ROSAT Status Report #18

Feb 11th 1992

A large transient in the ROSAT Z-gyro current occurred late on Feb 6th, reminiscent of that which immediately preceded the loss of the Y-gyro last May. The spacecraft was put into safe mode at that time and observations have been suspended.

The Z-gyro, although continuously with too high a current, still seems to give meaningful readings of angular velocities. However, its output signal seems to be scaled wrongly, indicating a factor of 3-4 slower speeds than the spacecraft actually performs. To pin down this value more exactly GSOC have to combine sun-sensor readings with the remaining two gyros (the S-gyro also has a Z-component), and nothing is known yet about the stability of this new scale factor. Tests on the ground with other gyros by Ferranti (the gyro manufacturer) and MBB (the AMCS contractor) are planned.

MBB is now producing a small software patch that enables a larger variation of the in-orbit scale factor compensation for the Z-gyro. They plan to test this software on the GSOC simulator on Feb 11-12th and, if successful, to link it up to the spacecraft. Afterwards they want to switch to pointing mode to see how the spacecraft reacts. Most optimistically GSOC could resume nominal timeline operations immediately. Maybe more realistically, ROSAT could go into another round of reduced pointing mode. The software development of the new one-gyro mode could then happen in a much less turbulent fashion.

Juergen and GSOC are preparing two sets of timelines for next week, a nominal and a reduced one. In the meantime GSOC are making PSPC background measurements with the PSPC out of focus (GSOC never measured the particle background with the reduced PSPC gain).

During the investigation of the safe-mode behavior a new problem with one of the reaction wheels (# 3) was noticed, which seems to have a higher static and dynamic friction than usual. This problem became evident because the wheels are rotating much less in safe mode than in pointing mode, but obviously was already present during the last pointing interval. Older data are investigated right now.

AO3 response

Preliminary statistics are available for the AO3 proposal. A total of 395 proposals were received by NASA; approximately 280 were received in Germany and approximately 120 were received in the UK. On the US side this corresponds to about the same number that was received last time. The breakdown by category is as follows:

Stars                    78
White dwarfs              7
CV's                     33
Neutron stars, etc.      34
Supernova remnants       26
Normal galaxies          38
AGN                     105
Clusters                 49
Diffuse                  15
Others                   10

As all the proposals have not yet been entered into the database, no estimate of time or funding oversubscription is available. These will be made available once they are compiled.

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