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

Figure 10-5. SQUID Noise Data - Payload TestTable 10-1. Calculations of true noise for each of the four SQUIDsSQUIDNoise @ 5.55 mHz(μΦ 0 /√Hz) measuredWhite Noise (μΦ 0 /√Hz)measured level at 0.5 HzNoise at 5.55 mHz (μΦ 0 /√Hz)after subtractionof aliased noise1 56 50 302 45 28 373 46 33 344 36 19 31To calculate the true noise at 5.55 mHz the excess (aliased) white noise in the data above must be subtractedfrom the measured noise at 5.55 mHz. The amount of noise to be subtracted from each FFT bin is the noisebetween the Nyquist sampling frequency (0.5 Hz) and the filter passband frequency (4 Hz). This is 7/8 of thepower as measured at 0.5 Hz. After subtraction of this aliased noise all four SQUIDs meet spec.In Figure 10-6 below, external AC magnetic fields applied to the payload show a response in the SQUIDs at theapplied frequency. For reference, the applied field is indicated on the charts in Figure 10-6 at the allowedamplitude at the roll frequency (nominally 5.5 mHz). This amplitude is below the SQUID noise floor at 5.5 mHzso measurements were performed at higher frequencies of 100 mHz and 50 mHz. The SQUID response at thesefrequencies shows the expected attenuation dependence on frequency and is consistent with meeting the ACshielding requirement at the roll frequency.Gravity Probe B — Post Flight Analysis • Final Report March 2007 289