Certainly many OEM GPS modules are DGPS ready. This means that there is a
fixed location broadcasting the correction info as has been said. This info
is received by the end user using a radio reciever tuned to the DGPS station
and the correction info input to the GPS module. As far as I remember (it's
been a while since I looked at this) you are supposed to use the nearest
DGPS station to you for best accuracy. Selective availability (intentional
errors introduced by the US military) was turned off in 2000 as far as I
remember. This could, of course, be turned on again at any time if they
needed to, so I would not be throwing out the DGPS rx unit yet...

Joe


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