Atmospheric Phase Correction Based on Sky Emission in the 210–248 GHz Band

Abstract

For millimeter radio astronomy, the instrumental performance is nearly always limited by the amount of fluctuating tropospheric water vapor in the line of sight. Several phase correction schemes for interferometers have been proposed, both for current observatories and the large arrays planned for the next century.We present some examples from the now operational real time phase correction system at the Plateau de Bure Interferometer (PdBI), which employs the stability of its dual frequency SIS receivers to monitor the atmosphere on timescales from one second to two minutes along the line of sight. The gain in coherent integration time was found to be considerable, and the phase noise became effectively independent of baseline length (with an optical path r.m.s. of 90 μm over one minute). However, the system works only during clear sky conditions and doesn’t allow to track the phase over source changes.Studies of the modelled phase show that on occasion (ground wind near 10 m/s), the atmospheric phase screen can move the phase at 230 GHz by 100° on timescales of two seconds (average over one hour, baseline 262 m) for an amount of precipitable water of 3 mm. The PdBI monitoring technique manages to transform this essentially random phase to an r.m.s. of about 25–35 degrees over one minute.KeywordsPhase NoisePrecipitable WaterPhase CorrectionBaseline LengthAtmospheric PhaseThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.