The in-situ measurement of spacecraft internal charging currents.

Abstract

Internal charging is a major cause of environmentally-induced malfunctions on satellites: the risk arises wherever light-to-moderately shielded dielectrics, or isolated metallic items, collect charge deposited by penetrating electrons so creating high potential differences between elements of the spacecraft. Observations from the US CRRES satellite in 1991/2 showed that electrostatic discharges are probable if the internal charging current, when averaged over ten hours, exceeds 0.1pAcm-2 and engineering practice today still relies heavily on this simple result. On CRRES internal charging currents had to be estimated from on-board electron flux measurements followed by radiation transport calculations. This thesis describes the first direct in-orbit measurements of internal charging currents: to do this a shielded current sensor (SURF) was invented and deployed into both geostationary transfer and medium Earth orbit. Currents were measured under shielding levels representative of those used to protect dielectrics in many real spacecraft. Being of the order of fAcm-2, measuring such currents presented practical challenges but these were overcome and SURF has now worked successfully for over a decade in medium Earth orbit. Data from the sensor can be used ‘in reverse’ to determine external electron fluxes via unfolding techniques. In this role SURF has some advantages over conventional electron counting instruments: first, when very intense fluxes occur there are no dead time effects so the readings can still be relied upon and second, SURF is largely immune from proton contamination in mixed environments, something which tends to plague traditional detectors. A disadvantage, however, is that the threshold of sensitivity for SURF is higher than for a traditional counting instrument. Overall it is concluded that shielded current sensors are a valuable new aid to quantifying the risk from internal charging and at the same time they are able to provide fresh insights into the external environment.