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

used. The star trackers are essentially pattern-matching cameras, located on opposite sides of the spacecraftframe. The payload magnetometers are like three-dimensional compasses that determine the spacecraft’sorientation with respect to the Earth’s magnetic field.During the 17-month GP-B flight mission, all of the above systems, in addition to data from the GyroSuspension Systems were used to “steer” the spacecraft in its orbit. Moment to moment changes in thespacecraft’s position were controlled by a combination of 16 micro thrusters (see below) and a set of magnetictorque rods—long electromagnets that push or pull against the Earth’s magnetic field to change the spacecraft’sorientation. After the helium was depleted from the dewar at the end of September 2005, these torque rods,controlled by feedback from the star trackers and magnetometers, are the only means of re-orienting thespacecraft in the post-mission phase.3.2.2.7 Star Tracker RedundancyBoth star trackers were turned on and have been running since the beginning of the mission, but only one pairof rate gyros was in use. During the science phase of the mission, we activated the other set of rate gyros, as well.Data from one set was sent to the ATC to control the spacecraft's position; data from the other set was collectedduring telemetry communications sessions and used for precise roll attitude calibrations to the science data. TheB-side switch-over of the CCCA computer in March 2005 also triggered a switch to the B-side rate gyros andstar tracker. We subsequently commanded the spacecraft to switch back to the A-side rate gyros and the A-sidestar tracker, both of which had been fine-tuned during the Initialization and Orbit Checkout (IOC) phase of themission and were performing slightly better than their backup counterparts.3.2.2.8 Micro Thruster RedundancyThe final redundant spacecraft system is the micro-thruster system. The spacecraft is outfitted with 8 pair ofopposing micro-thrusters, arranged in four clusters—two at the top of the spacecraft frame and two at thebottom. One thruster in each cluster is redundant, and a set of valves in the thruster system enables individualthrusters to be isolated and effectively disabled. This is precisely what we did early in the mission with twomicro-thrusters, whose nozzles became stuck open, apparently due to particle contamination shortly afterlaunch. The remaining 14 thrusters performed flawlessly throughout the science and post-science instrumentcalibration phases of the mission.3.2.3 GP-B’s Unique Technological Challenges and SolutionsWhen the GP-B experiment was conceived late in 1959-1960, much of the technology had not yet been inventedto accomplish this exacting test. Here are the some of GP-B’s unique technology inventions and clever solutionsto difficult challenges.80 March 2007 Chapter 3 — Accomplishments & Technology Innovations