GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

If the vehicle control system were in the observer mode (1A), the sinusoidal signal in the rate gyroscopes woulddominate the observer, and the vehicle would point in a sinusoidal pattern, rather than staying centered on theguide star. Pointing error in this setup would be much larger than the error resulting from the telescope noisethat enters the system in the direct pointing mode (1B).As a result, mode 1B has been used where no estimation is done on the proportional control term. The attitudeerror is sent straight from the telescope data to the controller with no signal processing other than a straightgain. Although this leaves the vehicle susceptible to the noise in the telescope signal, the overall pointingdisturbance is smaller. More importantly, the random telescope noise causes a disturbance much closer to whitenoise, rather than error at roll rate, the frequency that is most detrimental to science data.7.5.5.2 GS captures (rate capture mode)The method of capturing the guide star called the rate capture mode has also been more challenging thanoriginally thought due to the ARP motion. The rate capture mode, intended for guide star capture when the staris outside of the linear range of the telescope, is essentially a commanded rate in the direction of the guide star.In this case, the controller is commanded a constant rate in the correct direction, based on the telescope output.Once the star returns to the linear range of the telescope, the controller returns to PID control.A problem arises, however, when the gyroscope housing is misaligned with the telescope boresight, due to ARPmotion. There were cases where the ARP had moved so that when the gyros measured the commanded rate, thevehicle was actually moving in the wrong direction. This led to guide star captures in the worst case of over 20minutes.The solution to this problem has been to essentially stop using the rate capture mode. The implementation inplace is an expanded definition of the linear range of the telescope. It has been more successful to use theproportional term and modify the integral and rate path limits, even when the star is outside of the actual linearrange of the telescope.7.5.5.3 Roll rate disturbance/rate notch filterThe roll rate signal that enters through the gyroscopes causes another problem for the science data. The sameroll rate ARP motion that led to using mode 1B, still causes a problems in this mode, since the gyroscope signalstill enters the control law through the derivative term in the controller. Because the motion of the ARP is at rollrate, there is an artificial signal in the gyroscope output causing erroneous torque commands at roll frequency.This output is seen clearly in the spectral analysis of the attitude error and rate path error.The solution to this problem is to attack the roll rate signal in the gyroscope data directly. There is a filter on therate error path, between the gyroscopes and the control command generator. A notch filter at the roll rate hasbeen implemented to attenuate the signal at roll by 10X, basically throwing out as much of the false signal as isdeemed safe. Although the signal cannot be eliminated completely, this has been a very successful way ofreducing the roll rate output of the vehicle pointing.7.5.5.4 Gyro hold offsetThe orbit rate motion of the ARP has also provided challenges to the ATC system in the form of a large transienterror while the guide star is captured at the beginning of every guide star valid period. At the end of every guidestar valid period, the vehicle goes into gyro hold mode, where the telescope attitude error-nulling controller isoff. The vehicle controls its attitude using gyroscope data for rate and attitude.192 March 2007 Chapter 7 — Attitude & Translation Control Subsystem Analysis