Well in Geophysics we had the option to use hand held differential GPS systems that are accurate to a couple of centimetres.

It requires a base station receiver and the use of roaming receivers that use the base station as a calibration point.

The thing is it is not just the GPS receivers that are getting smaller it is the atomic clocks... from 2m cubes to 2cm cubes that can be hand held... that can really increase accuracies... and make satellites that are far lighter and hence cheaper to launch.