  Charges as small as 1 pC degrade the performance of high precision inertial
reference instruments when accumulated on their test masses (TMs). Non-contact
charge management systems are required for the most sensitive of these
instruments, with the TMs free-floating, and their charges compensated by
photoelectrons in a feedback loop with a TM charge measurement. Three space
missions have successfully demonstrated this technique: GP-B, LPF, and the
UV-LED mission. 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. Flight data was acquired
between December 2014 and December 2015 with the UV-LED instrument flown on
SaudiSat 4. 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. All results confirm the excellent reliability of the UV-LEDs
in the space environment, are fully consistent with the findings of ground
studies, and support the approach of using LEDs for passive charge management.
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 the TM and its enclosure, and thus on the exact configuration of the
instrument.

Space Physics