MAGION spacecraft attitude control:
life in the new reference system.



From NATAN EISMONT
October 30,1995

After all troubles caused by the undeployed boom and the small solar panel MAGION spacecraft was transferred to the new mode of the attitude control around new axis having maximum inertia moment. This axis is shifted by 40 degrees with respect to construction axis Z of spacecraft in ZX plane and by 13 degrees in ZY plane.

In spite of the huge changing of the inertia ellipsoid position with respect to the construction axes the natural demands of the people responsible for electric power unit was the same: to keep solar panels directed towards Sun, i.e. sustain -Z axis along the Sun direction. It may be possible if the s/c would have three axes stabilization what is not our case because for MAGION the spin stabilization has been chosen.

And according to the mechanics laws spin axis could be constant in space (and in spacecraft reference system) only if it would coincide with principal inertia axis.

But Z axis is no more principal inertia axis. So the decision has been taken to transfer to the new reference system and to use for this system the software developed for the nominal case.
It means that the solar sensors measurements are to be transformed for the new system as the control moments. But in this system the control moments vectors do not go along principal axes as it was supposed for the nominal case.
So applying the control impulse to rotate the angular momentum (i.e. giving moment orthogonally to the angular momentum vector) we can not avoid to change the value of this momentum so changing the spin rate. And this has happened during attitude control operations session at September the 25th.
During this session the principal axis Zp has been targeted towards Sun with the accuracy about 5 degrees. Simultaneously the period has been changed from 49 to 46 seconds.


Fig.1

Solar sensors measurements soon after control session (September the 29th) are illustrated by Fig.1. Nutation amplitude for this date is about 0.5 degrees. Immediately after the attitude control session at September the 25th nutation amplitude was 5 degrees.

Fig.2Fig.3Fig.4

Further figures Fig.2 - Fig.4 illustrate the diminishing of nutation with time and rising the angle of Zp axis with Sun according to Earth orbital motion i.e. about 1 degree per day.
It could be seen that the spin period is diminishing with time (Table 1). Supposed reason for this phenomena is Sun light pressure moments. To estimate the other components of this moments angular momentum vector has been calculated by processing Sun and Earth sensors measurements.

Right ascension and declination of this vector are given in Table 1.

DATE SPACECRAFT SPIN PERIODANGULAR MOMENTUM VECTOR
Right Ascension Declination
secdegdeg
2.10.9543.3181.8-3.9
6.10.9542.9182.0-2.4
14.10.9542.5182.4 -1.5

Table 1.

First impression of these results is that light pressure moments do influence the direction of angular momentum vector. But to estimate this influence reliably enough one needs more data.

Attitude motion parameters determination was based on Euler model approximation by trigonometric functions of time. Actually some amplitudes are calculated as well as angular velocities of s/c and angular momentum vector. The maximum deviation of the last one from the principal inertia axis Zp is a nutation amplitude.


Fig.5

Fig 5 presents the projections of this amplitude on two longitudinal (new) planes XZ,YZ as functions of ratio of nutation rate to s/c spin rate. Maximum amplitude corresponds to the best estimation of this ratio which is about 0.16.


Fig.6

In addition Fig 6 presents dependance of measurements rms from the mentioned ratio. The minimum as it is expected was reached for the same frequencies ratio.

  • N.Eismont neismont@esoc1.iki.rssi.ru , E.Ryazanova (Space Research Institute (IKI), Moscow)
  • J.Hum, V.Truhlik, P.Chennohuby (Institute of Atmospheric Physics (UFA), Czech Republic)


    The head of overall MAGION control operations is Pavel Triska (UFA).


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    Last updated 2 Nov 1995