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

Figure 11-8. Guide Star intensity versus timeThe dominant feature in the light curve is the 24.51 day period associated with star spots synchronized by thetidal and elliptical distention wave forced by an unseen binary companion. The stellar rotation and orbitalperiods are tidally locked. Occasionally these systems flare, although during the GP-B mission no flaring wasobserved. RS CVn class stars, of which IM Peg is a member, display photospheric spots that modulate the lightcurve and they are also magnetically active. Doppler Imaging and Zeeman Doppler Imaging are used formapping stellar surfaces of the RS CVn components.The annual linear drift in guide star brightness is less than the 10% requirement that was specified at thebeginning of the mission. Fortunately, we observed the star during a stable period when no flares or suddenchanges in brightness occurred. The comparison waveform is composed of an 8 term Fourier trigonometricseries with coefficients chosen by a least squares method (Matlab). The residual linear variation component ofstellar brightness was -0.31616%/annum.11.4.2 Telescope Pointing SignalThe telescope pointing signals for science are derived from the telescope detector slopes, which are calculatedon-board the space vehicle and telemetered to the ground where they are converted to detector currents+ / −, X / Y , A / Bi, where +/- refers to the +/- detectors of a pair, X/Y to the X or Y-axis, and A/B to the A or B sideX / Y , A / Belectronics. A normalized telescope pointing signal s Nis found using the detector currents and+ / −, X / Y , A / Bweighting factors w, which account for the different efficiencies for each of the optical andelectronic paths for the eight telescope detectors. For simplicity the X/Y and A/B designations are not includedin the expression. The symbol sign is plus for the X-axis, and minus for the Y-axis.Gravity Probe B — Post Flight Analysis • Final Report March 2007 319