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LISA and the next generation of space-based laser interferometers require gravitational reference sensors (GRS) to provide distance measurements with picometer precision. To meet this goal, we describe a stand-alone GRS structure that has high sensitivity and easy integration with a satellite. The stand-alone GRS integrates balanced optical probing of the proof-mass position in its housing, and provides a correlated reference surface to an independent external interferometer. The use of optical sensing allows for a large gap between the proof-mass and housing, which is the best way to reduce many proof-mass disturbances.

Proof mass electrical charge management is an important functionality for the ST-7-LTP technology demonstration flight and for LISA. Photoemission for charge control is accomplished by using deep ultraviolet (UV) light to excite photoelectron emission from an Au alloy. The conventional UV source is a mercury vapour lamp. We propose and demonstrate charge management using a deep UV light emitting diode (LED) source. We have acquired selected AlGaN UV LEDs, characterized their performance and successfully used them to realize charge management. The UV LEDs emit at a 257 nm central wavelength with a bandwidth of ~12 nm. The UV power for a free-space LED is ~120 µW, and after fibre coupling is ~16 µW, more than sufficient for LISA applications. We have directly observed the LED UV light-induced photocurrent response from an Au photocathode and an Au-coated GRS/ST-7 proof mass. We demonstrated fast switching of UV LEDs and associated fast changes in photocurrent. This allows modulation and continuous discharge to meet stringent LISA disturbance reduction requirements. We propose and demonstrate AC charge management outside the gravitational wave signal band. Further, the megahertz bandwidth for UV LED switching allows for up to six orders of magnitude dynamic power range and a number of novel modes of operations. The UV LED based charge management system offers the advantages of small-size, lightweight, fibre-coupled operation with very low power consumption.

One of the possible noise sources for the space-based gravitational wave detector LISA (the Laser Interferometer Space Antenna), associated with its test masses, is that due to spatial variations in surface potential (or patch effect) across the surfaces of the test mass and its housing. Such variations will lead to force gradients which may result in a significant acceleration noise term. Another noise source is that due to temporal variations in the surface potential, which in conjunction with any ambient dc voltage or net free charge on the test mass may also produce a significant acceleration noise term. The ST-7 demonstrator mission is designed to test technologies for LISA, including the gravitational reference sensor, which contains a gold-coated gold/platinum (Au/Pt) alloy test mass, surrounded by a housing that carries the electrodes for sensing and control. We have used a Kelvin probe at the Goddard Space Flight Center to make spatial and temporal measurements of contact potential differences for a selection of materials (Au/Pt, beryllia, alumina, titanium) and coatings (gold, diamond-like carbon, indium tin oxide, titanium carbide). Our investigations indicate that subject to certain assumptions all of these coatings appear to satisfy the ST-7 requirement that patch effect spatial variations should be less than 100 mV. The data also revealed evidence of behavioural trends with pressure and possible contamination effects. Regarding temporal variations, the current accuracy of the instrument is limiting the measurements at a level above the likely LISA requirements. We discuss our results and draw some conclusions of relevance to LISA.

The Laser Interferometer Space Antenna (LISA) and the Big Bang Observer (BBO) require angular sensing in their gravitational reference sensor (GRS), telescope pointing, and spacecraft control. The conventional angular sensing schemes utilize simple geometric reflections as the sensing mechanism. We propose and demonstrate the use of grating diffraction orders as angular sensing signal beams. The grating angular sensor can be far more sensitive than a simple reflection scheme for two reasons. First, the diffractive angles can vary more than the incident angle when the grating rotates. The grating thus magnifies the variation of the input angle. Second, the cross section of the diffracted beam is compressed by the oblique projection, resulting in a higher energy density. These two favorable effects become more pronounce for a normal incidence beam to diffract at grazing angles. We have conducted a preliminary experiment and demonstrated an angular sensitivity below 10 nanoradians per root hertz over a short working distance, meeting the Space Technology 7 (ST-7) GRS and LISA requirements for proof mass angular sensing. Our proposed grating-based angular measurement does not introduce additional optical elements between the sensed surface and the photodiode. Thus, it eliminates measurement uncertainty due to in-path optics. The proposed grating sensor can be generalized to build an integrated sensor for both angular and displacement sensing.

We present the Modular GRS (previously named as Stand-Alone GRS), in which the laser light from the remote spacecraft does not illuminate the proof mass. The modular GRS uses only a single spherical proof mass on each spacecraft and optical, as opposed to capacitive, position sensing. The use of a single sphere as the test mass avoids the issue of cross coupling that is inherent for the cubic proof mass, and allows true drag free flight with no forcing. Together, the modular design, optical sensing and a single spherical proof mass reduce the disturbances and the number of degrees of freedom that must be managed for future LISA and BBO.

A next generation gravitational reference sensor is being developed by Stanford University for the disturbance reduction system (DRS). The DRS will demonstrate the technology required for future gravity missions, including the planned LISA gravitational-wave observatory. The GRS consists of a freely floating test mass, a housing, sensing electrodes and associated electronics. Position measurements from the GRS are used to fly the spacecraft in a drag-free trajectory, where spacecraft position will be continuously adjusted to stay centred about the test mass, essentially flying in formation with it. Any departure of the test mass from a gravitational trajectory is characterized as acceleration noise, resulting from unwanted forces acting on the test mass. The GRS will have an inherent acceleration noise level more than four orders of magnitude lower than previously demonstrated in space. To achieve such a high level of performance, the interaction of the magnetized test mass with the magnetic fields produced by the spacecraft must be considered carefully. It is shown that a new noise source due to the interaction of the time-varying magnetic field gradient and the permanent dipole of the test mass must be added to the noise analysis. A simple current loop model shows that the design of the spacecraft and instrument electronics must be done with attention to the magnetic noise produced.

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 .

We describe a technique based on photoemission for controlling the charge of the Gravity Probe B electrostatically suspended gyroscopes, and three methods for measuring this charge. Charging is caused by cosmic radiation in orbit and by enhanced field emission in ground testing. Errors induced by disturbing torques require the potential of the gyroscope to be smaller than 15 mV (15 pC) during the space experiment. The disturbing drift rate produced by measuring and controlling the charge in orbit is smaller than 10−13 deg/h, as compared with the 10−11 deg/h systematic drift rate of the gyroscope. The charge control technique is based on ultraviolet photoemission of electrons from both the gyroscope and a charge control electrode on the gyroscope housing. We demonstrate the effectiveness of this method in ground testing and therefore its suitability for the space experiment. Calculations indicate that heating by absorbed photons is, in the worst case, smaller than 1 nW and thus not a problem for the experi...

Multilayer Ti-Cu films with three, five, and seven individual layers have been prepared by sputtering on fused quartz substrates. The multilayer films have been studied by electron field emission, X-ray diffraction, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy. Trilayer Ti-Cu-Ti electrode films exhibit reduced electron field emission. Ti-Cu-Ti trilayer films have been applied to the Gravity Probe B gyroscope housings. The thin films have been deposited by sputtering onto the fused quartz housings to form suspension electrodes and lands. The gyroscopes have been successfully spun to 170 Hz.

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.

We discuss two geosynchronous gravitational wave mission concepts, which we generically name gLISA. One relies on the science instrument hosting program onboard geostationary commercial satellites, while the other takes advantage of recent developments in the aerospace industry that result in dramatic satellite and launching vehicle cost reductions for a dedicated geosynchronous mission. To achieve the required level of disturbance free-fall onboard these large and heavy platforms we propose a "two-stage" drag-free system, which incorporates the Modular Gravitational Reference Sensor (MGRS) (developed at Stanford University) and does not rely on the use of micro-Newton thrusters. Although both mission concepts are characterized by different technical and programmatic challenges, individually they could be flown and operated at a cost significantly lower than those of previously envisioned gravitational wave missions. We estimate both mission concepts to cost less than 500M US$ each, and in the year 2015 we will perform at JPL a detailed selecting mission cost analysis., Paper submitted for publication to the journal: "Review of Scientific Instruments". It is 13 pages long and contains 2 figures

Deep ultraviolet (UV) LEDs have recently been used in AC charge management experiments to support gravitational reference sensors for future space missions. The UV LED based charge management system offers compact size, light weight, and low power consumption compared to plasma sources. The AC charge management technique, which is enabled by easy modulation of UV LED output, achieves higher dynamic range for charge control. Further, the high modulation frequency, which is out of the gravitational wave detection band, reduces disturbances to the proof mass. However, there is a need to test and possibly improve the lifetime of UV LEDs, which were developed only a year ago. We have initiated a series of spectral and power stability tests for UV LEDs and designed experiments according to the requirements of AC charge management. We operate UV LEDs with a modulated current drive and maintain the operating temperature at 22 °C,28 similar to the LISA spacecraft working condition. The testing procedures involve measuring the baseline spectral shape and output power level prior to the beginning of the tests and then re‐measuring the same quantities periodically. As of the date of submission (August 28th, 2006), we have operated a UV LED for more than 2,700 hours.

Modular Gravitational Reference Sensor (modular GRS) was proposed by the Stanford Team in 2004. In a modular GRS, the laser beam from the remote the sensor does not illuminate the proof mass directly. The internal measurement from the housing to proof mass is separated from the external interferometry. A double‐sided grating further simplifies the structure and may better preserve the measurement precision. We review the recent progress in developing the modular GRS at Stanford. We are developing optical sensors with picometer resolution, capable of operating with a large gap for high precision readout. We have conducted an initial experiment incorporating RF heterodyne detection and thus lowered the optical power compared with direct detection. We have demonstrated sub‐nanoradian sensitivity of a grating angular sensor. We have successfully demonstrated fabrication of localized grating patterns on dielectric and gold surfaces. We have made critical progress in optical measurement of the mass center (MC) ...

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 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...

We have designed combined passive and active thermal control system to achieve sub microkelvin temperature stability and uniformity over an optics bench size enclosure, which has an analogous structure to the LISA spacecraft. For the passive control, we have constructed a new thermal enclosure that has a multilayer structure with alternative conducting and insulating layers, which enables the temperature uniformity and ease the burden of the active control. The thermal enclosure becomes an important test facility for Modular Gravitational Reference Sensor (MGRS) development. For the active control, we have developed a model predictive control (MPC) algorithm, which will regulate temperature variations of the proof-mass (PM) down to sub-microkelvin over the LISA science band. The LISA mission requires extremely tight temperature control, which is as low as 30 μK/ over 0.1 mHz to 1 Hz. Both temporal stability and spatial uniformity in temperature must be achieved. Optical path length variations on optical bench must be kept below 40 pm/ over 0.1 mHz to 1 Hz. Temperature gradient across the proof mass housing also must be controlled to reduce differential thermal pressure. Thermal disturbances due to, for example, solar radiation and heat generation from electronics, are expected to be significant disturbance source to the LISA sensitivity requirements. The MGRS will alleviate the thermal requirement due to its wider gap between the proof-mass and the housing wall. However, a thermally stable and uniform environment is highly desirable to achieve more precise science measurement for future space science missions.

The Modular Gravitational Reference Sensor (MGRS) is targeted as a next generation core instrument for both space gravitational wave detection and an array of other precision gravitational experiments in space. The objectives of the NASA funded program are to gain a system perspective of the MGRS, to develop key component technologies, and to establish important test platforms. Our original program was very aggressive in proposing ten areas of research and development. Significant advancements have been made in these areas, and we have met or exceeded the goals for the program set in 2007-2008. Additionally, we have initiated research projects for innovative technologies beyond the original plan. In this paper we will give a balanced overview of progress in MGRS technologies: the two layer sensing and control scheme, trade-off studies of GRS configurations, multiple optical sensor signal processing, optical displacement and angular sensors, differential optical shadow sensing, diffractive optics, proof mass center of mass and moment of inertia measurement, UV LED charge management, proof mass fabrication, thermal control and sensor development, characterization for various proof mass shapes, and alternative charge manage techniques.

We propose to perform a null-gravitational redshift experiment by comparing a mechanical gyroscope clock with atomic clocks. The Gravity-Probe-B Relativity Gyroscope Experiment provides the opportunity for this co-experiment. The goal is to measure the effect to an accuracy of 0.01% of the gravitational redshift due to the eccentricity of the orbit of the earth about the sun. This corresponds to an integrated frequency measurement over one year of Av/v=3∗10−14. A major disturbance torque on the gyroscope is due to fluctuations in the molecular drag of the residual gas caused by temperature variations. We propose to use a low temperature bake-out technique in order to achieve the required vacuum of 10−17torr.

We propose and test a method for controlling the charging of the Gravity Probe B(GP‐B) electrostatically suspended gyroscopes using electrons generated by field emission cathodes. The GP‐B Gyroscope Experiment is designed to measure for the first time the geodetic and the frame‐dragging effects predicted by Einstein’s general theory of relativity. The expected accuracy of ∼0.3 marcsec/yr (10−11 deg/h) will allow for a 0.01% measurement of the geodetic effect and a 1% measurement of the frame‐dragging effect. Gyroscope charging is caused by cosmic radiation, by field emission, and by the separation of dissimilar metals. The expected charging rate for the gyroscopes is ∼1 nC/yr and consequently above the 50 pC limit dictated by disturbing torque considerations. The present charge control technique is based on ultraviolet photoemission of electrons from both the gyroscope and an auxiliary electrode. Experiments have shown this method to be effective at room temperature in ground testing, and calculations ind...

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 Einstein's General Theory of Relativity, to 0.01 percent (geodetic) and 1 percent (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.

LISA will be the first space-home gravitational wave observatory. It aims to detect gravitational waves in the 0.1 MHz+1 Hz range from sources including galactic binaries, super-massive black-hole binaries, capture of objects by super-massive black-holes and stochastic background. LISA is an ESA approved Cornerstone Mission foreseen as a joint ESA-NASA endeavour to be launched in 2010-11. The principle of operation of LISA is based on laser ranging of test-masses under pure geodesic motion. Achieving pure geodesic motion at the level requested for LISA, 3×10^(−15) ms^(−2)/√Hz at 0.1 mHz, is considered a challenging technological objective. To reduce the risk, both ESA and NASA are pursuing an in-flight test of the relevant technology. The goal of the test is to demonstrate geodetic motion within one order of magnitude from the LISA performance. ESA has given this test as the primary goal of its technology dedicated mission SMART-2 with a launch in 2006. This paper describes the basics of LISA, its key technologies, and its in-flight precursor test on SMART-2.