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

voltage vs. flux characteristics (less than 1% deviation from a straight line over the expected operating range of0.02 Φ o ) and by requiring and achieving sufficiently high open-loop gain of the electronics (greater than 100 at1 kHz).Tests were also performed to evaluate the readout system’s performance in the presence of EMI. We divided theEMI into 2 broad frequency ranges: below and above 1 GHz. EMI below 1 GHz was coupled via pickup oncables entering the Probe. We were able to reduce readout EMI sensitivity for these frequencies byimplementing a variety of cable upgrades including 2 layers of electrostatic shielding, solid cable shields, andimproved cable filters. Signals above 1 GHz were found to pass through the telescope window reaching theProbe interior. Shielding the window significantly reduced high frequency EMI related effects.On orbit the GP-B spacecraft made repeated passes through the South Atlantic Anomaly (SAA). While in theSAA, as many as 10 4 protons/cm 2 /s with energies in excess of 10 MeV passed through the spacecraft. Duringcomponent level ground testing we bombarded SQUIDs (Muhlfelder et al., 1995) with high energy protons tosimulate this environment. The flux-locked loop electronics showed no observable sensitivity to the protonbombardment. Some of the SQUIDs that we tested did show step-like bias shifts. For the SAA situationdescribed above, these shifts corresponded to a few mΦ o every 5 minutes. We analyzed the impact of these biasshifts upon the determination of the relativistic drift rates and found that the predicted bias shifts should notcause significant degradation. We also subjected the SQUIDs and electronics to approximately 10 times the totalproton dose we expect for the GP-B mission. Neither was damaged by this exposure.The as-built GP-B readout system performed to its design specifications during ground testing. Its noise at 5.5mHz was < 0.2 arcsec/√Hz. The SQUID temperature coefficient was < 10 mΦ o /K, and the SQUID temperaturewas stabilized to 2 μK at the spacecraft’s roll frequency. The temperature coefficient of the electronics was < 20μΦ o /K and the temperature of the electronics was controlled to 1 mK at roll frequency. The system hadundergone thermal, proton, magnetic and EMI testing. Improvements in the SQUID’s input filter andimprovements in the shielding of the telescope window allowed us to meet the EMI requirement. All of theperformance and environmental tests showed that the GP-B readout system met the flight requirements. Onorbitdata indicate that the flight performance is similar.10.4 On-Orbit Performance DataThis section summarizes on-orbit performance of the SRE system in terms of SQUID noise, SQUID brackettemperature control, and SRE temperature control.10.4.1 SQUID NoiseFigure 10-10 below shows that SQUID noise data collected shortly after launch and initial system set-up areconsistent with pre-launch results. The gyroscopes were electrostatically caged in their housings in order tominimize magnetic flux motion in the input circuit and make a measurement of only the SQUID sensor noise.The green horizontal line indicates the noise requirement at the roll frequency; the red line is a least squares fitto a power law tail. All SQUIDs meet the noise requirement at the roll frequency. The spectra for SQUIDs 2 and4 also confirm that the ac magnetic shielding meets the requirement at the roll frequency.Gravity Probe B — Post Flight Analysis • Final Report March 2007 293