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The Gravity Probe B relativity mission experiment is designed to measure the frame dragging and geodetic relativistic precessions in a 650 km polar orbit. We describe some of the advanced experimental techniques used to achieve the required gyroscope accuracy of between 0.05 and . The subjects discussed are: (i) the development of high-precision gyroscopes with drift rates of less than , (ii) a low-temperature bake-out procedure resulting in a helium pressure of less than at 2.5 K, (iii) a read-out system using DC SQUID magnetometers with a noise figure of at 5 mHz and (iv) AC and DC magnetic shielding techniques which produce an AC attenuation factor in excess of and a residual DC field of less than .

The Gravity Probe B mission provided two new quantitative tests of Einstein's theory of gravity, general relativity (GR), by cryogenic gyroscopes in Earth's orbit. Data from four gyroscopes gave a geodetic drift-rate of −6601.8 ± 18.3 marc-s yr−1 and a frame-dragging of −37.2 ± 7.2 marc-s yr−1, to be compared with GR predictions of −6606.1 and −39.2 marc-s yr−1 (1 marc-s = 4.848 × 10−9 radians). The present paper introduces the science, engineering, data analysis, and heritage of Gravity Probe B, detailed in the accompanying 20 CQG papers.

This paper gives a detailed account of the Gravity Probe B cryogenic payload comprised of a unique Dewar and Probe. The design, fabrication, assembly, and ground and on-orbit performance will be discussed, culminating in a 17 month 9 day on-orbit liquid helium lifetime.