Checking Time Dilation due to gravity

[Ray McCarthy]

-- Again

By accident
http://www.theregister.co.uk/2015/11/10/borked_eu_gps_satellites_braincheck_einstein/

Albert Einstein predicted a century ago that time would pass more slowly close to a massive object. It has been verified experimentally, most significantly in 1976 when a hydrogen maser atomic clock on Gravity Probe A was launched 10 000 km into space, confirming the prediction to within 140 parts in a million.

The two navigation satellites will give perhaps x4 more accurate measurement.