Calibration of the PRONAOS/SPM submillimeter photometer

Aims. PRONAOS is a stratospheric balloon-borne project dedicated to submillimeter astronomy. Sensitive to low emission gradients, PRONAOS allowed pointed observations in four photometric bands between 200 mu m and 1.2 mm with an angular resolution of 2' to 3.5'. We present here the PRONAOS calibration strategy and the accuracy achieved. Methods. An absolute calibration scheme based oil an Internal Calibration System provided original photometric data. The accurate determination of the spectral dependence Of the measured fluxes is a key to the understanding of the cold universe. Results. We demonstrate that the PRONAOS absolute photometric accuracy is 8% (1-sigma) and that the channel to channel relative accuracy is 6% (1-sigma). This calibration is checked against observations of Saturn. We show that the PRONAOS calibration is in good agreement with the present knowledge of the Saturn submillimeter emission. PRONAOS observations also agrees with the observations of several compact and extended Sources by the COBE/DIRBE or the ISOPHOT instruments.