Availability of Calibration Sources for Measuring Spacecraft Angular Position with Sub-Nanoradian Accuracy

Precision measurements are now capable of determining the angular position of spacecrafts with accuracies of 2-5 nanoradians. To achieve this level of precision, compact radio sources with flux density of at least a few hundred milli-Jansky (at 8.4 GHz) are used for calibration purposes. Further improvements in position measurement accuracy may be possible with use of appropriate calibrators near the direction of the spacecrafts even if the calibrators are much weaker (a few milli-Jansky) in flux density. In this paper we discuss the calibrator flux density required to achieve sub-nanoradian astrometric accuracy and attempt to estimate the density of suitable calibrators, using existing source count surveys. We point out, however, that the fraction of these sources that are suitable for use as calibrators is not well understood and requires further study.