| We describe the use of very-long-baseline interferometry (VLBI) in two experiments designed to measure different relativistic effects of gravitation. In the first experiment we used VLBI to measure the solar gravitational deflection of radio waves. For the second experiment, the NASA-Stanford Gravity Probe B (GP-B) relativity gyroscope experiment, we established a procedure for astrometric VLBI observations of radio-bright stars.; Our measurement of the solar gravitational deflection of radio waves is based on VLBI observations of the extragalactic radio sources 3C273B and 3C279 during a ten-day period surrounding the October 1987 solar occultation of 3C279. Cross-correlation of data recorded at antennas in California and Massachusetts at 2, 8, and 23 GHz yielded group delays correctable for the dispersive effects of the solar corona and ionosphere. We used a water-vapor radiometer and a spectral hygrometer at each site to account for the effects of atmospheric water vapor. We analyzed the group delays with a Kalman-filter estimator that modeled atmospheric and frequency-standard fluctuations as Gauss-Markov processes. We obtained for the parameterized post-Newtonian parameter {dollar}gamma{dollar} a value of 0.9996 {dollar}pm{dollar} 0.0017 (estimated standard error), corresponding to a gravitational deflection 0.9998 {dollar}pm{dollar} 0.0008 times that predicted by general relativity. Our largest sources of error are the uncertainties in the position of the Earth's pole, in the propagation delays through the Earth's atmosphere, and in the fluctuations of the timing signals from each site's frequency standard.; The GP-B experiment is designed to measure the motional and geodetic precessions of gyroscopes in orbit about the Earth. For the measurement-accuracy goals of the experiment to be met, VLBI observations are needed to determine, with standard error {dollar}le{dollar}0.3 mas/yr, the proper motion of the GP-B "guide star," whose position on the sky serves as the reference relative to which the precessional motions are measured. Starting with a set of requirements that the guide star must meet, we narrowed the field of candidates from several thousand stars to three: HR 1099, HR 5110, and HR 8703. Through making VLBI observations of HR 8961 ({dollar}lambda{dollar} And), a former candidate star, we established a procedure for measuring with VLBI the proper motions of these three stars. |
