Comment on Long-Theorized GPS Weakness Exploited on Large Scale by Paul

In reply to Mystick.

GPS IS supplemented by terrestrial stations, at least in North America. Local WAAS ground station measurements are used to transmit corrections by the WAAS satellites. Receivers then combine that with the GPS data to refine their position estimate.

DGPS, at least the survey-grade instruments, skips the satellite hop and generates position fixes relative to a (very) local reference site. They can get sub-centimeter precision.