You might be aware that surveyors have been using GPS to do extremely precise surveys for years, fixing points with relative accuracy on the order of millimeters.Relative positions
To the person on the street GPS is a navigation resource. It locates you in terms of latitude and longitude somewhere on the planet.
But in many surveying applications the job isn't locating a point according to Lat and Lon, but rather fixing the positions of a group of points in relation to each other.
These points are often eventually tied into a "control" point that locates them with respect to the rest of the world, but the important relationship is really among the points themselves.
This distinction is important when it comes to accuracy. Relative positions are easier to measure accurately.
They get this accuracy by using GPS in a very specialized way. In fact it's really a form of interferometry.Interferometry
A measurement technique based on the fact that two waveforms will constructively or destructively interfere with each other if they arrive slightly out of phase.
The large effect of the interference is easier to measure than the signals themselves and so provides a very sensitive way to compare two signals.
They use multiple receivers like the differential systems we've been discussing but the technique is much more involved - so involved that only trained geodesists with expensive machines and lots of time can do it.
But even though the system surveyors use is too finicky for most users of GPS, one of its underlying principles is starting to find its way into general receivers.
It's called "carrier-phase GPS," and it can be orders of magnitude more accurate than the "code-phase GPS" that most mortals use. Continue to see how.