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

4 JUNE 2004—MISSION UPDATE: DAY 45After six weeks in orbit, the spacecraft continues to be healthy, with allsubsystems performing well. Over the past two weeks, with the arrivalof summer in the Northern Hemisphere, the spacecraft emerged frombeing partially eclipsed by the Earth each orbit to being in full sunlightcontinuously. The spacecraft's orbit is stable and meets ourrequirements for transition into the science phase of the mission. Allfour gyros remain digitally suspended, and all are spinning veryslowly. Progress in locking the on-board telescope onto the guide star,IM Pegasi, has been slower than anticipated, but this afternoon, asthese highlights were being posted, we achieved this importantmilestone.This past week, the team started performing a highly methodical andpainstaking series of calibration tests on the four science gyroscopes.These tests begin at very low gyro spin rates of 0.333 Hz (20 rpm) orless. This enables the team to exercise and check out the softwarecommand templates that control the tests, without risk of damage tothe gyro rotors or housings, which could occur at higher speeds. Weaccomplish these calibration tests by briefly applying voltagesasymmetrically to the suspension electrodes on a given gyro, causingthat gyro rotor (sphere) to move off center by a few micrometers (10-6meters) or less, and then re-centering it. Photo: A backlit housing halfof a gyroscope. The three oval shapes (at 1 o'clock, 5 o'clock and 9o'clock, on the picture) are where the electrodes are placed.During these initial calibration tests, the team discovered that theperformance characteristics of the gyros on orbit are slightly differentfrom the simulator predictions on which the original commandtemplates were based. As a result of these differences, during one ofthe initial tests, gyro #4 touched one of its electrodes and stoppedspinning. A simple modification to the spin-up command templates isnow in process. Once the on-orbit performance characteristics of eachgyro is well understood, the team will repeat these tests at graduallyincreasing spin rates.The team also exercised the Ultraviolet Discharge procedure to reducethe electrostatic charge, which builds up on the gyro rotors over time.The rotors build up a charge in two ways: first, the process ofsuspending the rotors electrically deposits some charged particles onthem, and second, protons from the sun are constantly bombardingthe spacecraft, especially over the South Atlantic Ocean-the so-called“South Atlantic Anomaly”- and some of these protons strike the rotorsand leave a residual charge on them.We use ultraviolet light to remove the build-up of electrostatic chargefrom the gyro rotors. Each gyro housing is fitted with fiber optic cablesthat run from the gyro housings, up through the top hat of the Probeand out to an ultraviolet light source in the Experiment Control Unit(ECU) box, mounted on the spacecraft frame. UV light is beamedthrough the fiber optics onto the gyro rotors to discharge them.Reducing the level of charge is important because it increases thesensitivity and accuracy of the Gyroscope Suspension System (GSS).11 JUNE 2004—MISSION UPDATE: DAY 52On its 52nd day in orbit, the spacecraft continues to be in good health,with all subsystems performing very well. The spacecraft’s orbit, whichwill remain in full sunlight through August, is stable and meets ourrequirements for transition into the science phase of the mission. Allfour gyros are digitally suspended and have passed several very slowspeedcalibration tests. Furthermore, the science telescope is lockedonto the guide star, IM Pegasi, and we have verified that it is lockedonto the correct starOver the past two weeks, through a combination of softwaremodifications, revised procedures, and commands sent directly to thespacecraft, considerable progress has been made in adjusting theAttitude and Translation Control system (ATC) to properly maintainthe spacecraft’s attitude (pitch, yaw, and roll) in orbit. The ATCsystem accomplishes this important job by controlling the flow ofhelium gas, continually venting from the dewar, through thespacecraft’s micro thrusters. This system is critical to the success of themission because it maintains the required roll rate of the spacecraft, itkeeps the spacecraft and science telescope pointed at the guide star,and it keeps the spacecraft in a drag-free orbit. Thus, the team isparticularly gratified to now have the ATC functioning reliably, withthe science telescope locked onto IM Pegasi.The process of locking the science telescope onto IM Pegasi startedwith star trackers on either side of the spacecraft locating familiarpatterns of stars. Feedback from the star trackers was used to adjustthe spacecraft’s attitude so that it was pointing to within a few degreesof the guide star. The telescope’s shutter was then opened, and a seriesof increasingly accurate “dwell scans” was performed to home in onthe star. Since the spacecraft is rotating along the axis of the telescope,imbalance in the rotation axis can cause the guide star to move in andout of the telescope’s field of view. Feedback from the telescope wassent to the ATC system, which adjusted the spacecraft’s attitude untilthe guide star remained focused in the telescope throughout multiplespacecraft roll cycles. The ATC was then commanded to “lock” ontothe guide star. Photo: The GP-B Star Tracker before installation ontothe spacecraft, circa April 1999.Finally, to verify that the telescope was locked onto the correct guidestar, the micro thrusters were used to point the spacecraft/telescope ata known neighboring star, HD 216635 (SAO 108242), 1.0047 degreesabove IM Pegasi. When the telescope was pointed at this location, theneighboring star appeared with anticipated brightness, and there wereno other stars in the immediate vicinity. Thus, the sighting of the star,HD 216635, confirmed the correct relationship between the locationsof the two stars, ensuring that the telescope is indeed locked onto thecorrect guide star. In addition, the telescope has also seen the star HRPeg (HR 8714), a brighter and redder star, located less than half adegree to the left of IM Pegasi.This past week the team continued performing calibration tests of allgyros, spinning at less than 1 Hz (60 rpm). In addition, the teamsuccessfully tested a back-up drag-free mode of the spacecraft withthree of the gyros for an entire orbit, and, more significantly, the teamcompleted its first successful test of the primary drag-free mode sincere-configuring the micro thrusters, using gyro #3.In primary drag-free mode, the Gyro Suspension System (GSS) isturned off on one of the gyros, so that no forces are applied to it. TheATC uses feedback from the position of this gyro in its housing to“steer” the spacecraft, keeping the gyro centered. Back-up drag-freemode is similar, but in this case the GSS applies very light forces on thegyro to keep it suspended and centered in its housing. The ATC usesfeedback from the GSS to “steer” the spacecraft so that the GSS forcesare nullified or canceled, thereby keeping the gyro centered. Applyingforces with the GSS to suspend the drag-free gyro adds a very small,but acceptable, amount of noise to the gyro signal, and thus, eitherprimary or back-up drag-free mode can be employed during thescience experiment. Upcoming milestones include maintaining thespacecraft in a drag-free orbit, and beginning gyro calibration tests atspin rates of up to 5 Hz (300 rpm).476 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission