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

Star Trackers. A star tracker is basically a telescope with a camera and pattern matching system that usesconstellations and stars to determine the direction in which a satellite is pointing. The GP-B spacecraft containstwo star trackers (also called star sensors) for redundancy. The star trackers have a field of view on the order ofeight degrees, and they can focus to a position within 60 arcseconds (0.017 degrees)—about the same as thewhole field of view of the on-board telescope, which can pinpoint the guide star’s position to within amilliarcsecond. With such a small field of view, it would be nearly impossible to locate the guide star using onlythe onboard telescope, so the star trackers functioned like “spotting scopes” for initially pointing the spacecrafttowards the guide star. During the mission, once the star tracker had oriented the spacecraft within 60arcseconds of the guide star, the onboard telescope then took over the job of maintaining the precise alignmentrequired for measuring gyroscope drift.Control Gyroscopes. GP-B contains two pairs of standard, flight-qualified rate-sensing gyroscopes, equivalentto those found on other spacecraft (and also airplanes, ships, and other vehicles). These gyroscopes are part ofthe general Attitude & Translation Control System (ATC) of the spacecraft.Electro-mechanical Control Systems. Surrounding the dewar, is a lattice of trusses that forms the structure ofthe space vehicle. Attached to these trusses are a number of electrical and mechanical systems that control theoperation of the spacecraft and enable the relativistic measurements to be carried out. These control systemsinclude the following:• Attitude & Translation Control System (ATC)—Uses feedback from the gyro suspension system(GSS), the SQUID readout system (SRE), the telescope readout system (TRE), the star trackers,magnetometers, and rate-sensing navigational gyroscopes to controls the proportional micro thrustersand magnetic torque rods that determine and maintain the spacecraft’s precise attitude and position.• Magnetic Torquing System (MTS)—A set of long electromagnets that push or pull against the Earth’smagnetic field to orient the spacecraft’s attitude. During the GP-B flight mission, the magnetic torquerods were used in conjunction with 16 proportional micro thrusters to control the spacecraft’s pointingwith extreme precision. Following the mission, with no helium propellant remaining to operate themicro thrusters, these magnetic torque rods are the only means of controlling the spacecraft’s attitudeand position in orbit.• Mass Trim Mechanism (MTM)—A system of movable weights that can be adjusted during flight torestore rotational balance of the space vehicle (similar to spin balancing the tires on an automobile)• Gyro Suspension System (GSS)—The electronics that levitate and precisely control the suspension ofthe four gyroscopes at the heart of the Gravity Probe B experiment. The GSS control boxes are mountedin the truss work, outside the dewar. The wiring goes through the top hat section of the Probe and downto each gyroscope.• Gas Management Assembly (GMA)—A very complex set of valves, pipes, and tubing, that runs from atriangular assembly on the truss work, through the top hat and down the Probe to each of thegyroscopes. The critical job of the GMA is to spin up each of the four gyroscopes by blowing a stream of99.99999% pure helium gas over them, through a channel built into one half of each gyroscope’s quartzhousing.• Experiment Control Unit (ECU)—The ECU controls many of the systems onboard the space vehicle,including the GMA, the UV system, and various thermal devices.Gravity Probe B — Post Flight Analysis • Final Report March 2007 41