William J. Bencze

The Gravity Probe B test of general relativity

The Gravity Probe B mission provided two new quantitative tests of Einsteins theory of gravity, general relativity (GR), by cryogenic gyroscopes in Earths 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.

A separation principle for hybrid control system design

A method is presented here, based on automatic control system design practice, for the synthesis of hybrid control systems, controllers which contain both real-time feedback loops and logical decision-making components. In the proposed framework, the overall design task is separated into three component parts: 1) design of the real-time control loops; 2) synthesis of the decision-making logic; and 3) construction of appropriate boolean/real-time translation routines. This is an effective partitioning of the control system design task, as shown through two examples: 1) control of a highly flexible structure and 2) a robotic manipulator control task. This framework was found to be compatible with expert system-based intelligent control systems and can employ expert system techniques when necessary or effective.<<ETX>>

Gravity Probe B data analysis: II. Science data and their handling prior to the final analysis

The results of the Gravity Probe B relativity science mission published in Everitt et al (2011 Phys. Rev. Lett. 106 221101) required a rather sophisticated analysis of experimental data due to several unexpected complications discovered on-orbit. We give a detailed description of the Gravity Probe B data reduction. In the first paper (Silbergleit et al Class. Quantum Grav. 22 224018) we derived the measurement models, i.e., mathematical expressions for all the signals to analyze. In the third paper (Conklin et al Class. Quantum Grav. 22 224020) we explain the estimation algorithms and their program implementation, and discuss the experiment results obtained through data reduction. This paper deals with the science data preparation for the main analysis yielding the relativistic drift estimates.

Precision attitude control of the Gravity Probe B satellite

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 experiments liquid Helium cryogen as propellant. This paper summarizes the attitude control systems design and on-orbit performance.

The design and testing of the Gravity Probe B suspension and charge control systems

The Relativity Mission Gravity Probe B (GP-B), is designed to verify two rotational effects predicted by gravitational theory. The GP-B gyroscopes (which also double as drag free sensors) are suspended electrostatically, their position is determined by capacitative sensing, and their charge is controlled using electrons generated by ultraviolet photoemission. The main suspension system is digitally controlled, with an analog backup system. Its functional range is 10 m/s2 to 10−7 m/s2. The suspension system design is optimized to be compatible with gyroscope Newtonian drift rates of less than 0.1 marcsec/year (3×10−12 deg/hr), as well as being compatible with the functioning of an ultra low noise dc SQUID magnetometer. Testing of the suspension and charge management systems is performed on the ground using flight gyroscopes, as well as a gyroscope simulator designed to verify performance over the entire functional range. We describe the design and performance of the suspension, charge management, and gyros...

The expected performance of Gravity Probe B electrically suspended gyroscopes as differential accelerometers

Four cryogenic gyroscopes on the Gravity Probe B satellite will be used to measure the precession of the local inertial reference frame with respect to a distant inertial reference frame. One of these four gyroscopes will serve as the drag-free sensor for the satellite. The other three gyroscopes, which are separated from each other by 8.25 cm, will be electrostatically supported by a digital control system. Although the gyroscopes and the electrostatic suspension system are designed to measure a precession as small as 0.1 mas/yr, any pair of these gyroscopes may also be used as a differential accelerometer. This paper analyzes the expected performance of these gyroscopes as differential accelerometers for accelerations in the frequency band from 2×10−3 to 2×10−2 Hz. The three contributions to the specific force on any one of the gyroscopes are the residual acceleration of the spacecraft, the specific forces acting between the gyroscope and the satellite, and the noise in the capacitance bridge which sens...

The Gravity Probe B electrostatic gyroscope suspension system (GSS)

A spaceflight electrostatic suspension system was developed for the Gravity Probe B (GP-B) Relativity Missions 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 instruments 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 spacecrafts 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.

Flux flushing for the GP-B gyroscopes

We conduct several experiments aimed at reducing the amount of magnetic flux trapped in superconducting gyroscopes. Most of experiments are conducted in an ambient field of 0.2±0.1 μG. We notice that the amount of flux trapped in the superconducting gyroscopes are sensitive to the thermal cycling procedures. We report our observations and discuss its implication for the Gravity Probe B Experiment.

Gravity Probe B cryogenic payload

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.

Proportional helium thrusters for Gravity Probe B

The Gravity Probe B (GP-B) satellite used electrostatically suspended gyroscopes to test two predictions of general relativity. Here, we describe the satellites proportional thrusters, which utilized boil-off helium gas for attitude and translation Control (ATC). The evolution of the design and its successful effect on orbit performance is reported.