Compact radio source density and precision spacecraft tracking

Compact radio sources are routinely used as reference calibrators in VLBI observations of radio sources to accurately determine the latter´s position. Similar techniques have been developed over the years to determine the position of spacecrafts with remarkable accuracy at the level of a few nano-radians. We have carried out measurements at the DSN (Deep Space Network) that show great promise to further improve the accuracy below one nano-radian by removing many sources of calibration errors. This level of improvement is achieved by taking advantage of calibrators that are within one degree of the radio source. However, such improvement can be counted on only if there are sufficient numbers of compact radio sources near the position of the spacecraft. Over the last two years, we have carried out small surveys of the sky with the VLA and subsequently with the VLBA to empirically determine the compact radio source density at 8 GHz for sources with flux density above 1 mJy. We will report the results of the latest observations and their implication for precision spacecraft astrometry.