Harmonics in gyroscope drifts
Hipparcos was equipped with 5 rate integrating gyroscopes, which have fixed input axes and angular momentum vectors with respect to the spacecraft structure. The gyroscope readouts have been examined relative to the reconstructed rotation rates reconstructed for the payload.One element of the analysis concerned the modulation of the drift with respect to the rotation phase of the satellite, where the phase is measured from the great circle through the spin axis of the satellite and the direction of the Sun. Clear and repetitive modulation was observed, as can be seen in the chart below for Gyroscope 2. Note that during a period when the satellite was operating in so-called Sun-pointing mode (orbits 1569 to 1578), the modulation entirely disappeared.
The other gyroscopes showed similar behaviour. No significant change was observed when gyroscope 5 took over from gyroscope 4. The amplitude of the strongest feature for gyroscope 2 was in addition analysed as a function of time, and shows a modulation as would be expected from the variation in solar flux due to the ellipticity of the Earth orbit. This is shown on the last tab of the display.