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

transferred to the electrons of the superconductive photo voltaic (PV) cells. The electrons in the PV cellsbecome excited enough to escape their atoms and begin flowing, producing the current that powers the GP-Belectronics.Because the GP-B spacecraft always points at the guide star, rather than at the Sun, the angle between thespacecraft's main axis (also the telescope axis) and the Sun—called the gamma angle—varies throughout theyear. Thus, the solar arrays do not receive consistent direct sunlight, and this is the reason for the unusual anglesat which the solar arrays are canted.Figure 3-20. GP-B’s annual orientation patterns with respect to the sunEngineers at Lockheed Martin, who designed and built the GP-B solar arrays, had to ensure that the arrayswould produce sufficient electricity in the worst-case gamma angle of approximately 22 degrees, whichoccurred once during the mission, on 12 September 2004. The Lockheed Martin engineers also had to designand mount the solar arrays in such a way that there would be sufficient power during the maximum 36-minuteeclipse times, which occur when the gamma angle is at its maximum and minimum values (158 degrees and 22degrees, respectively). At these angles, GP-B's orbit plane directly faces the sun—in other words, the Sunappears directly below the guide star, instead of off to one side or the other. This alignment only happened twiceduring the actual mission—on 12 September 2004 and 20 March 2005—and the greatest power usage anddeepest depth of discharge (DOD) by the batteries occurred at these times.Taking these worst-case alignments into account, the Lockheed Martin designers arranged the orientations ofthe solar panels on the spacecraft to optimize the amount of incident sunlight. Looking down on the spacecraftfrom the top of the telescope (that is, looking down the spacecraft's z-axis), the arrays are arranged 90 degreesapart, forming an “X” pattern. Panels 1 and 3 are canted 90 degrees into the Z-plane of the spacecraft, so thatyou can only see their edges looking down on the spacecraft. Panel 2 is canted 25 degrees forward in the Z-plane, towards the top of the spacecraft, and Panel 4 is canted in the opposite direction, 25 degrees backwards inthe Z-plane, towards the bottom of the spacecraft. Because of this asymmetry, the spacecraft is free to rotateabout the Z-axis of the telescope with minimal perturbations due to the dragging or torquing of the solar panels.GP-B maintains a Low Earth Orbit (LEO) at an altitude of only 640 km (400 miles) about the Earth's poles. Atthis altitude, it has an orbital period of 97.5 minutes, eclipsing behind the Earth 14-16 times a day--almost 5,000times a year. While in these eclipses, GP-B relies on its Super NiCAD batteries to provide power for all of theGravity Probe B — Post Flight Analysis • Final Report March 2007 89