Gravity Probe A (GP-A) was a space-based experiment to test the equivalence principle, a feature of Einstein's theory of relativity.

Masses cause distortions in spacetime, which leads to the effects of length contraction and time dilation, both predicted results of Albert Einstein's theory of general relativity.

The probe was launched on June 18, 1976 from the NASA-Wallops Flight Center in Wallops Island, Virginia.

The probe was carried via a Scout rocket, and attained a height of 10,000 km (6,200 mi), while remaining in space for 1 hour and 55 minutes, as intended.

This distortion causes time to pass more slowly in the vicinity of a massive object, compared to the rate experienced by a distant observer.

Such time dilation appears in the Rindler coordinates, attached to a uniformly accelerating particle in a flat spacetime.

From this apparent variance in time, Einstein inferred that change in velocity affects the relativity of simultaneity for the particle.

In this way, the Gravity Probe A was a test of the equivalence principle, matching the observations in the inertial reference frame (of special relativity) of the Earth's surface affected by gravity, with the predictions of special relativity for the same frame treated as being accelerating upwards with respect to free fall reference, which can thought of being inertial and gravity-less.

A hydrogen maser produces a very accurate signal (1.42 billion cycles per second), which is highly stable—to one part in a quadrillion (1015).

The transponder signal received on the ground was Doppler shifted due to the speed of the rocket and was gravitationally blue-shifted by the same amount that it was red-shifted on the way up.

The probe was launched nearly vertically upward to cause a large change in the gravitational potential, 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, or about one second every 73 years.