Comment by burnt-resistor

RTK = Trimble Navigation.

Even back in 1999 prior to Selective Availability (SA) zeroing[0], fixed base station-assisted (pre-WAAS DGPS RTK) could achieve 10 mm horizontal accuracy and 1 m vertical accuracy. It was good enough that farming, mining (above ground), and earth-moving equipment could combine 2 receivers to determine tool angle and cut depth.

0. SA was an injected random error for security reasons that was constant for a local area, so it was weak security because it could be easily defeated if a fixed base station's position was known by subtracting it in the RTK receiver via DGPS update.