Sub-degree CMB anisotropy from space .2. In-flight calibration

In the context of the COBRAS/SAMBA mission study(1), we discuss in-flight calibration of extended sky maps of the microwave sky using celestial sources. We simulate the observations in order to assess the accuracy obtainable for absolute and relative calibration of the Low Frequency Instrument (LFI), operating in the 30 - 130 GHz range. Accurate calibration 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 similar to 0.7% (limited by the COBE-DMR uncertainty on Delta T-D) on time-scales of about 10 days at all frequencies and for the entire mission lifetime. Long-term calibration with accuracy < 0.2% can be obtained using the spacecraft orbital velocity. Additional, independent calibration will be provided by the observation of external planets. We also 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.