Gravity Probe A
Gravity Probe A (GP-A) was a space-based experiment to test the theory of general relativity, performed jointly by the Smithsonian Astrophysical Observatory and the National Aeronautics and Space Administration. It sent a hydrogen maser, a highly accurate frequency standard, into space to measure the rate change of a clock in lower gravity with high precision.
 Experimental setup
The 100 kg Gravity Probe A spacecraft housed the atomic hydrogen maser system that ran throughout the mission, and a microwave repeater to measure the Doppler shift of the maser signal. The satellite was launched nearly vertically upward to cause a large change in the local gravity seen by the maser, reaching a height of 10,000 km (6,200 mi). At this height, general relativity predicted a clock should run 4.5 parts in 1010 faster than one on the Earth.
According to the 1976 press release by Joyce B. Milliner: "The interaction of the electron and proton in the hydrogen atom generates a microwave signal (1.42 billion cycles per second) stable to one part in a quadrillion (1 x 1015), or the equivalent of a clock that loses less than two seconds every 100 million years."
The clock rate was measured from the ground by comparing the microwave signal from the clock to a maser on the ground and subtracting a signal from the spacecraft that measured the Doppler shift. The clock rate was measured for most of the duration of the flight and compared to theoretical predictions. The stability of the maser permitted measurement of changes in the rate of the MASER of 1 part in 1014 for a 100-sec measurement.
The experiment was thus able to test the equivalence principle. Gravity Probe A confirmed the prediction that gravity slows the flow of time, and the observed effects matched the predicted effects to an accuracy of about 70 parts per million.
 See also
- "Fundamental Physics of Space - Technical Details - Gravity Probe A". Nasa JPL. May 2, 2009. Retrieved May 2013.
- Milliner, Joyce B. (June 10, 1976). "Space Probe to Test Einstein's "Space-Time Warp" Theory". Retrieved May 2013.
- Than, Ker (May 05, 2011). "Einstein Theories Confirmed by NASA Gravity Probe". National Geographic Society. Retrieved May 2013.