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.

Charges as small as 1 pC degrade the performance of high precision inertial reference instruments when accumulated on their test masses (TM). Non-contact charge management systems are required for the most sensitive of these instruments, with the TM charges compensated by photoelectrons in a feedback loop with a TM charge measurement system. Three missions have successfully demonstrated this technique: GP-B, the LISA Pathfinder, and the UV-LED mission, launched in 2004, 2015, and 2014 respectively; with the first two using the 254 nm Hg line and the last one a set of 255 nm UV-LEDs. UV-LEDs represent a significant improvement over the discharge sources, in terms of reliability, lifetime, switching speeds, power consumption, weight, and volume. Charge management techniques that eliminate the charge measurement and feedback systems, referred henceforth as passive, reduce the complexities and disturbance effects introduced by these systems, and are thus the subject of active research and development work. Passive charge management depends critically on the stability and reproducibility of the photoemission properties of a given system. In support of this work, we present comprehensive flight characterization data for a suite of 16 UV-LEDs in various configurations and 255 ± 1 nm center wavelength. We back up our results with ground-based measurements performed in configurations comparable to the flight one between September 4, 2020, and October 8, 2020. Results confirm the reliability of the UV LEDs in space environment and are fully consistent with the findings of ground studies. We find that the equilibrium potential of the TM, under illumination by the 255 nm LEDs, is independent of the UV intensity and reproduceable to about ± 6 mV, or ± 6 fC/pF, over periods of up to six months. The value of the equilibrium potential is dependent on the geometry of the electric field between TM and enclosure.

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