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 254 nm 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 UV LEDs operating at 255 nm are superior to Mercury vapor lamps because of their smaller size, lower 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 percent change in performance after 12 months in orbit.

There are very few direct experimental tests of the inverse square law of gravity at distances comparable to the scale of the Solar System and beyond. Here we describe a possible space mission optimized to test the inverse square law at a scale of up to 100 AU. For example, sensitivity to a Yukawa correction with a strength of 10(-7) times gravity and length scale of 100 AU is within reach, improving the current state of the art by over two orders of magnitude. This experiment would extend our understanding of gravity to the largest scale that can be reached with a direct probe using known technology. This would provide a powerful test of long-distance modifications of gravity including many theories motivated by dark matter or dark energy.

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.

Engineers have constantly put efforts into improving the precision of machining processes. Ultraprecision requirements often make designs such machine tools fundamentally different from traditional ones. In addition to design changes, error correction capability is essential to increase the precision of new machines from current technologies. Therefore, the developments of an ultraprecision machine will often require the integration of new actuation means, novel sensors, and feedback control. In this paper, the integration of the carriage positioning system of an ultraprecision diamond turning machine is presented.

We compared the effects of reinforcing compliance with either positive reinforcement (edible items) or negative reinforcement (a break) on 5 participants' escape-maintained problem behavior. Both procedures were assessed with or without extinction. Results showed that compliance was higher and problem behavior was lower for all participants when compliance produced an edible item rather than a break. Treatment gains were achieved without the use of extinction. Results are discussed regarding the use of positive reinforcement to treat escape behavior.

We developed a fast, short stroke hydraulic actuator, suitable for toolpost actuation, for spindle motion error correction or for non-circular cutting. The actuator has a throw of 180 μm at 2 MPa. The dynamic response is limited by the hydraulic resonance (≊ 20,000 rad/s). The choice of servovalve and feedback design govern the dynamic response. It shows rise times of better than 25 ms, with accuracies suitable for ultraprecision machining. In a facing operation, a surface finish of better than 25 nm Ra was achieved, and for non-circular cutting, about 40 nm Ra.

Gravity Probe-B is the relativity gyroscope experiment being developed by NASA and Stanford University to test two extraordinary, unverified predictions of Albert Einstein's general theory of relativity. The experiment will check, very precisely, tiny changes in the directions of spin of four gyroscopes contained in an Earth satellite orbiting at 400-mile altitude directly over the poles. So free are the gyroscopes from disturbance that they will provide an almost perfect space-time reference system. They will measure how space and time are warped by the presence of the Earth, and, more profoundly, how the Earth's rotation drags space-time around with it. These effects, though small for the Earth, have far-reaching implications for the nature of matter and the structure of the Universe.

A general procedure to determine accurately the true relationship between the commanded and the actual force output of a set of thrusters is presented. This relationship is determined from data generated from a spacecraft while it is on orbit. On orbit thruster calibration measures the true outputs of the thrusters in the actual space environment in which they operate. The feasibility of the calibration technique is verified by a digital simulation of the Gravity Probe B (GP-B) spacecraft dynamics. GP-B is an Earth orbiting experiment which will test several aspects of general relativity. A set of 18 proportional thrusters generates forces and moments for three degree of freedom attitude and three degree of freedom drag free translation control of the GP-B spacecraft. A total of 108 parameters are needed to characterize the magnitude and direction of both the force and moment outputs of the 18 thrusters. The ability to calibrate these parameters to an accuracy of better than 1%is demonstrated.

Space Station Freedom topics addressed include: general design issues; issues related to utilization and operations; issues related to systems requirements and design; and management issues relevant to design.

We describe the development of sensors and actuators for spacecraft control. Sensors include gyros and other inertial sensors, horizon sensors, sun sensors, star sensors and magnetometers. Among the actuators are reaction wheels, control moment gyros, magnets and propulsion systems. The timelessness of some principles and requirements are as interesting as the evolution of technology which has changed how we apply others. Combined with our recent ability to compute on orbit the selection of sensors, control system life and how we structure our control systems continues to change.

Thruster locations and control schemes which make the most efficient use of the available propellant of a constant flow propulsion system are investigated. A set of three necessary and sufficient conditions are established for a thruster system to be optimal in the sense of maximizing the smallest magnitude force which can be generated by the thrusters. Analytical expressions are derived for the 'the envelope of least authority" which is defined as the smallest force magnitude vs. moment magnitude which can be generated by any given thruster system. Several thruster configurations are proposed for the GP-B spacecraft and their performance is compared with respect to their envelopes of least authority. The results are applicable to any system composed of thrusters which saturate at some finite value.