Systematic observations of anomalous refraction at millimeter wavelengths

It is well known that the water vapor in the troposphere plays a fundamental role in radio propagation. The refractivity of water vapor is about 20 times greater in the radio range than in near-infrared or optical regimes. As a consequence, phase fluctuations at frequencies higher than about 1 GHz are predominantly caused by fluctuations in the distribution of water vapor, and thus radio seeing at these frequencies is predominantly caused by tropospheric turbulence. Radio seeing shows up on filled-aperture telescopes as an anomalous refraction (AR), i.e. an apparent displacement of a radio source from its nominal position, corrected for large-scale refractive effects. The magnitude of this effect, as a fraction of the beam width, is bigger on larger telescopes and thus its impact on the pointing is likely to become critically important in the next generation of electrically large filled-aperture radio telescopes ($D/\lambda > 10^4$) and in particular on the Large Millimeter Telescope. AR effects are expected to reduce the total effective observing time at the highest frequencies and will affect on-the-fly mapping. Here we present the results of systematic AR measurements carried out with the 13.7-m telescope of the Five College Radio Astronomy Observatory. The measured AR pointing errors range from $1''{-}3''$ (winter) to about $20''$ (summer) and most of the events last less than about 4 s. We analysed the structure function, power spectrum and Allan variance of the data and we have carried out a statistical analysis to identify correlations of the statistical functions with selected observing parameters such as precipitable water vapor, time of day, season and elevation angle. Our results suggest that uncompensated AR may be the most important dynamic environmental source of pointing errors on the new large radio telescopes (ALMA, GBT, LMT, SRT) and may guide the design of active AR-compensation devices and help allocating suitable observing time through dynamic scheduling