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Mission Status Report #83      Star Date: March 25, 2005

On the Way Back!

Caption: FUSE Satellite Control Center, during recovery operations on March 22, 2005. (Photo by Bill Blair, FUSE project, JHU.)

(Click image above to see larger version.)

I am extremely pleased to report that the FUSE satellite is well on its way toward the recovery of science operations. Major steps toward recovery were made this week, after spending nearly three months in a safe mode configuration due to a hardware problem that occurred on Dec. 27, 2004. (See Background info on Reaction Wheel Anomaly and this earlier Status report.)

Over the last month and a half, the JHU operations team and engineers at Orbital Sciences Corporation (the spacecraft manufacturer) have undertaken an intensive research and development effort to revise the flight control software so that it will properly control the satellite for science operations even though only one reaction wheel remains. After testing several variants of the control algorithm and extensive testing through software simulation, the best algorithm was adopted and the new flight code was developed and tested.

On Tuesday, March 22, 2005, this revised attitude control software was uplinked to the FUSE satellite, and the satellite was commanded out of its safe mode and into an inertial pointing mode (i.e. pointing continuously at a specific point on the sky). The satellite was then slewed to the south orbital pole, where it remained pointed nominally at the few arcsecond level. The behavior of the slew to the pole followed expectations very closely. The rest of the day was used to monitor the momentum unloading and attitude control algorithms, and perform a few small satellite maneuvers.

For the remainder of the week, we have performed other tasks to ready the satellite for science operations. Onboard heaters were used to bring all elements of the FUSE satellite up to operating temperatures, and all sub-systems were powered on. This could not be done until the satellite was in a stable, inertial pointing mode with the solar panels held continuously on the sun (to provide full power). The FES guide camera was put through an "annealing" process to prepare it for use in science operations after the long hiatus.

Additional slew tests were performed, including a slew to a position 20 degrees off the orbit pole and back. Three orbits (~4.5 hours) were spent at the offset position with nominal pointing control the entire time. (We could have stayed at that position longer, but it was time to get on with other tests!) Assessment of telemetry from tests such as this one is providing confidence that the system performance closely matches our software simulations. This is important because extensive simulations will be needed in normal operations to keep FUSE pointed stably in the new operations mode.

Caption: More views of Control Center during recovery operations on March 22, 2005. (Photos by Bill Blair, FUSE project, JHU. Click to enlarge.)

Over the next week, we have some important goals, which include ramping up the high voltage on the spectrograph detectors, opening the telescope doors, and performing our first target acquisitions and observations since December! However, before that (while the doors are still closed for safety), we will perform more aggressive slew tests that will provide information about our pointing capabilities in different parts of the sky. This will help us answer the next important question: How much of the sky we can get to for science observing? Initially, we are staying close to the southern continuous viewing zone (close to the orbit pole), where stability is easier to maintain. Then we will "learn as we go," trying more sophisticated maneuvers and more difficult pointing directions. I note that, because of the 25 degree tilt of the FUSE orbit, being able to observe at 20 degrees away from the orbit pole bodes well for potential observation of targets above an absolute declination 45 degrees, and were just getting started! We have a lot of learning still to do, but we are encouraged by the early performance with the new system.

I will post the next update in approximately one week. Stay tuned!

Thanks to all of the FUSE Sciences Operations team, including our partners from the Mission Operations Team from Honeywell Technology Solutions, Inc., and our colleagues at Orbital Sciences Corporation, for their tremendous effort so far, and for their continuing efforts as we learn to use the new system.

Reported by: Bill Blair, FUSE Chief of Observatory Operations

Last Update: March 25, 2005

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