freebsd-ports/astro/dgpsip/pkg-descr

On a stationary GPS without a differential correction signal, you
should see a 20 m average radius "drunken walk" pattern. On the same
receiver with DGPS corrections and a good view of the sky, the
error the same receiver with DGPS corrections and a good view of
the sky, the error should be reduced to approximately 2 m average
radius. If you've always wanted to know exactly where your house
is located without that annoying GPS "wander," but didn't want to
spend the money for a DGPS radio, here is your big chance!

I'd like to announce a fun DGPS hack. I've written a small Un*x
server and client for redistributing DGPS correction signals over
the Net. Basically the server grabs the serial byte stream from my
DGPS radio and sends it off over a TCP connection. The client does
the same thing but in reverse.  The result is that you can receive
the local DGPS corrections from absolutely anywhere by using the
Internet as the world's largest extension cord.  You'll still  need
to be within 1000 miles or so of San Francisco, California, USA
for best results. However chances are better than not that the GPS
error will still be reduced if you are 2000 to 3000 miles away.
Several respondents from 2000 miles away have noted that the remote
differential signals have diminished the SA-induced position and       
velocity errors by approximately 3 times.

WWW: http://www.wsrcc.com/wolfgang/gps/dgps-ip.html