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

Constraint 3: Keep the spacecraft pointed at a selected guide star (IM Pegasi) to within 50 milliarcsecondswhenever the spacecraft has an unobstructed view of the guide star (Guide Star Valid periods). When thespacecraft moves behind the Earth so that the guide star is eclipsed from its view (Guide Star Invalid periods),maintain its pointing orientation so that when the guide star comes back into view, the spacecraft’s pointing canlock back onto the precise guide star pointing orientation within 1-2 minutes or less. This constraint requiresvery precise attitude control (pitch, yaw, and roll).Constraint 4: Fly the entire spacecraft drag-free around one of the science gyroscopes, which have a clearanceof only 32 micrometers (~0.001 inches) between the gyro rotor and its housing. In other words, the spacecraftmust continuously “chase” this gyro in orbit without ever allowing the rotor to touch its housing wall. Thisrequires that the attitude control system also have 3 degrees of translational control—thus the name Attitudeand Translation Control System or “ATC.”Solution: Many of the unique technologies described earlier in this section were combined to meet thischallenge. The actual precision attitude control was provided by the 8 pairs of proportional micro-thrusters,described in section 3.2.3.3. These thrusters were arranged so that they could control the spacecraft in all threetranslational degrees of freedom, as well as three additional degrees of freedom attitude control.The helium gas that constantly boiled off from the liquid helium in the dewar yielded a clever solution toproviding the propellant for powering these micro-thrusters, and the porous plug, described in section 3.2.3.1provided a means of controlling this helium gas as it escaped from the dewar. The design of the dewar, itself, wasalso part of the solution. It had to be large enough to hold the 650 gallons of liquid helium that would maintainthe cryogenic environment for the gyros to spin for at least 16 months; yet the dewar had to be physically smallenough to fit in the fairing of a Boeing Delta II rocket.The science telescope, with its Image Divider Assembly, described in section 3.2.3.8, was also an important partof the solution, since it provided the precise pointing reference for the spacecraft throughout the mission.Likewise, the Gyro Suspension System (GSS) provided feedback on the rotor position of the “proof mass” gyro,around which the spacecraft was flown drag-free.Finally, a number of more traditional spacecraft orientation and control devices, including the star trackers,navigational rate gyros, magnetometers, magnetic torque rods, and GPS receiver were all components ofspacecraft’s attitude control system. (See section 2.1.7.2 for a description of these components.) Notably absentfrom the ATC system were reaction wheels and momentum wheels. The concern to the experiment was thatthese wheels could have a mass imbalance, which could interfere with the zero-g gyroscope performance.Result: The attitude and translation control system (ATC) designed by Lockheed Martin for the GP-Bspacecraft was truly an engineering “tour-de-force.” It took a considerable amount of time during IOC, and partof the science phase of the mission for the GP-B and Lockheed Martin Mission Operations Team to master allthe subtleties of this complex system. But once it was properly tuned up, the GP-B ATC system was able to meetthe mission requirements, and it enabled the GP-B program to collect over a terrabyte of data during the sciencephase of the mission.92 March 2007 Chapter 3 — Accomplishments & Technology Innovations