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

10.3 Pre-Launch Ground-Based TestsWe performed ground-based readout system noise measurements in both isolated, quiescent conditions as wellas in the full GP-B payload configuration. Several years ago (Lockhart et al., 1994), we achieved a single-sidedSQUID noise of 30 μΦ o /√Hz (equivalent to 120 marcsec/√Hz) at a frequency of 5.5 mHz. The readout noiserequirement of 190 marcsec/√Hz (equivalent to 50 μΦ o /√Hz) was met in the full GP-B configuration duringpayload testing. Improved EMI rejection allowed us to eliminate a lossy SQUID input filter which had beenused in earlier tests. Removing this filter reduced SQUID noise and increased gyroscope coupling efficiency.Both of these effects translate into improved readout noise performance. In addition, the SQUID packages weretemperature controlled to limit thermally induced drift and the use of normal metal near the pickup loop waslimited to minimize Johnson noise.Ground tests confirmed the coupling between the rotor’s London moment and the output of the readoutsystem, thus confirming Equation 10-1. Furthermore, on-orbit environments were simulated to ensure that thereadout system would continue to perform adequately. Specifically, we tested the readout system for magneticand electromagnetic effects, thermal sensitivities, proton induced upsets and nonlinear behaviors.The earth’s magnetic field could couple into the readout system to mimic the science data. There are separateimpacts for AC and DC fields. We demonstrated flux trapping in the gyro rotor of less than 1 μG (uniform dcfield equivalent). This performance met our needs at dc. Some of our earlier tests had shown that theattenuation of ac magnetic fields was limited by leakage into the SQUID package. In tests of an isolated SQUIDpackage, a Pb-Sn gasket placed between the SQUID package and lid reduced this leakage by a factor of 50yielding a residual leakage of approximately 1 mΦ o /G. An end-to-end readout system ac shielding testdemonstrated adequate performance.Figure 10-5 shows a plot of SQUD noise data obtained during a payload test of the SRE system. The data werecollected through the SRE DAS. The sampling rate was 1 Hz but the low pass digital filter was not active. Evenwithout the low pass filter active, SQUIDS 2, 3 and 4 meet spec. SQUID 1 is 20% above spec. Proper filteringbrings all SQUIDS into spec (see the third column of data shown directly below the figure).288 March 2007 Chapter 10 — SQUID Readout Subsystem (SRE) Analysis