SUB-DEGREE CMB ANISOTROPY FROM SPACE .2. IN-FLIGHT CALIBRATION

In the context of the COBRAS/SAMBA mission study(1), we discuss in-fli ght calibration of extended sky maps of the microwave sky using celest ial sources. We simulate the observations in order to assess the accur acy obtainable for absolute and relative calibration of the Low Freque ncy Instrument (LFI), operating in the 30 - 130 GHz range. Accurate ca libration can be achieved using the CMB dipole signal, Delta T-D. With conservative assumptions on the effect of Galactic contamination, we find that the CMB dipole will provide absolute calibration accuracy si milar to 0.7% (limited by the COBE-DMR uncertainty on Delta T-D) on ti me-scales of about 10 days at all frequencies and for the entire missi on lifetime. Long-term calibration with accuracy < 0.2% can be obtaine d using the spacecraft orbital velocity. Additional, independent calib ration will be provided by the observation of external planets. We als o describe the capability of the proposed scanning technique to detect and remove long-term instrumental drifts, and show that these effects , if present, can be controlled and removed with an overall negligible impact on the data uncertainty.