Welll.....that's not strictly true. If you've got an environment free of multipath, you use differential or WAAS-enabled GPS receivers, you can get pretty good accuracy.

Source: http://www.oc.nps.edu/oc2902w/gps/gpsacc.html

Please note that all of these things depend on many factors and can be considered nearly "heroic" measures when it comes to position accuracy, but the Navy has historically been pretty damn concerned about knowing where their assets are and is somewhat obsessive about error-factors where that's concerned.

With orbit and clock corrections delivered to the receiver in ~real time (such as Fugro's G2), you'll get <10cm (95%) on a standalone receiver.

translation: I didn't know about RTK before I posted. Thanks for schooling me.

It is, using RTK (real time kinematic) and 2 different receivers: http://www.kickstarter.com/projects/swiftnav/piksi-the-rtk-g...

And there you have it.

This is from the datasheet of the Trimble R10 surveying receiver:

real time Kinematic surveying

Single Baseline <30 km

Horizontal ...................................... 8 mm + 1 ppm RMS

Vertical ....................................... 15 mm + 1 ppm RMS

Network RTK3

Horizontal ..................................... 8 mm + 0.5 ppm RMS

Vertical ...................................... 15 mm + 0.5 ppm RMS

RTK start-up time for specified precisions4 .....................2 to 8 seconds

Trimble CenterPoint RTX

Horizontal .................................................. 4 cm

Vertical .................................................... 9 cm

RTX convergence time for specified precisions12 ........... 30 minutes or less

RTX QuickStart convergence time for specified precisions12 .... 5 minutes or less Trimble xFill5

Horizontal ................................. RTK6 + 10 mm/minute RMS

Vertical ................................... RTK6 + 20 mm/minute RMS

Yes but that somewhat limits the practicality because you need two receivers, a clear signal path between them and one has to be relatively static.

It's fine for surveying but for dynamic applications which are far more common, it's a pain.

You can receive RTK corrections over IP (via a cellular connection). Government bodies across the US push the data over IP to you if you register for a free account.

http://igs.bkg.bund.de/ntrip/index

http://en.wikipedia.org/wiki/Networked_Transport_of_RTCM_via... nternet_Protocol

Then you engineer another source of correction and incorporate that into your project.

The US Navy has been doing accurate position determination using ZERO satellite availability. How do you think a submarine knows where it is?

http://www.liveleak.com/view?i=aba_1332656862

In agriculture, it is not uncommon to get correction data over mobile data connections, where the static receiver is provided by a third-party service provider.