ROSAT Gyro Status: Pointing Problems

--This summary was compiled from MPE Status Reports #25 and #26

After Christmas and in early January there were a number of occasions during which problems in ROSAT's attitude measurement and control system (AMCS) caused the satellite to drift away from its nominal pointing direction. This situation caused the safe mode to be triggered. It soon became clear that the problem was related to the new attitude- control strategy, which became active after the Z-gyro failure that occured in November 1993.

ROSAT originally had four gyros to measure rotations around three spatial axes: the X-axis pointing to the Sun, the Z-axis the telescope pointing direction, and the Y-axis perpendicular to X and Z; the axis of the fourth (redundant) gyro was inclined with respect to all of the other three axes. The Y-gyro was lost earlier in the mission. With the failure of the Z-gyro, the attitude solution had to be derived from the following information:

If the star tracker had identified a star pattern, this information was exclusively used for the attitude solution.
If no star tracker information was available (during slew, Earth block, or when the star pattern could not be identified) the 3-D attitude solution could no longer be derived from the readings of three gyros (as was the case previously), but had to be deduced from the combined readings of the X-gyro and S-gyro, the Sun sensor(s), and the magnetometers.

All critical situations occured when ROSAT was in EarthÕs shadow (i.e. no Sun-sensor readings could be used) and also no star tracker information was available (e.g. during an Earth block of the sky region to which the telescope was pointing). In these cases, the attitude solution had to be derived from the two gyros and the magnetometers. Since the gyro measurements had been routinely used during the whole mission, the error was isolated to be in the magnetometers.

In the first instances, drifts occuring during Earth blocks on the night side were typically of the order 25 degrees. When ROSAT left EarthÕs shadow the fine Sun sensor provided the information that the maximum Sun angle (20 degrees) was exceeded and triggered the safe mode. It then was believed that only slow drifts occured during the night side, and that the Sun angle could not be exceeded by much. As a quick measure to prevent frequent safe modes, the attitude-control software was modified in such a way that it allowed for 2.5 - 5 minutes during which the fine Sun sensor could try to move the satellite back to nominal Sun angle before triggering safe mode.

This measure did not help when a drift of 110 degrees occured, such that after sunrise the coarse Sun sensor (with a field of view of 45 deg radius) did not `see' the Sun and triggered safe mode. After it was clear that such large drifts could occur, it was decided that ROSAT was not to be recovered from safe mode before a detailed understanding of the problem was gained. Otherwise, there was a possibility of damaging the focal plane instruments.

What caused the problem?

A detailed analysis of the drift events revealed the following results:

For operational reasons, magnetometer readings can be used only in intervals of two minutes. In the critical shadow-and-Earth-block situation, two spatial coordinates constantly are given by the gyros, but the third component is available only in two-minute intervals. If some kind of external torque is present (see below) the satellite begins to drift around some spatial axis, and for two minutes, only two components of this drift (parallel to the Xand S-gyro) are measured and compensated for. The third drift component (perpendicular to the X-S-plane) is only recognized two minutes later. In some unfortunate situations, the satellite is oriented such that Earth's magnetic field vector is lying near the plane defined by the axes of the X- and S-gyros. Then the third component is not available from the magnetometers, the drift is not recognized and can continue.

While it was clear from the beginning that these critical geometrical situations can occur, it was not realized that torques strong enough to cause significant drifts in a few minutes are being exerted on the satellite. The source of these torques now has been identified as the magnetic torquers that are used to prevent the reaction wheels from accumulating overly high rotation rates and saturating. The torquers produce a magnetic field that tends to align (by rotation of the satellite) with the terrestrial field, thereby making it possible to slow the rotation rate of the reaction wheels (counter-rotation). To make this discharge of the reaction wheels as smooth as possible, it is performed almost permanently in small portions. Thus during the two minutes without magnetic field measurements, a number of discharge events can take place. As simulations performed by DASA (the company responsible for the AMCS system) show, the strength of these torques is of the right order to produce the observed drifts.

Measures taken to solve the problem

The identification of the source of the disturbances lead to a quite straightforward cure of the problem, i.e. the suppression of magnetic reaction wheel discharge during night time. The capacity of the reaction wheels is sufficient to restrict their discharge to the times of Sun presence. However, substantial modifications of the onboard software were required that had to be tested in a simulator on ground.

A temporary reduced pointing phase

To make use of the time while new software was being prepared, a reduced pointing phase was conducted, with the following constraints:

During full-Sun orbits (a few days once a month) the Sun sensor could be used together with the gyros so that a regular, automatic timeline could be applied.
During times when there was a continuous viewing zone in the sky (also a few days per month) targets located in that zone could be observed, because attitude reference was given without interruption.
At all other times, regions in the sky were identified that do not go into Earth block during the shadow phase. Targets located in those regions were identified.

The solution to the problem:

A new operating mode has now been activated for ROSAT in which the satellite is angular-momentum stabilized if there is no startracker information available in the Earth shadow. In this case the magnetometers (the sources of the attitude control irregularities) are not used by the control procedure.

This stabilized mode was tested during the last week in February and appears to work in a satisfactory manner. Since midnight of February 26/27 (UT) MPE has continued an observing program, beginning with five days of PSPC observations according to the timeline. The resumption of HRI observations was scheduled for March 4.

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