Multi-band Radiometer for Measuring the Cosmic Microwave Background

High precision measurements of the Cosmic Microwave Background (CMB) is the scientific core of the PLANCK mission of the European Space Agency. The Low Frequency Instrument (LFI), one the two instruments on board, is a multi-band radiometer array operating in the frequency range 27-110 GHz that will measure with unprecedented accuracy the CMB angular power spectrum and will be sensitive to the linear polarisation component. To achieve the required accuracy, the LFI receivers are cooled to 20 K and the measured sky radiation is compared to a reference load at 4 K. This paper presents the LFI system, the expected performance and calibration strategy based on the prototype activity currently on-going.