VERY LONG BASELINE ARRAY ASTROMETRIC OBSERVATIONS OF MARS ORBITERS

This paper presents astrometric observations of Mars that are reduced from Very Long Baseline Array (VLBA) measurements of Mars-orbiting satellites. These observations provide angular positions for Mars in the International Celestial Reference Frame (ICRF). Nine observing epochs were used: eight from 2008 and one from 2013. For each epoch, observed R.A. and decl. are provided with associated uncertainties. The post-fit rms residuals of these measurements against JPL's DE430 ephemeris are 0.13 mas and 0.18 mas for R. A. and decl., respectively, with average uncertainty of 0.24 mas in R. A. and 0.32 mas in decl. The results are generally in good agreement with single-baseline Very Long Baseline Interferometry and range measurements of Mars-orbiting satellites. The VLBA measurements of Mars are used to determine the orientation of the dynamical system of Earth and Mars relative to the ICRF with uncertainty of 0.23 mas.