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 , 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 \pm 0.09 mas yr ^ { -1 } in \alpha and -27.27 \pm 0.09 mas yr ^ { -1 } in \delta 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 \pm 0.07 mas , corresponding to a distance of 96.4 \pm 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 .