Also worth investigating is WAAS (in north america...).

http://www.garmin.com/aboutGPS/waas.html

Surveyors, etc. who need high precision can be licensed to use equipment 
that uses the military bands available from GPS satellites. This allows 
for greater precision, although there are accuracy problems. The 
accuracy problems are solved by "averaging" multiple readings, thus, it 
typically involves a couple of hours with very expensive equipment to 
get centimeter types of accuracy.

Additionally, many of these systems use post-processing to further 
refine the numbers.

In other words, even the military grade signal does not give 
instantaneously useful highly accurate readings.

Atmospheric conditions are the primary cause of accuracy issues now with 
both WAAS, and GDPS.

Rolf

Jan-Erik Soderholm wrote:
> Michael Rigby-Jones wrote :
>
>   
>>> Aren't most sold GPS devices today actualy D-GPS devices ?
>>>
>>> Jan-Erik.
>>>       
>> I thought that differential GPS required two GPS receivers, 
>> one fixed at a known location and the other mobile receiver.
>>
>> Regards
>>
>> Mike
>>     
>
> Yes, but you don't have to own both yourself.
>
> In Sweden that state runs "fixed" GPS'es that resends
> the diff-info and many/some GPS receivers sold in Sweden
> has an built in recevier for this correction info, so they
> are true DGPS devices. There are also option receivers
> for this correction signal that can be attached to a
> non-DGPS receiver, if I'm not wrong.
>
> The correction signal is also sent over the RDS (Radio
> Data System) on the FM band all over Sweden.
>
> Anyway, I've also read that DGPS isn't realy needed
> anymore since the "error" in the GPS signal has been
> removed.
>
> Jan-Erik.
>
>
>
>   


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