Radio source calibration for the VSA and other CMB instruments at around 30 GHz

Accurate calibration of data is essential for the current generation of CMB experiments. Using data from the Very Small Array (VSA), we describe procedures which will lead to an accuracy of 1 percent or better for experiments such as the VSA and CBI. Particular attention is paid to the stability of the receiver systems, the quality of the site and frequent observations of reference sources. At 30 GHz the careful correction for atmospheric emission and absorption is shown to be essential for achieving 1 percent precision. The sources for which a 1 percent relative flux density calibration was achieved included Cas A, Cyg A, Tau A and NGC7027 and the planets Venus, Jupiter and Saturn. A flux density, or brightness temperature in the case of the planets, was derived at 33 GHz relative to Jupiter which was adopted as the fundamental calibrator. A spectral index at ~30 GHz is given for each. Cas A,Tau A, NGC7027 and Venus were examined for variability. Cas A was found to be decreasing at $0.394 pm 0.019$ percent per year over the period March 2001 to August 2004. In the same period Tau A was decreasing at $0.22pm 0.07$ percent per year. A survey of the published data showed that the planetary nebula NGC7027 decreased at $0.16pm 0.04$ percent per year over the period 1967 to 2003. Venus showed an insignificant ($1.5 pm 1.3$ percent) variation with Venusian illumination. The integrated polarization of Tau A at 33 GHz was found to be $7.8pm 0.6$ percent at pa $ = 148^circ pm 3^circ$.}