DGPS may provide a solution for keeping GPS accuracy reasonable (within 1-5m CEP with a < 100km baseline) which will be somewhat better than the satellite based WAAS or EGNOS PRN as long as the baseline is within a suitable range to the ground station. DGPS receivers such as the CSI MBX3 are pretty expensive and bulky so here is a link to a home brew solution for a low cost solution;

http://web.abnormal.com/dgps/bix1/ (makes for a nice home electronics project )

Your GPS model will need RS232 port comms rather than USB though to recieve the RTCM-SC104 message.

Even the cheap Garmin eTrex Units can have improved accuracy by ensuring WAAS or EGNOS PRN is enabled for differential accuracy as long as the firmware is up to date. But I don't think the eTrex models have HDOP and velocity filtering though or even the ability to discard any satellite PRN below a certain angular height above the horizon.