[dingaling]

I remember this question arising on HN about a year ago. GR is a necessary consideration in the design of the satellite system.

See page 5 of DTIC document: Relativistic Effects in the Global Positioning System

http://www.dtic.mil/dtic/tr/fulltext/u2/a158720.pdf

Without correction the drift due to combined coarse relativistic effects would be on the order of 100,000 nanoseconds per DAY.

In fact the first NTS-2 satellite was launched with the clock unadjusted for GR, due to doubters, but after three weeks the syntheziser had to be activated to compensate. The calculated uncorrected drift on that clock alone was 38,000 nanoseconds per day.

As the document notes, to further refine Navstar to greater levels of precision would require tackling yet another set of tougher relativistic effects, plus more mundane atmospheric influences.