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 performance of the LISA gravitational wave detector depends critically on limiting spurious accelerations of the fiducial masses. Consequently, the requirements on allowable acceleration levels must be carefully allocated based on estimates of the achievable limits on spurious accelerations from all disturbances. Changes in the allocation of requirements are being considered, and are proposed here. The total spurious acceleration error requirement would remain unchanged, but a few new error sources would be added, and the allocations for some specific error sources would be changed. In support of the recommended revisions in the requirements budget, estimates of plausible acceleration levels for 17 of the main error sources are discussed. In most cases, the formula for calculating the size of the effect is known, but there may be questions about the values of various parameters to use in the estimates. Different possible parameter values have been discussed, and a representative set is presented. Improvements in our knowledge of the various experimental parameters will come from planned experimental and modelling studies, supported by further theoretical work.

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Much as the Global Positioning System has ushered in an era of autonomous navigation on a global scale, X-ray Navigation (XNAV) offers the possibility of autonomous navigation anywhere in the solar system. X-ray astronomers have identified a number of X-ray pulsars whose pulsed emissions have stabilities comparable to atomic clocks. X-ray Navigation uses phase measurements from these sources to establish autonomously the position of the detector, and thus the spacecraft, relative to the solar system barycenter.This paper describes the development of a general noise model for X-ray Navigation instruments. Key noise terms are identified and simple analytic expressions provided for each. This noise model is used to predict the performance of a typical XNAV system that could be used as the primary navigation resource on missions, including those beyond the orbit of Jupiter.

Abstract.The UV-LED mission demonstrates the precise control of the potential of electrically isolated test masses that is essential for the operation of space accelerometers and drag-free sensors. Accelerometers and drag-free sensors were and remain at the core of geodesy, aeronomy and precision navigation missions as well as gravitational science experiments and gravitational wave observatories. Charge management using photoelectrons generated by the 254nm UV line of Hg was first demonstrated on Gravity Probe B and is presently part of the LISA Pathfinder technology demonstration. The UV-LED mission and prior ground testing demonstrates that AlGaN UVLEDs operating at 255 nm are superior to Mercury vapor lamps because of their smaller size, lower power draw, higher dynamic range, and higher control authority. We show flight data from a small satellite mission on a Saudi Satellite that demonstrates AC charge control (UV-LEDs and bias are AC modulated with adjustable relative phase) between a spherical test mass and its housing. The result of the mission is to bring the UV-LED device Technology Readiness Level (TRL) to TRL-9 and the charge management system to TRL-7. We demonstrate the ability to control the test mass potential on an 89 mm diameter spherical test mass over a 20 mm gap in a drag-free system configuration. The test mass potential was measured with an ultra-high impedance contact probe. Finally, the key electrical and optical characteristics of the UV-LEDs showed less than 7.5% change in performance after 12 months in orbit.