Now pick one up. The one on the ground is your fixed point, the other unit is what you use for measuring. I wonder if doing it like that (possibly with two or three "anchors") -- would give increased precision? I haven't really looked in to what makes cheap gps "unreliable".
The units could communicate via bluetooth/mesh wifi for low latency, or over the internet (wifi/gprs/4g/dial-up...) -- if one just needed "point-to-point" measurements. Eg measuring a foundation or something related for construction.
I wonder if it would be "repeatable" (possibly only with the same units). And if one might get cm precision that way.
[ed: Not sure which part is "hard". Maybe (just for science experiment fun) one could drop a phone at one end of a 100m running track, an another at the other end - to get an exact 100m baseline (+/- 10cm?). I wonder if that might be useful in order to calibrate two or more units to work in pairs -- if there is some kind of predicable source of error in addition to just gps/signal noise etc].
If you wanted to experiment, there's no need to do it online. If you log the output of the two receivers you can align the timestamps and try different ways of post-processing.
However, I wish some of these guys would show us the initialization procedures. In my experience trying to get an RTK level solution with an L1 only GPS receiver (read: cheap) is impractically hard. Then once you have it, you can lose it with the slightest change in the number of satellites available (i.e. if you go under a tree or next to a building). However, my experience is now over 3-4 years old, maybe we've come a long way.