[dzhiurgis]

Friend of mine is doing ocean bottom surveys. He uses pretty decent GPS receiver, but also gets real-time (or applies it later) "correction" data from ground based stations that calculate error caused by atmosphere, which gets him down to 1cm accuracy.

[tamcap]

That's SBAS/WAAS! another cool thing

This is trying to measure ionosphere disturbances which affect signal propagation rates via ground reference stations. The real-time model is then calculated and distributed using ground stations and geostationary satellites.

GPS is way, way cool, and it's a pity the extent of its engineering is not better known.