Gregory M. Gutt

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.

Development of the Gravity Probe B flight mission

Abstract 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 Einsteins 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.

Optimization of a SQUID system for space

We have optimized a sensitive SQUID measurement system for use in a space flight experiment in the presence of significant interference from other sub-systems and the perturbing effects of EMI and thermal fluctuations. We will describe developments including a sapphire carrier for the SQUID chip, a precision temperature controller for the SQUID, control electronics with high bandwidth and enhanced thermal stability, and special shielding and filtering techniques used to increase EMI resistance.

The Gravity Probe B gyroscope readout system

Abstract We describe the superconducting gyroscope readout system to be used for measuring to a precision of 1 marcsecond in 10 hours of integration time the spin axis orientation of the Gravity Probe B (GP-B) gyroscope. The cryogenic portion of the readout system uses a dc SQUID to measure the gyroscopes London magnetic moment. Room temperature electronics appropriately bias the dc SQUID, allowing the detection and amplification of the gyroscope signal. We will describe recent advances in the system hardware including improved electronics and packaging. We will show flight quality noise performance and will discuss measurements of the systems rejection of simulated on-orbit environmental influences.

Effects of high energy proton bombardment (50-280 MeV) on dc SQUIDS

Three thin film dc SQUIDs of varied construction were bombarded with energetic protons in the energy range of 50 to 280 MeV. Measurements of the voltage output of the dc SQUIDs were taken in open loop, as well as flux locked mode, in an environment of proton flux that was varied from 10/sup 4/ to 10/sup 7/ protons/cm/sup 2//s. Discrete voltage jumps corresponding to 0.01 to 0.001 flux quanta were observed in two of the three SQUIDs in the flux locked mode; discrete changes in the open loop SQUID output voltage were also observed. Some data appear to be consistent with proton-induced flux motion in the body of the SQUID loop.<<ETX>>

Gravity Probe B payload verification and test program

Abstract 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

A method for simulating a flux-locked DC SQUID

The authors describe a computationally efficient and accurate method for simulating a DC superconducting quantum interference devices (SQUIDs) V- phi (voltage-flux) and I-V characteristics which has proven valuable in evaluating and improving various SQUID readout methods. The simulation of the SQUID is based on fitting of previously acquired data from either a real or a modeled device using the Fourier transform of the V- Phi curve. This method does not predict SQUID behavior, but rather is a way of replicating a known behavior efficiently with portability into various simulation programs such as SPICE. The authors discuss the methods used to simulate the SQUID and the flux-locking control electronics, and present specific examples of this approach. Results include an estimate of the slew rate and linearity of a simple flux-locked loop using a characterized DC SQUID.<<ETX>>

An ultralow noise amplifier for superconductive detectors

The design and construction of an AC-coupled, ultralow noise amplifier (equivalent input noise voltage En=0.33 nV/ square root Hz, optimum source impedance 1 k Omega at 100 kHz) is presented. The amplifier employs nine Sony 2SK 152-4 JFETS in parallel to yield the low noise result. Measurements were taken to characterize the amplifiers bandwidth, the additive voltage noise versus frequency, and the current noise versus frequency. SPICE modelling parameters for the JFETs used will also be discussed. Finally, the authors will review the performance of the amplifier in a flux-locked SQUID application.

Gravity Probe B gyroscope readout system

We describe the Gravity Probe B London-moment readout system successfully used on-orbit to measure two gyroscope spin axis drift rates predicted by general relativity. The system couples the magnetic signal of a spinning niobium-coated rotor into a low noise superconducting quantum interference device. We describe the multi-layered magnetic shield needed to attenuate external fields that would otherwise degrade readout performance. We discuss the ~35 nrad/yr drift rate sensitivity that was achieved on-orbit.

Comparative study of bias-reversing schemes for low frequency noise reduction in dc SQUIDS

Abstract We have investigated the effectiveness for low frequency noise reduction of three different schemes of periodic bias current reversal in dc SQUIDs operated in the flux-locked mode. The three techniques incorporate different frequency and phase relationships between the bias current waveform and the flux modulation waveform. We find that some SQUIDs show significant low frequency noise reduction under bias reversing, while other SQUID devices of the same design inherently achieve the lower level of low frequency noise and hence do not benefit from bias reversal.