Wouter,
I don't think it is fair to compare the accuracy of DGPS to GPS w/o SA. Once SA is gone the largest single source of error is atmospheric distortion and DGPS is able to compensate for this but of course these systems are pretty spendy. However DGPS can get you sub-meter and even sub-cm accuracy which is way better than just GPS without SA. Now WAAS is a poor mans version of DGPS but is a good alternative given the cost.

Your idea of using a fixed reciever to subtract out the atmospheric error and get poor mans DGPS is interesting, it would be an interesting expt to set 2 GPS recievers next to each other, log the data, subtract them and see how much error you get. If you do this please let us know how accurate this is. BTW this is the way they used to do DGPS back in the old days...

Wow now that I think about it for a minute it has been 10 years since I worked in this field, sure doesn't feel like it. But some of my details might have become fuzzy with time...

-Eric
SC 20 without GPS