TDRSS OVLBI experiment - Data correlation and results

The use of an orbiting antenna in a VLBI experiment complicates the data correlation, due to the spacecraft motion and the presence of the satellite-to-ground link. For this experiment, using a TDRSS satellite, a multistep method was used to calculate delay and phase for correlation and fringe-fitting. Data from three sources: 1510-089, NRAO 530 (1730-130), and 174l-038 were successfully correlated. The maximum projected baseline achieved was 1.4 earth diameters, for NRAO 530, with baselines longer than one earth diameter obtained for all three sources. Fringe visibilities for the maximum projected baselines were: 0.15 (1510-089), 0.05 (NRAO 530), and 0.50 (1741-038). The coherence of baselines to the orbiting antenna was 84 percent for 700 seconds. For a dedicated orbiting VLBI observatory (e.g., QUASAT), excellent coherence can be expected for integration times of several hundred seconds and observing frequencies as high as 20-30 GHz.