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 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.

Based on the analyses described above, we devised and tested a set of commands which will correct the sign error. We also modified and tested the TERRIERS ground station code to include a mode in which we can transmit only, so that we can upload the commands as soon as contact is established. Furthermore, we set up another ground station at Poker Flats, Alaska to increase our coverage.

Through July and August we received no "truly" positive indications that the spacecraft had booted. We had support from Poker for about 3 weeks during this time as well. Analysis assuming that we were tumbling randomly across the sky, indicated that the spacecraft would be in a power positive position above a ground station once every few days. This led us to believe that the spacecraft was either phase locked away from the sun (a small possibility given our simulations) or something else had failed.

In September, Michael Comberiate at GSFC suggested that the south pole albedo during Antarctic summer, might be enough to boot the spacecraft. The idea seemed plausible and preparations began shortly thereafter to send some equipment down to the McMurdo ground station to attempt contact in December, 1999.

The TERRIERS, south pole recovery effort took place between December 15th and December 24, 1999. This was the culmination of efforts by Michael Comberiate (GSFC), Greg Huffman (BU RF engineer), and GSFC Wallops and McMurdo personnel. It was sponsored in combination by USRA, NASA and NSF.

The team assembled and tested TERRIERS ground station specific equipment at Wallops Flight Facility in mid November. The equipment was subsequently packed up and then delivered to NASA's McMurdo Ground Station (MGS). Mike Comberiate and Greg Huffman arrived at MGS on December 15, 1999 to integrate the equipment into the GS. They were able to get the system up and running in only a few hours. Thanks to the excellent support from the McMurdo GS team, they covered nearly 60 passes over the 9 day period. Unfortunately, in all attempts to contact and receive date from the spacecraft, no sign of a downlink was detected from TERRIERS. A more detailed report ( pdf format or ps format ) of the operations was written by Greg Huffman.

From the operation, we can conclude that the spacecraft did not boot in response to the Antarctic albedo during the height of Antarctic summer. This is either because there is not enough energy coming from the surface or there is something else preventing the spacecraft from communicating. In his report, Greg presents a calculation of the albedo, which indicates that there may not have been enough reflected sunlight for the spacecraft to boot. However, there should have been enough energy for the transmitter to send a carrier signal even if the system was in a browned out state. Test data indicate that this is highly likely to happen. Because this signal was not detected, it is more likely that there is something else preventing the spacecraft from booting.

As of updating this page (1-24-99), we have not reestablished direct contact with the spacecraft. Below is a summary of the mission operations to date.

Mission Operations Log