|ACIS Users Guide, Rev. A|
It is expected that ACIS will be subject to high radiation conditions under two scenarios. The first is when the spacecraft passes through the Van Allen belts as part of its orbit. The second is radiation due to solar flares.
Although the ACIS hardware is fairly robust with respect to radiation, repeated exposure to high levels of radiation will gradually degrade the hardware. The effect of the damage to the hardware can be reduced by powering off sensitive subsystems during exposures to high radiation levels. The most sensitive parts of ACIS, with respect to radiation damage, are the CCDs, the Analog-to-Digital Converters, and the Digital-to-Analog Converters; both of the latter are on the DEA video boards.
During each orbit, the spacecraft, and therefore ACIS, will pass through radiation belts. The location and levels of radiation in these belts will be fairly deterministic, and can be factored into the operations scheduling system. To reduce the degradation of the CCDs due to routine exposure to the belts, the video boards to the CCDs should be powered off during belt passage. Since knowledge of when the spacecraft will enter and exit the belts can be predicted by ground control, the ground should not schedule observations during belt passage, and should issue commands to power-down the video boards when entering the belts, and power them up again once the belts have been traversed. Refer to Section 5.7 for a description of ACIS operations during radiation belt passage.
The spacecraft contains a science instrument, known as an Electron, Proton, Helium Instrument (EPHIN), which monitors the levels of different types of radiation. If, during a solar flare or other unanticipated period of high-radiation, an ACIS-specific level is exceeded, the spacecraft invokes an on-board command sequence, which will issue a hardware serial digital command to ACIS. This command will cause the DPA hardware to set a radiation monitor flag. The ACIS System Configuration task polls this flag once per second. If the flag is asserted, the ACIS software suspends the current science observation, and issues commands to power off all of the video boards (takes about 10 seconds). Once the radiation level subsides, the spacecraft will invoke another "all-clear" command sequence, which issues another hardware serial digital command. This command clears the DPA radiation flag. The System Configuration task will then restore power to the video boards , and re-start the suspended science run, or start a new science run, depending on the ACIS commands that may have been received while the monitor was asserted.
Radiation Alert Hardware Serial Digital Command Mnemonics1RMONIRM (v=1) - Assert Radiation Monitor Flag 1RMONIRM (v=0) - De-assert Radiation Monitor Flag
As the science run is stopped, and as the video boards are powered off, the electric current levels will drop. See Appendix A for a table of current levels under different instrument configurations.
When the radiation monitor flag is asserted, the instrument reports the following to Software Housekeeping:SWSTAT_SCI_INHIBIT_ON - Inhibited science runs SWSTAT_DEACCD_POWEROFF - Power off to the DEA video boards
Once the radiation subsides and the monitor flag is deasserted, ACIS reports the following housekeeping:SWSTAT_SCI_INHIBIT_OFF - Science runs are allowed again SWSTAT_DEACCD_POWERON - Re-powering the video boards (if necessary)
If a science run was in progress when the radiation monitor is asserted, the run will be aborted. The resulting "Science Report" telemetry packet (see Section 4.1) will report a termination code of SMTERM_RADMON, indicating that the run was terminated due to the radiation monitor.
If a command is issued to start a science run while the monitor flag is asserted, the start will be deferred until the flag is de-asserted. The Command echo will indicate this in the "result" code with a CMDRESULT_INHIBITED.
If a command is issued to stop a science run while the monitor flag is asserted, the current run will not resume once the flag is de-asserted. The command echo will supply a "result" code of CMDRESULT_INHIBITED.
Because the radiation may corrupt the contents of the FEP bias maps, these maps are always re-computed at the start of the resumed run, or next science run. This may affect the start-up time of an observation.
When the video boards are autonomously powered off, the DEA will get a little colder.
If a DEA Housekeeping run is in progress which queries one or more of the video boards, the corresponding entries will report 0xffff while the boards are off.
ACIS remembers the most recent science run command when the radiation flag is asserted. Once it is deasserted, ACIS attempts to place the instrument into the science configuration that was desired for that moment in the observatory timeline.
For example, if ACIS is performing science run 1, and the monitor goes off, 1 is aborted. Once the monitor subsides, science run 1 is re-started, with a forced recomputation of its bias map. If, however, a stop command followed by a start run 2 command is received while the monitor is active, science run 2 will be started when the monitor flag is de-asserted.