Post Processing DGPS
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 latter, 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 sources for corrections in your area. Some academic institutions are experimenting with the Internet as a way of distributing corrections.
There's another permutation of DGPS, called "inverted DGPS," that can save money in certain tracking applications.
Let's say you've got a fleet of buses and you'd like to pinpoint them on street maps with very high accuracy (maybe so you can see which side of an intersection they're parked on or whatever).
Anyway, you'd like this accuracy but you don't want to buy expensive "differential-ready" receivers for every bus.
With an inverted DGPS system the buses would be equipped with standard GPS receivers and a transmitter and would transmit their standard GPS positions back to the tracking office. Then at the tracking office the corrections would be applied to the received positions.
It requires a computer to do the calculations, a transmitter to transmit the data but it gives you a fleet of very accurate positions for the cost of one reference station, a computer and a lot of standard GPS receivers. Such a deal!