Gravity Probe B, launched 20 April 2004, is a space experiment testing two fundamental predictions of Einstein's theory of general relativity (GR), the geodetic and frame-dragging effects, by means of cryogenic gyroscopes in Earth orbit. Data collection started 28 August 2004 and ended 14 August 2005. Analysis of the data from all four gyroscopes results in a geodetic drift rate of -6601.8±18.3  mas/yr and a frame-dragging drift rate of -37.2±7.2  mas/yr, to be compared with the GR predictions of -6606.1  mas/yr and -39.2  mas/yr, respectively ("mas" is milliarcsecond; 1  mas=4.848×10(-9)  rad).

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.

Gravity Probe B (GP-B) is a landmark physics experiment in space designed to yield precise tests of two fundamental predictions of Einstein's theory of general relativity, the geodetic and frame-dragging effects, by means of cryogenic gyroscopes in Earth orbit. Launched on 20 April 2004, data collection began on 28 August 2004 and science operations were completed on 29 September 2005 upon liquid helium depletion. During the course of the experiment, two unexpected and mutually-reinforcing complications were discovered: (1) larger than expected ‘misalignment’ torques on the gyroscopes producing classical drifts larger than the relativity effects under study and (2) a damped polhode oscillation that complicated the calibration of the instrument's scale factor against the aberration of starlight. Steady progress through 2006 and 2007 established the methods for treating both problems; in particular, an extended effort from January 2007 on ‘trapped flux mapping’ led in August 2007 to a dramatic breakthrough, resulting in a factor of ∼20 reduction in data scatter. This paper reports results up to November 2007. Detailed investigation of a central 85-day segment of the data has yielded robust measurements of both relativity effects. Expansion to the complete science data set, along with anticipated improvements in modeling and in the treatment of systematic errors may be expected to yield a 3–6% determination of the frame-dragging effect.

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A simple pre-flight strategy of the Gravity Probe B (GP-B) data analysis has evolved in the elaborate multi-level structure after the discovery of the complex polhode motion, and of the patch effect torques. We describe a cascade of estimators (filters) that reduce the science data (SQUID and telescope signals) to the estimates of the relativistic drift rates. Those estimators, structured in two "floors", are based on the polhode-related models for the readout scale factor and patch effect torque. Results of the 1 st Floor processinggyro orientation profiles-manifest clearly the strong geodetic effect but also the presence of classical torque. Modeling of the patch effect torque at the 2 nd Floor provides a successful compensation of the torque contributions, and leads to consistent estimates of the relativistic drift rates.