I seem to remember that the original satellites (which were the first highly accurate clocks in orbit) had a hedge--if the effects of general relativity turned

I seem to remember that the original satellites (which were the first highly accurate clocks in orbit) had a hedge--if the effects of general relativity turned out to not be correct the system could drop that part of the correction. But I'm on janky hotel internet right now so I can't find a reference. Edit: The comment from jcr is correct--this was not in GPS, but in an earlier system. http://www.leapsecond.com/history/Ashby-Relativity.htm http://www.leapsecond.com/history/Ashby-Relativity.htm > At the time of launch of the first NTS-2 satellite (June 1977), which contained the first Cesium clock to be placed in orbit, there were some who doubted that relativistic effects were real. A frequency synthesizer was built into the satellite clock system so that after launch, if in fact the rate of the clock in its final orbit was that predicted by GR, then the synthesizer could be turned on bringing the clock to the coordinate rate necessary for operation. The atomic clock was first operated for about 20 days to measure its clock rate before turning on the synthesizer. The frequency measured during that interval was +442.5 parts in 1012 faster than clocks on the ground; if left uncorrected this would have resulted in timing errors of about 38,000 nanoseconds per day. The difference between predicted and measured values of the frequency shift was only 3.97 parts in 1012, well within the accuracy capabilities of the orbiting clock. This then gave about a 1% validation of the combined motional and gravitational shifts for a clock at 4.2 earth radii.