Automatic charge control system for geosynchronous satellites

Abstract

An autonomous system to detect both absolute and differential spacecraft charging aboard high-altitude satellites, and to reduce those potentials before hazardous arcing levels are reached, is now being developed under the Air Force program on Space Systems Environmental Interaction Technology. Operation of the system is based on the empirical results of the Space Test Program SCATHA (P78-2) and NASA ATS-6 satellites, both of which successfully demonstrated the principle of safely reducing spacecraft charging levels by the emission of a low-energy neutral plasma —effectively shorting the spacecraft and charge dielectric surfaces to the ambient space plasma. The Charge Control System now being built at Hughes Research Laboratories in Malibu, CA, will utilize a xenon-based plasma source that can be ignited without any warm-up, and that will produce a plasma capable of furnishing a neutralizing ion current greater than 1 mA. The spacecraft charging level will be detected by sensors similar to those that operated aboard SCATHA. Satellite frame potential (relative to the ambient space plasma) will be determined by an elestrostatic analyzer capable of detecting both ions and electrons in the energy range 50 eV - 20 keV. Differential charging (relative spacecraft frame ground) will be determined by a surface-potential monitor utilizing two different dielectric surfaces, and covering a range of ±20kV with a response time of one second. A transient pulse monitor will detect the presence of exterior spacecraft arcing by measuring its electrostatic field pulse. Automatic operation of the system will be accomplished by a microprocessor controller which interpret the sensor data and activate the plasma source when predetermined threshold levels are exceeded. The gas supply will provide for more than 2000 hours of plasma source operation in space, which under nominal conditions, would be sufficient for up to 10 years of on-orbit spacecraft protection. The breadboard model has already operated successfully under simulated spacecraft charging conditions — with sensors and plasma source in a vacuum chamber at 5×10 −6 Torr, all irradiated by energetic electrons (up to 10 keV). Each sensor individually, as well as with all the sensors operating collectively, responded to the threshold charging level and activated the plasma source automatically, reducing the charging level to zero. The flight system will be completed by mid-1988, and could be flight-tested at geosynchronous orbit in the 1989–1992 time period.