We review the radio very long baseline interferometry (VLBI) observations of the guide star, IM Peg, and three compact extragalactic reference sources, made in support of the NASA/Stanford gyroscope relativity mission, Gravity Probe B (GP-B). The main goal of the observations was the determination of the proper motion of IM Peg relative to the distant Universe. VLBI observations made between 1997 and 2005 yield a proper motion of IM Peg of -20.83 ± 0.09 mas yr-1 in α and -27.27 ± 0.09 mas yr-1 in δ in a celestial reference frame of extragalactic radio galaxies and quasars virtually identical to the International Celestial Reference Frame 2 (ICRF2). They also yield a parallax for IM Peg of 10.37 ± 0.07 mas, corresponding to a distance of 96.4 ± 0.7 pc. The uncertainties are standard errors with statistical and estimated systematic contributions added in quadrature. These results met the pre-launch requirements of the GP-B mission to not discernibly degrade the estimates of the geodetic and frame-dragging effects.
