3.2.4 Operational Constraints

The Spacecraft contains a radiation monitor (the EPHIN). Whenever the EPHIN measured radiation exceeds a threshold which is settable via ground control (presumably at the direction of the ASC), the Spacecraft tells ACIS that the monitor's threshold has been exceeded. When the radiation level subsides, the Spacecraft will tell ACIS that the condition has subsided. This information appears to the ACIS software as a single radiation flag.

During periods of unexpectedly high radiation, the radiation monitor's level will trip, and the ACIS radiation monitor flag will be asserted. The software will then issue a command to the DEAs to disable power to the CCDs, DEAs and FEPs. When the radiation environment returns to acceptable levels, the flag will be de-asserted. ACIS will sample the flag for a configurable period of time (specified as one of the System Configuration Parameters). If the sampled flag remains de-asserted for the specified time, ACIS will issue a command to the DEAs to allow power to be restored to the CCDs.

Handling of the spacecraft radiation monitor is done by software, so if flight experience dictates, ACIS can choose to ignore this signal or provide a more sophisticated decision tree to decide whether to shut down or not.