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The Gravity Probe B satellite used ultra-precise gyroscopes in low Earth orbit to compare the orientation of the local inertial reference frame with that of distant space in order to test predictions of general relativity. The experiment required that the Gravity Probe B spacecraft have milliarcsecond-level attitude knowledge for the science measurement, and milliarcsecond-level control to minimize classical torques acting on the science gyroscopes. The primary sensor was a custom Cassegrainian telescope, which measured the pitch and yaw angles of the experiment package with respect to a guide star. The spacecraft rolled uniformly about the direction to the guide star, and the roll angle was measured by star trackers. Attitude control was performed with sixteen proportional thrusters that used boil-off from the experiment’s liquid Helium cryogen as propellant. This paper summarizes the attitude control system’s design and on-orbit performance. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Gravity Probe B (GP-B) satellite was equipped with a pair of redundant Global Positioning System (GPS) receivers used to provide navigation solutions for real-time and post-processed orbit determination (OD), as well as to establish the relation between vehicle time and coordinated universal time. The receivers performed better than the real-time position requirement of 100 m rms per axis. Post-processed solutions indicated an rms position error of 2.5 m and an rms velocity error of 2.2 mm s−1. Satellite laser ranging measurements provided independent verification of the GPS-derived GP-B orbit. We discuss the modifications and performance of the Trimble Advance Navigation System Vector III GPS receivers. We describe the GP-B precision orbit and detail the OD methodology, including ephemeris errors and the laser ranging measurements. [ABSTRACT FROM AUTHOR]

This paper provides detailed descriptions of the numerical estimation algorithms used to fit physics-based models to the data from the Gravity Probe B spacecraft, as well as the scientific results of the experiment, and the statistical and systematic uncertainties. The first paper in this series of three data analysis papers derives the mathematical expressions for the signals to be analyzed, and the second paper deals with science data acquisition and their preparation for the relativistic drift rate estimation. The data from each of the four gyroscopes are partitioned into six segments, each spanning several weeks to several months. These segments are first analyzed individually to check the validity of the mathematical models and the accuracy of the estimation routine by examining the consistency of the relativistic drift rate estimates from each of these 24 gyro-segments. Then, the drift rate estimates and uncertainties are calculated for each individual gyroscope and for the four gyroscopes combined. These results are presented and compared with each other and with the prediction of general relativity. [ABSTRACT FROM AUTHOR]

A spaceflight electrostatic suspension system was developed for the Gravity Probe B (GP-B) Relativity Mission’s cryogenic electrostatic vacuum gyroscopes which serve as an indicator of the local inertial frame about Earth. The Gyroscope Suspension System (GSS) regulates the translational position of the gyroscope rotors within their housings, while (1) minimizing classical electrostatic torques on the gyroscope to preserve the instrument’s sensitivity to effects of General Relativity, (2) handling the effects of external forces on the space vehicle, (3) providing a means of precisely aligning the spin axis of the gyroscopes after spin-up, and (4) acting as an accelerometer as part of the spacecraft’s drag-free control system. The flight design was tested using an innovative, precision gyroscope simulator Testbed that could faithfully mimic the behavior of a physical gyroscope under all operational conditions, from ground test to science data collection. Four GSS systems were built, tested, and operated successfully aboard the GP-B spacecraft from launch in 2004 to the end of the mission in 2008. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]