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Most of the Flight Payload hardware for the Gravity Probe B Relativity Mission is currently being manufactured. The design, fabrication, and integration of this hardware has already been subjected to an extensive program of full scale prototyping and testing in order to provide maximum assurance that the payload will meet all requirements. Full scale prototyping is considered to be a crucial aspect of the payload development because of the complexity of the payload, the stringency of its requirements, and the necessity for integration of a warm cryostat probe into a dewar maintained at liquid helium temperature. This latter requirement is derived from the fact that the dewar contains a superconducting ultralow magnetic field shield which provides an ambient magnetic field environment for the probe of

Gravity Probe B is an experiment to measure the geodetic and frame-dragging precessions, relative to the “fixed” “stars”, of a gyroscope placed in a 650 km altitude polar orbit about the earth. For Einstein's general relativity, the precessions are calculated to be 6.6 arcsec/yr for the geodetic precession and 0.042 arcsec/yr for the frame-dragging precession. The goal of the experiment is to measure these precessions to better than 0.01% and 1%, respectively. This paper gives an overview of the experiment and a discussion of the flight hardware development and its status. This paper also includes an estimate of the geodetic and frame-dragging errors expected for the experiment.

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 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.

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.

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.

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.

NASA’s Gravity Probe B Mission is a test of two predictions of Einstein’s General Theory of Relativity based on observations on very precise cryogenic gyroscopes in a satellite in a 650 km polar orbit about the Earth. Construction and the first round of testing of the flight payload was completed in December 1999. Of the 32 planned qualification tests 28 were passed with complete success, meeting or in several instances surpassing the program requirements. However, one test very unexpectedly revealed a problem in the thermal performance of the Dewar/Probe system which has required a significant redesigin and rework, now successfully completed. Gravity Probe B is scheduled for launch on April 1, 2002. This article reviews from the physicist's viewpoint the experience of living through a space flight program.

The relativity mission, also known as gravity probe B (GP-B), uses high-precision electrostatically suspended cryogenic gyroscopes for measuring the relativistic precessions of the frame of reference in a 650 km polar orbit. A 2 K environment is used to ensure the thermal stability and to implement the readout technique based on the magnetic dipole moment generated by a rotating superconductor. Analysis and results from more than 100 000 h of gyroscope operation show that the residual Newtonian drift is less than 0.17 marcsec/yr for a supported gyroscope in 10 −9 m/s 2 , and less than 0.020 marcsec/yr for a gyroscope in a fully inertial orbit.

Techniques based on superconductivity are crucial in providing the means of achieving the high accuracy and low noise required by experimental tests of gravitational theories. We discuss applications of superconductivity to two space-based experiments: the Gravity Probe B Relativity Mission (GP-B), and the Satellite Test of the Equivalence Principle (STEP). Superconducting shields attenuate the dc magnetic field to less than 10−11 T and provide an ac shielding factor in excess of 1012. The readout of the GP-B gyroscopes is based on the London magnetic dipole generated by a rotating superconductor and detected with state-of-the-art dc SQUIDs, which are also used in STEP.

Linear and nonlinear attitude control law synthesis for spinning aerospace vehicles, including analog simulation for control designs based on frequency symmetry