Part of it is the difference in the receiver hardware. Survey-grade GPS uses the phase of the signals as well as their timestamps to measure sub-wavelength reso

Part of it is the difference in the receiver hardware. Survey-grade GPS uses the phase of the signals as well as their timestamps to measure sub-wavelength resolution. This requires larger antennas and much heavier-duty processing hardware, and generally only works when stationary as well, so it's not really feasible for a mobile device.