On-orbit measurement of JFET leakage current and its annealing as functions of dose and bias at Jupiter

Excessive source-pin leakage current was found to cause the Galileo spacecraft gyroscope slew sensor to report incorrect spacecraft slew and thereby cause the computer to command inappropriate slewing. At normal bias, ground tests had found similar leakage to occur at dose levels similar to in-space levels. Controlled bias versus dose is used on-orbit in order to induce radiation-annealing and minimize growth of the leakage current in a DG181 JFET device. The growth of leakage current is minimized by turning the JFET power voltage off while it is in the radiation belts, thus, allowing for operation of the gyroscope attitude control system (ACS) when it is outside the radiation belts. During several 10- to 30-krad passes through the radiation belts, the leakage-associated damage was seen to slightly recover while power was turned off, but dramatically increase while turned on. The data provide perspective on the relations between ground test/design and optimization of performance in radiation and it suggests ways that procedures for ground testing could be improved.