NASA has successfully switched to backup hardware on the Hubble Space Telescope after a failure that was first put down to a fault memory module.
Instead, engineers at ASA needed to move to the backup Power Control Unit (PCU) and the backup Command Unit/Science Data Formatter (CU/SDF) on the Science Instrument and Command & Data Handling (SI C&DH) unit. The PCU distributes power to the SI C&DH components, and was the cause of the initial fialure, while the backup CU/SDF was switched to their alternate interfaces to connect to the SI C&DH. Once these steps were completed, the backup payload computer on this same unit was turned on and loaded with flight software and brought up to normal operation.
The Hubble team is now monitoring the hardware to ensure that everything is working properly. The team has also started the process for recovering the science instruments out of their safe mode configuration. This activity is expected to take more than a day as the team runs various procedures and ensures the instruments are at stable temperatures. The team will then conduct some initial calibration of the instruments before resuming normal science operations.
When the payload computer halted last month, Hubble’s science instruments were automatically placed into a safe configuration. A series of multi-day tests, which included attempts to restart and reconfigure the computer and the backup computer, were not successful, but the information gathered from those activities has led the Hubble team to identify the PCU as the problem.
The PCU sits on the SI C&DH unit and provides a steady voltage supply to the payload computer’s hardware. The PCU contains a power regulator that provides a constant five volts of electricity to the payload computer and its memory. A secondary protection circuit senses the voltage levels leaving the power regulator. If the voltage falls below or exceeds allowable levels, this secondary circuit tells the payload computer that it should cease operations. The team’s analysis suggests that either the voltage level from the regulator is outside of acceptable levels (thereby tripping the secondary protection circuit), or the secondary protection circuit has degraded over time and is stuck in this inhibit state.
Because no ground commands were able to reset the PCU, the Hubble team switched over to the backup side of the SI C&DH unit that contains the backup PCU. The team performed a similar switch in 2008, which allowed Hubble to continue normal science operations after a Command Unit/Science Data Formatter (CU/SDF) module, another part of the SI C&DH, failed. A servicing mission in 2009 then replaced the entire SI C&DH unit, including the faulty CU/SDF module, with the SI C&DH unit currently in use.
Replacing the module is no longer an option, but the next generation space telescope, the James Webb, is scheduled for launch in the autumn.
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