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

Result: The on-orbit gyroscope spin rates were lower than baselined—70 Hz or 4,200 rpm on average, ratherthen 130 Hz or 7,800 rpm— which increased the integration time necessary to estimate the gyroscope spindirection to the desired precision. Partially in response to the low gyro spin rates, the final vehicle roll rate wasset higher than baselined (12.9 mHz or 0.77 rpm rather than 5.5 mHz or 0.33 rpm) which decreased theintegration time necessary. The net result is a decrease in measurement time needed. On-orbit noise floor datashows that the SQUID readout system is capable of measuring the gyro spin axis to a precision of 1milliarcsecond in 6 hours, and to a precision of 0.1 milliarcseconds in 4 weeks.3.2.3.10 Dual-Sided Solar ArraysFigure 3-19. Checking the GP-B solar arrays on the spacecraft at Lockheed Martin & solar arrays deployed on ascale model of the spacecraftChallenge: How do you provide continuous solar power to a spacecraft that has the unique requirements ofalways pointing at a single distant star, rather than the sun, while rolling continuously in orbit for 16 months orlonger?Solution: Most satellites are designed to face the sun year around, or they have moveable solar panels that canbe re-oriented to face the sun, thus maintaining positive power throughout each orbit. This is not the case withGP-B. Due to the nature of the GP-B science mission, GP-B always points at the guide star, IM Pegasi; it neverpoints directly at the Sun, because sunlight could overwhelm the telescope's ultra-sensitive photon detectors.Furthermore, the GP-B spacecraft itself must be as free as possible from any motion disturbances that couldaffect the gyros and SQUID readouts. Thus, once opened, the solar arrays on the GP-B spacecraft had to belocked into fixed positions. For these reasons, GP-B's solar arrays are unique, both in their design and in theirpositioning on the spacecraft.GP-B is equipped with four fixed, double-sided solar array panels, canted at such angles as to always be able toreceive some solar power regardless of the season. Additionally, these double-sided panels were designed tominimize the thermal effects of the spacecraft entering and exiting eclipses, when the Earth blocks the Sun'slight for up to 36 minutes per orbit. Each GP-B array contains 9,552 individual single-junction GalliumArsenide (GaAs) solar cells that convert sunlight into electrical power with 18.5% efficiency, the standard forthis type of eclipsing satellite. As sunlight is absorbed by the solar cells, its photons, or bundles of energy, are88 March 2007 Chapter 3 — Accomplishments & Technology Innovations