Not all DGPS applications are created equal. Some don't need the radio link because they don't need precise positioning immediately.

It's one thing if you're trying to position a drill bit over a particular spot on the ocean floor from a pitching boat, but quite another if you just want to record the track of a new road for inclusion on a map.

For applications like the later, the roving receiver just needs to record all of its measured positions and the exact time it made each measurement.

Then later, this data can be merged with corrections recorded at a reference receiver for a final clean-up of the data. So you don't need the radio link that you have to have in real-time systems.

If you don't have a reference receiver there may be alternative source for corrections in your area. Some academic institutions are experimenting with the Internet as a way of distributing corrections.