Conclusive evidence for internal dielectric charging anomalies on geosynchronous communications spacecraft

When the ANIK El and £2 communications spacecraft suffered serious failures of their momentum wheel control systems, it was postulated that the satellites were subjected to bulk (internal dielectric) charging followed by discharge that disabled key circuitry. This paper effectively confirms the hypothesis by linking the events to a well-established pattern of operational anomalies on another spacecraft. Since March 1991, a commercial geosynchronous satellite has experienced over 50 specific, but relatively trivial, mode switches. These are analyzed in relation to energetic electron fluences measured at GOES-7 and METEOSAT-3; without exception they coincide with periods of relatively high flux. Combining the measurements, the critical directed energy fluence is calculated to be 1 x 10 MeV cm". This figure is consistent with the theoretical breakdown threshold and CRRES data characterizing the discharge process; it fits with an effective shielding thickness of less than 0.2 mm of aluminum. Three switches in the days preceding the ANIK failures strongly argue for a common explanation, although it may be impossible to even identify actual discharge sites. Modern communications spacecraft employ tried and tested techniques for electrostatic discharge protection, but it is clear that the recognized hazard of internal dielectric charging has often been underestimated and shielding guidelines overlooked.