pycalc11: A Python Interface to the CALC VLBI Delay Model

Accurate and precise delay models are essential to performing VLBI, especially in geodetic and astrometric applications. The CALC11 program has been a standard delay model in the VLBI world since the 1970s, aiming to provide geometric delays that are correct to picosecond precision. The difxcalc11 program provides an interface to CALC11 designed to work with the DiFX correlator, and updated the delay model to include new IERS parameters. Here, we present pycalc11, a Pythonic interface to the CALC11 program, derived from the difxcalc11 code base. In addition to providing a simpler interface that uses well-maintained astropy utilities, pycalc11provides users with direct access to underlying functions, and can lead to much more rapid development. We demonstrate pycalc11's consistency with difxcalc11 and with basic geometric delay calculations.