Advanced LIGO and Advanced Virgo are actively monitoring the sky and collecting gravitational-wave strain data with sufficient sensitivity to detect signals routinely. In this paper we describe the data recorded by these instruments during their first and second observing runs. The main data products are the gravitational-wave strain arrays, released as time series sampled at 16384 Hz. The datasets that include this strain measurement can be freely accessed through the Gravitational Wave Open Science Center at http://gw-openscience.org, together with data-quality information essential for the analysis of LIGO and Virgo data, documentation, tutorials, and supporting software.

We present a method for 3D sub-nanometer displacement measurement using a set of differential optical shadow sensors. It is based on using pairs of collimated beams on opposite sides of an object that are partially blocked by it. Applied to a sphere, our 3-axis sensor module consists of 8 parallel beam-detector sets for redundancy. The sphere blocks half of each beam's power in the nominal centered position, and any displacement can be measured by the differential optical power changes amongst the pairs of detectors. We have experimentally demonstrated a displacement sensitivity of 0.87nm/Hz at 1 Hz and 0.39nm/Hz at 10 Hz. We describe the application of the module to the inertial sensor of a drag-free satellite, which can potentially be used for navigation, geodesy and fundamental science experiments as well as ground based applications.

This white paper describes a space gravitational wave mission concept consisting of three dragfree spacecraft at the Earth-moon L3, L4, and L5 Lagrange points. It also describes a Gravitational Reference Sensor (GRS) instrument concept, an Interferometric Measurement System (IMS) concept, and enabling technologies for drag-free propulsion, charge management and material coatings. The primary submitter is John W. Conklin for the team members listed above. We are willing to present this concept at the workshop and there is no sensitive or controlled information herein.

The Space Time Asymmetry Research (STAR), recently proposed as a NASA Small Explorer Mission (SMEX) will test isotropy and symmetry of space time at unprecedented precision. We will use precision molecular iodine stabilized Nd:YAG laser interferometers to search for small deviations from Lorentz Invariance, a cornerstone of relativity and particle physics and thus our understanding of the Universe. A Lorentz violation would have profound implications for cosmology and particle physics. An improved null result will constrain theories attempting to unite particle physics and gravity. We have previously submitted a Science White Paper to Astro 2010. While self contained, this White Paper mainly outlines technology development for the STAR mission, with emphasis on the science payload and spacecraft. With a funding level compatible with SMEX, we plan to develop a high performance, high reliability science payload in a 4~5 year time frame. The STAR mission is designed to work one-year in space, with possible extension to indefinitely longer as long as the payload remains functional.

Space-borne gravitational wave observatories like the Laser Interferometer Space Antenna (LISA) and those beyond, which may utilize a Modular Gravitational Reference Sensor (MGRS), greatly benefit from precise knowledge of the mass center location and moment of inertia tensor of the test mass prior to launch. The motion of the mass center of a drag-free test mass, which follows a pure geodesic, must be inferred from measurements of the surface. Therefore, knowledge of the mass center is critical for calibration of the cross-coupling between rotational and translational degrees of freedom. Together with the moment of inertia tensor, the mass center can also provide an estimate of the material density inhomogeneity to quadratic order, and the gravitational potential to second order, which improves modeling of self gravitation forces. These benefits, which are independent of the test mass shape, motivate the development of three new techniques for improving mass center and moment of inertia measurements beyond the current state of the art. A static pendulum is proposed to determine the mass center of a cubic test mass to ∼ 1 μm by measuring the equilibrium position with the cube in up to 24 different orientations relative to the pendulum platform. Measuring the natural frequency of a dynamic torsion pendulum can determine both the mass center and moment of inertia tensor of arbitrarily shaped objects to ∼ 5 μm and 1 part in ∼ 104 respectively. The velocity modulation technique for measuring the mass center of a sphere has raised the bar in precision to ∼ 150 nm, a factor of 20 improvement over the work presented at the LISA 6th symposium. This new technique involves rolling the sphere down a set of parallel rails to spectrally shift the mass center offset information to the rolling rate frequency, in order to avoid the 1/f noise that typically prevents other techniques from achieving precision below 1 μm.

We review state of the art of the gravitational reference sensor (GRS) for the Laser Interferometer Space Antenna (LISA). LISA consists of three identical spacecraft placed at the corners of an equilateral triangle with a 5 million kilometer baseline. In the LISA baseline design, the spacecraft at each corner will have two optical assemblies subtending an angle of 60 degrees. A proof mass (PM) is housed in a GRS located at the center of each assembly. LISA measures the distance variation between PMs separated by 5 million kilometers to a precision of 40 pm/Hz(1/2). The GRS must shield the PM from external disturbances such as solar wind and functions as a drag-free sensor for spacecraft control. The GRS must minimize the back action and cross talk exerted by measurements themselves. Significant progress has been made in the design, fabrication and testing of the GRS. LISA Pathfinder will fly a test GRS system scheduled around 2009. In addition, there have also been new architectures proposed to simplify the LISA payloads by using a single PM and therefore only one GRS per spacecraft. Further a modular GRS (MGRS) structure is proposed to reduce complexity. Optical sensing and large gap size between the PM and the MGRS housing are proposed to lower the disturbance level. Many experimental, engineering design, and trade off studies are underway.

A moment of inertia measurement apparatus typically attempts to produce a pure rotation about one degree of freedom. The measurements of rotation can have uncertainties when there are significant other degrees of freedom. Bifilar and trifilar pendulums, for example, do not constrain the swinging or lateral translation modes. The five-wire design reduces errors due to tilt and horizontal translational degrees of freedom.

The acceleration generated by the gradient of the mass attraction field between the spacecraft and proof mass is one parameter critical to drag-free performance. The gravitational self-attraction properties between two distributed bodies is characterized by the mass, mass center and moment of inertia for each body. Mass property measurements can therefore be used to indirectly measure the mass attraction properties. Since the ultimate goal is to demonstrate the ability to predict the system gravitational mass attraction force and force gradients to a precision below that of the LISA requirements, the corresponding proper-ties of mass, mass center, and moment of inertia must be precisely determined for the proof mass and satellite components. This work introduces a new method for measuring the moment of inertia using a novel five-wire torsion pendulum, which reduces errors due to translational degrees of freedom. The five-wire pendulum is integrated with optical angular sensing using diffraction grating angular magnification to provide a sensor with both a large dynamic range and high resolution.

Future space-borne gravitational experiments such as the Laser Interferometer Space Antenna (LISA) and the Space Test of the Equivalence Principle (STEP) require test masses whose mass center (MC) location must be known with unprecedented accuracy. A new technique potentially capable of determining the mass center offset from the geometric center (GC) of a spherical test mass to 0.1 mu m or better has been developed. Previous methods use the pendulous technique, which is typically limited to MC offsets greater than 1 mu m. The new technique involves rolling the sphere down a set of parallel rails so that the MC offset modulates the sphere's velocity at the rolling rate frequency. This technique uses a novel optical sensing system to measure the sphere's trajectory and a Monte Carlo parameter search to recover the magnitude and phase of the MC offset. Initial validation of the method using a spherical test mass with a known MC offset achieved a measurement error of < 3 mu m. Later, the apparatus may be modified to accommodate cylindrical or faceted test masses like those proposed for the STEP and LISA missions.

INTRODUCTION Future space-borne gravitational experiments such as the Laser Interferometer Space Antenna (LISA) and the Space Test of the Equivalence Principle (STEP) require test masses whose mass center (MC) location must be known with unprecedented accuracy. Motion of the MC is the primary science signal in these experiments, because it is the MC of a drag-free test mass that follows a perfect geodesic.

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.

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 RIF 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) position of a spherical proof mass to a precision of a few micrometers. We are studying a method to experimentally determine the selfgravitational attraction via measurement of the moments of inertia. We have further demonstrated over 2700 hours of operation of a UV LED under typical AC charge management conditions. We are modeling the electrostatic field surrounding gapped housing wall, cubic and spherical proof masses. We have studied surface potential of metallic proof masses using a Kelvin probe and UV photoelectric current.