From: Goddard Space Flight Center
Posted: Sunday, August 10, 2003
It is just over four years now since the FUSE satellite was launched into its roughly 500 mile circular orbit above the earth. The science instrument, a far-ultraviolet spectrograph, continues to operate extremely well, and its overall sensitivity has diminished only slightly (about 15% on average) over the lifetime of the mission. (Given that some pre-launch estimates were that it would lose 20% sensitivity per year, this is an outstanding result.)
Unlike the Hubble Space Telescope, FUSE is not "servicable." We can't send astronauts up to the telescope to make changes or updates to it. But that doesn't mean that ground controllers are helpless in the face of problems! Many of you regular readers followed our earlier rescue of the science mission following problems with our reaction wheels back in late 2001. (See Mission Report #52 and following reports.) That rescue involved both changes to flight software and also using different elements of the satellite hardware (Magnetic Torquer Bars) in new and clever ways.
Over the last couple of years, FUSE scientists and engineers have been working hard to fix a problem BEFORE it became a problem, and this work has been successfully completed. The potential problem was the loss of one or more ring-laser gyroscopes that were part of the satellite's Attitude Control System. Almost every satellite that requires stable pointing has gyros that provide information on pointing and drift rates to the control software. FUSE's gyros started showing some evidence of "aging" very early in the mission, and this has been a concern for a long while.
Well, our clever scientists and engineers have done it again. They came up with a modified flight software system that will use any gyros that are present in the system, but will also compensate for any missing gyros automatically and keep on operating the satellite! The satellite can actually now operate without ANY gyros if it has to (although performance is somewhat improved as long as there is at least one operational gyro in the system). This amazing feat of satellite trickery involved software changes to all three computers onboard, plus changing the "philosophy" of how FUSE gets from one target to the next. After extensive ground testing, the software was uplinked back in April and has been used since that time, with various on-orbit testing and check-outs largely being done even while on-going science observations were being carried out. The system has been declared operational, and many news services carried stories based on our recent press releases. (See the FUSE On-line! page for links to these stories.)
As luck would have it, the ultimate test of the new flight software occurred last week. On July 31, 2003, the "yaw" axis gyro failed in the IRU-B package. This caused the system to...keep working as if nothing had happened! The software automatically sensed that the gyro had failed and switched into the "compensating" mode that was built in. It just kept right on working, with no loss of science data taking that was in progress. While we had tested this any number of times, to see it happen for real and to have the system work as intended was very gratifying for all those involved.
Just a reminder, NASA has released of the Call for a new round of proposals, Cycle 5. Details are posted at the FUSE GI Support Site. The deadline for receipt of proposals is September 19, 2003! Good luck to all!
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