D. DeBra

The gravity-probe-b relativity gyroscope experiment: Development of the prototype flight instrument

The Gravity-Probe-B Relativity Gyroscope Experiment (GP-B) will measure the geodetic and frame-dragging precession rates of gyroscopes in a 650 km high polar orbit about the earth. The goal is to measure these two effects, which are predicted by Einsteins General Theory of Relativity, to 0.01% (geodetic) and 1% (frame-dragging). This paper presents the development progress for full-size prototype flight hardware including the gyroscopes, gyro readout and magnetic shielding system, and an integrated ground test instrument. Results presented include gyro rotor mass-unbalance values (15–86 nm) due the thickness variations of the thin niobium coating on the rotor, interior sphericities (163–275 nm peak-to-valley) of fused-quartz gyro housings produced by tumble lapping, gyro precession rates (gyroscopes at 5 K) which imply low mass-unbalance components parallel to the gyro axis (23–62 nm), and demonstration of a magnetic shielding factor of 2×1010 for the gyro readout system with one shielding component missing (the gyro rotor). All of these results are at or near flight requirements for the GP-B Science Mission, which is expected to be launched in 1995.

Vibration isolation of precision machine tools and instruments

Abstract Successful precision engineering is the balance of robustness of the machine and how benign the environment can be made through isolation to minimize the strains caused by vibration that compromise a machines accuracy. This paper discusses principally the process of isolating a machine from the disturbances which come from the ground, air and utilities which serve it. Principles are reviewed and the intrinsic dependence on frequency established. Requirements are discussed and the hardware realizations currently available are reviewed. A number of examples of systems with unusually demanding requirements are given to illustrate the diverse nature of the solutions in practice. Some new developing areas are identified as the various topics are presented.

Seismic isolation of Advanced LIGO: Review of strategy, instrumentation and performance

The new generation of gravitational waves detectors require unprecedented levels of isolation from seismic noise. This article reviews the seismic isolation strategy and instrumentation developed for the Advanced LIGO observatories. It summarizes over a decade of research on active inertial isolation and shows the performance recently achieved at the Advanced LIGO observatories. The paper emphasizes the scientific and technical challenges of this endeavor and how they have been addressed. An overview of the isolation strategy is given. It combines multiple layers of passive and active inertial isolation to provide suitable rejection of seismic noise at all frequencies. A detailed presentation of the three active platforms that have been developed is given. They are the hydraulic pre-isolator, the single-stage internal isolator and the two-stage internal isolator. The architecture, instrumentation, control scheme and isolation results are presented for each of the three systems. Results show that the seismic isolation sub-system meets Advanced LIGOs stringent requirements and robustly supports the operation of the two detectors.

Design of Laminar Flow Restrictors for Damping Pneumatic Vibration Isolators

Pneumatic isolators provide excellent isolation of machine tools and measuring machines from seismic floor disturbances. Damping of the machine mass/air spring behavior becomes essential when automatic height adjustment is added. Air flow from the isolator to a tank can dissipate the oscillatory energy if it is passed through a restrictor. Commercially available isolators use a simple orifice as a restrictor, but flow through an orifice is non-linear with pressure. This non-linearity Insures that the damping is correct at only one amplitude of oscillation which can in turn lead to limit cycles as large as 1 mm. Conventional solutions to this instability problem include lowering the sensitivity of the self leveling valves, increasing the spacing between the isolators and lowering the center of mass of the machine with respect to the isolators. Laminar flow restrictors that provide optimum damping of an all amplitudes can be implemented with shim stock and spacers with no impact on the machine installation. Design formulas that accurately predict the experimental results will be described and some guidelines based on practical experience will be discussed.

Measuring Form and Radius of Spheres with Interferometry

Abstract The geometry of a nearly spherical surface, for example that of a precision optic, is completely determined by the radius-of-curvature at one point and the deviation from the perfect spherical form at all other points of the sphere. Measurements of radius and form error can now be made with interferometers to remarkable accuracy. We describe measurements of radius and form error of a precision silicon sphere, having a nominal radius of 46.8 mm, with the “extremely accurate CALIBration InterferometeR” (XCALIBIR) at the National Institute of Standards and Technology (NIST). For these measurements XCALIBIR is configured as a spherical Fizeau interferometer providing a field of view of 44°. To measure the radius, a variant of the well known interferometric radius bench method is used. Careful alignment of phase measuring and displacement measuring interferometers enables us to achieve a standard measurement uncertainty for the sphere radius of about 5 parts in 10 7 . The measurement of the form error is complicated because the entire sphere surface cannot be imaged in one measurement. Instead, 138 overlapping areas of the sphere surface are measured. A “stitching” algorithm is then employed to assemble these measurements into a form error map for the entire surface. We show that form errors can lead to considerable uncertainty in the radius of a sphere obtained through a radius-of-curvature measurement with the radius bench method.

Advanced gravitational reference sensor for high precision space interferometers

LISA and the next generation of space-based laser interferometers require gravitational reference sensors (GRS) to provide distance measurements to picometre precision for LISA, and femtometre precision for the proposed Big Bang Observatory (BBO). We describe a stand-alone GRS structure that has the benefits of higher sensitivity and ease of fabrication. The proposed GRS structure enables high precision interferometric links in three-dimensional directions. The GRS housing provides the optical reference surface onto which the transmitted laser beam, and the independent received laser beam are referenced. The stand-alone GRS allows balanced optical probing of the distance of the proof mass relative to the housing at a power and wavelength that differ from the transmitted and received wavelengths and with picometre sensitivity without radiation pressure imbalance. The single parameter that reduces proof mass disturbance forces is the gap spacing. Optical readout allows the use of a large gap between the GRS housing and proof mass. We propose using rf-modulated optical interferometry to measure both relative displacement and absolute distance. Further we propose to use a reflective grating beamsplitter within the GRS and on the external optical bench. The reflective grating design eliminates the in-path transmissive optical components and the dn/dT related optical path effects, and simplifies the optical bench structure. Inside the GRS, a near-Littrow mounted grating enables picometre precision measurement at microwatts of optical power. Preliminary experimental results using a grating beamsplitter interferometer are presented, which demonstrate an optical sensing sensitivity of 30 pm Hz−1/2.

Damping dilution factor for a pendulum in an interferometric gravitational waves detector

Mechanical loss in pendulums is a subject of great importance to gravitational waves detectors being built and being planned, as this determines the level of thermal noise associated with the detector suspensions. Relationships between the mechanical loss of the pendulum and the mechanical loss of the suspending fibres or wires can be derived in two apparently contradictory ways which give answers different by a factor of two. In this paper the differences are resolved and it is shown that both methods lead to the same answer.

Drag - free spacecraft as platforms for space missions and fundamental physics

A short review of the technology employed in the drag-free flights to date, on the Transit programme and planned missions is presented. The sensing, actuation, spacecraft design and control laws are discussed. The requirements for several quite different missions are presented and their comparison helps illustrate some aspects of drag-free design.

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.

An Optimal Thruster Configuration Design and Evaluation For Quick STEP

Abstract A procedure to calculate the margin of safety of a thruster system is presented. The margin of safety is the excess control authority of a thruster system available to overcome the worst case disturbances and is based on calculating the minimum control authority of the thruster system normalized by the worst case disturbances. Various controllers are presented for both one-sided and two-sided thrusters and their minimum control authorities are compared. Finally, a procedure to optimize a thruster system configuration in the sense of maximizing the margin of safety is presented and applied to the design of a thruster system for the Quick STEP spacecraft, an orbiting gravitational physics experiment. The techniques presented in this article are generally applicable to many other multi-degree of freedom actuation systems like the primary flight control effectors on a highly maneuverable aircraft for example.