The effects of simulated and observed quasar structure on the VLBI reference frame

Radio-loud quasars making up the Celestial Reference Frame are dynamic objects with significant structure that changes on timescales of months and years. This is a problem for geodetic VLBI, which has so far largely treated quasars as point sources in analysis. We quantify the effects of various levels of source structure on the terrestrial (TRF) and celestial (CRF) reference frames using the source structure simulator recently implemented in the Vienna VLBI Software (VieVS) package.We find that source structure affects station positions at the level of 0.2–1mm. While quasar structure contributes only ~10% to the total TRF error budget, which is dominated by tropospheric turbulence; the effect of quasar structure on the CRF is discernible even in present-day observations. Astrophysical properties of quasars are related to their structure and geodetic stability, and we discuss several quasar structure mitigation strategies. These include: (1) astrophysically-based quasar selection techniques; (2) scheduling sources by taking into account source structure; and (3) analyzing geodetic observations using knowledge of source structure. We find that for observed highly variable quasars, flux density is strongly anti-correlated with structure and position stability, suggesting that such quasars should preferentially be observed in their bright phase. We use simulations to investigate new scheduling strategies which avoid unfavourable jet—baseline orientations. Improvement is seen at the millimetre level on the longest baselines when our new scheduling strategy is used in simulations that only include quasar structure. This improvement disappears in the full simulations including the troposphere, because we are compromising sky coverage in order to mitigate source structure effects. This again confirms that, at present, tropospheric turbulence dominates the accuracy of TRF determination. However, the contribution of quasar structure will become more important as tropospheric effects decrease in future broadband observations.