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

electrostatic patches, located at opposite poles of the rotors, this would create a significant, classical torqueproblem in our gyros. To address this issue and determine the sizes of the patches, we performed two types oftests:1. a visual inspection,2. a laboratory measurement of the size and magnitude of the patches.The visual testing measured the grain size of the sputtered niobium coating, and found that the patches on theGP-B gyros were very small (less than 1 micrometer in size) and were scattered about the entire surface of eachrotor. With such small patches randomly distributed, it was our expectation that the patch effects would averageout and not be of any consequence to the experiment.To ensure this was the case, we performed a laboratory test. We sent flat samples of niobium-coated surfaces(produced in our same Stanford lab were the gyro rotors were coated) to a lab in France where they scanned thesamples with a Kelvin probe and showed that there was no problem with patches in the flat samples. (The lab inFrance could not handle spherical objects in their scanner, so we created flat samples using our same gyrocoating machinery.)Figure 15-11. A slide from the GP-B plenary talk that Francis Everitt will deliver at the APS meeting on April 14,2007. This slide shows the succession of investigative steps and observations that led to the determination thatpatch effects are the root cause of both the polhode variation and misalignment torques on the gyros.Thus, once the spacecraft was in orbit and we began observing the changing polhode periods of the gyros andthe misalignment torques, we did not initially suspect that patch effects might be the underlying cause. However,having eliminated other causes through all of the tests and calibrations performed during the mission, we havedetermined that patch effects are, indeed, the underlying cause of our two surprises. Furthermore, throughrecent gyro coating tests performed in our labs here at Stanford, we now know that we overlooked a critical issuein our method of coating the gyro rotors.436 March 2007 Chapter 15 — Preliminary Results