 1:25,000
scale map - for what it's worth very few people can navigate to a 100m square
anyway (even WITH a GPS!)
SA will very likely NEVER be switched on again as the Military (and
authorised Civilian Forces) use a different (encrypted) channel in addition to the
channel used by civilian devices and the Military can selectively degrade the
signal from ANY of the 24 satellites in the GPS constellation to deny it "In Area"
to those it doesn't want to use it.
Using the American WAAS system (or European EGNOS System) ground based
stations you can now "correct" SA or other errors down to sub 2m accuracy.
But GPS has generally become so indespensible to millions of users that they
would find it very hard to do so again.
Anyway the Europeans are just launching their OWN system (GALLELIO) in first
quarter 2005 - satellite 44 is transmitting now!
S**t do I really know all this crap!