Determination of Angular Separation Between Spacecraft and Quasars with the Very Long Baseline Array

The interferometric technique of phase referencing was used to determine the relative angular positions of the Mars Exploration Rover B (MER-B) spacecraft with respect to angularly nearby quasars. The final cruise state of MER-B was ob- served in three sessions by the Very Long Baseline Array (VLBA) as part of a larger feasibility study to determine the accuracy of this technique. This article summa- rizes the VLBA observations and reductions of the nominal 10-station (45-baseline) observables, the incorporation of the delay data within the Orbit Determination Program, and the comparison of the VLBA and the Deep Space Network (DSN)- based delta differential one-way ranging (∆DOR) results. The pathway from VLBA observations to navigation use of the data is well-defined. The analysis shows that the formal accuracy of the VLBA-determined approximately declination-projected position of the spacecraft is 1.2 nanoradians (nrad) when the correlation among different-station delay observables is ignored. This formal accuracy, deduced from observed residual delay scatter, is about two times smaller than the corresponding result from the DSN ∆DOR observations. While the effect of quasar position and station location errors was included as a priori error in the estimate of formal errors, note that, to highlight the impact of improved measurement precision, the formal accuracy values do not include the larger Mars ephemeris and planetary-to-inertial- frame tie and the effects of possible modeling errors. These contributions, as well as those from tropospheric refraction errors and the assumed 0.7-nrad uncertainty of the particular quasar position, must be reduced in order to obtain more accurate positions in the future.