Predicting residual levels of atmospheric sky noise in ground-based observations of the cosmic background radiation

Abstract

Observations of anisotropies in the cosmic background radiation (CBR) require instruments with angular resolutions of tens of arcminutes to several degrees. At centimetre wavelengths, interferometers using short baselines to achieve low angular resolution offer substantial reductions in ground spillover and atmospheric emission, when compared to beam-switched experiments at the same frequency. However, there are few experimental data available concerning the effects of low-frequency fluctuations in antenna temperature caused by water vapour drifting through the beam. This paper predicts likely levels of fluctuations for both interferometers and beam-switched experiments at centimetre wavelengths. The effects of wind-speed and the turbulent outer scale size on the expected temporal power spectrum of fluctuations are also explored, and comparisons with available experimental data are presented. It is shown that, in good observing conditions and with a wind-speed of 2ms^-1 or more, residual fluctuations in antenna temperature arising from atmospheric emission are well below the expected receiver noise of a short-baseline 15-GHz interferometer. Consequently, such an instrument can be operated at sea level in good conditions without loss of sensitivity.