J.M. Lockhart

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.

Ultralow magnetic fields and Gravity Probe B gyroscope readout

Abstract We describe the generation of an ultralow magnetic field of −11 Tesla in the flight dewar of the Gravity Probe B Relativity Mission. The field was achieved using expanded-superconducting-shield techniques and is maintained with the aid of a magnetic materials control program. A high performance magnetic shield system is required for the proper function of gyroscope readout. The readout system employs a dc SQUID to measure the London moment generated by the superconducting gyro rotor in order to resolve sub-milliarcsecond changes in the gyro spin direction. In addition to a low residual dc magnetic field, attenuation of external field variation is required to be 10 12 at the gyro positions. We discuss the measurement of the dc magnetic field and ac attenuation factor and the performance of the readout system.

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.

Remanent magnetization of instrument materials for low magnetic field applications

We report remanent magnetization and magnetic susceptibility measurements made on materials used in the construction of cryogenic instruments. SQUID based magnetometers were used to make the measurements over a range of background fields from 102 to 10−7 Gauss. Although the materials tested are generally regarded as nonmagnetic, some samples have sufficiently high magnetization values, or values which vary with foundry lot and heat, that use in low field or magnetically sensitive applications is contraindicated.

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>>

Timing system design and tests for the Gravity Probe B relativity mission

In this paper, we discuss the timing system design and tests for the NASA/Stanford Gravity Probe B (GP-B) relativity mission. The primary clock of GP-B, called the 16fo clock, was an oven-controlled crystal oscillator that produced a 16.368 MHz master frequency3. The 16fo clock and the 10 Hz data strobe, which was divided down from the 16fo clock, provided clock signals to all GP-B components and synchronized the data collection, transmission, and processing. The sampled data of science signals were stamped with the vehicle time, a counter of the 10 Hz data strobe. The time latency between the time of data sampling and the stamped vehicle time was compensated in the ground data processing. Two redundant global positioning system receivers onboard the GP-B satellite supplied an external reference for time transfer between the vehicle time and coordinated universal time (UTC), and the time conversion was established in the ground preprocessing of the telemetry timing data. The space flight operation showed that the error of time conversion between the vehicle time and UTC was less than 2 μs. Considering that the constant timing offsets were compensated in the ground processing of the GP-B science data, the time latency between the effective sampling time of GP-B science signals and the stamped vehicle time was verified to within 1 ms in the ground tests.

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

Applications of superconductivity to space-based gravitational experiments

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.