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TERRIERS - O verview > > Mission Operations/Status History...

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Overview >> Mission Operations/Status History Timeline Radio Interview TERRIERS Location Mission Control/Responsibilities Space Weather Publications Mission Operations/Recovery Status TERRIERS launched successfully from VAFB on May 18 05:09:47 GMT. It hit a nearly perfect orbit (550 X 530 km and an inclination of about 97.72 degrees). To see the TERRIERS position go to the position page. For a quick update on the TERRIERS status see the mission operations log below . Mission operations for TERRIERS are taking place at Boston University's Center for Space Physics . TERRIERS operations began on 5-18-99 with the first contact of the spacecraft around 7:07 EDT. Unfortunately, after the first 3 contacts it was clear that the the spacecraft's attitude control system was not functioning properly, since the solar array was pointing away from the sun and the system was nutating. Sometime shortly thereafter the spacecraft ran out of battery power and effectively went to sleep. That week a recovery team was formed by NASA/USRA to devise and implement a plan to wake the spacecraft and get it back to the original mission. Led by Steve Battel, the team took on several tasks in pursuing this goal: Determine the cause of the ACS problem, Assess the state of the system and its ability to wake up, and Model the spacecraft dynamics to determine if and when the solar array will be illuminated due to natural disturbances Devise and test a command load that will aide in the recovery of the satellite. Within a week or so we had pruned our fault tree down to the lone cause of the failure: a sign flip in one of the 3 torque coil actuators. The fix for this is fairly simple given the flexibility of the flight software. All the telemetry from the spacecraft indicates that it was working properly, except for the sign flip. Furthermore, the system should not degrade much in its current configuration, since the spacecraft is powered down and in a reasonable thermal state. The big question is whether the system can reboot itself from this state. Simulations on our engineering system were run and it appears there is a good probability the system will reboot. Unfortunately the dynamics of the spacecraft was not easy to model, due to uncertainties in TERRIERS' inherent magnetic moment, center of pressure, and eddy current/hysteresis damping characteristics. Initially we thought that the gravity gradient torque would tie our angular momentum
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vector (and solar array vector) towards the orbit normal, which is always pointed away from the sun. However, data points taken by NORAD of our angular momentum direction and spin period showed that this model was not adequate. Further modeling efforts over the weeks have not been able to match the data either. However, we could see simply from the data that TERRIERS was spinning down at a linear rate and that in about 70 days (late July) from launch, its motion should become "erratic" such that the solar panel should get illuminated for periods on the order of 10 minutes.
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