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

The first, and only other 3-axis drag-free controller ever flown was called DISCOS (DISturbance COmpensationSystem). It was developed in the late 1960s as an offshoot of GP-B under the leadership of Professor Dan DeBra,then head of the Guidance & Control Laboratory in the Stanford Aero-Astro Department, and a Co-PrincipalInvestigator on GP-B. DISCOS was the central drag-free control module in the three-module TRIAD I satellite,a test vehicle developed to improve the accuracy of the U. S. Navy’s Transit satellite navigation system. Transitsatellites enable ships to locate their positions on the Earth's surface by reference to orbit data stored on boardthe satellite, but earlier ones had been limited by uncertainties in orbit prediction arising from atmosphericdrag. The DISCOS module was specifically designed to address this problem. The other two modules in theTRIAD I satellite were developed by the Applied Physics Laboratory (APL) at Johns Hopkins University, underthe leadership of Robert Fischell. The TRIAD I satellite, with its central DISCOS module, was launched onSeptember 2, 1972 and flew in a polar orbit at 750 km for a little more than a year and achieved drag-freeperformance of 5x10 -12 g for the entire mission. Subsequently, the Navy built two more satellites in the TRIADseries, followed by three satellites in the NOVA series. All of these subsequent satellites contained only singleaxisdrag-free control, the development of which also included significant contributions by Stanford faculty andgraduate students.3.2.1.5 Telescope & Guide StarIn the GP-B science instrument, enclosed within the Probe, along the central axis of the dewar, a 36 centimeter(14 inch) long Cassegrain reflecting astronomical telescope, with a focal length of 3.8 meters (12.5 feet), isbonded, using a hydroxide catalyzed bonding process, to the end of the quartz block that houses the gyroscopes.Together, the telescope and the quartz block form the Science Instrument Assembly (SIA). Hydroxide catalyzedbonding is a method of fusing together quartz parts, without the use of any glue or fasteners, that was developedand patented by the GP-B team at Stanford. This was necessary for the SIA not to distort or break when cooledto the cryogenic temperatures required for superconductivity used by the gyroscopes. The telescope's line ofsight provided a reference for measuring any drift in the gyroscopes' spin axis over the life of the experiment.During the science phase of the mission, the telescope had to remain focused on a distant stable referencepoint—a guide star. Furthermore, it had to remain fixed on the center of this guide star within a range of +/- 20milliarcseconds throughout the science phase of the mission. The resulting telescope signal was continuouslysubtracted from the gyroscope signal at the 0.1 milliarcsecond level to determine the amount of spin axisprecession of each gyroscope.Figure 3-7. A pre-flight prototype of the Cassegrain telescope on-board the GP-B spacecraftGravity Probe B — Post Flight Analysis • Final Report March 2007 75