The REFIR (Radiation explorer in the far infrared) project is a study, funded by European Union, of feasibility of a novel space-borne instrument that will measure the atmospheric spectral radiance of the Earth in the broad spectral range 100-1100 cm-1 from space with sufficient spectral resolution (0.5 cm-1) and signal-to-noise ratio (SNR > 100). The main scientific objectives of the REFIR experiment are the measurement of the outgoing FIR radiation at the top of the atmosphere and the improvement of our knowledge of the principal drivers of this flux, e.g. temperature structure, water vapor, and clouds throughout the troposphere-surface system. The REFIR concept consists of a far infrared Fourier transform spectrometer (FTS) as the core instrument, of an embedded imager operating in an infrared 'window', sharing the same bore-sight as FTS, for scene/cloud signature identification in the FIR, of an add-on imager to provide multi-channel imagery, and of an absolute single-pixel radiometer with a single broad-band channel, used to measure the emitted radiation contextually with the spectral measurements. The integration of all the systems leads to a very compact satellite instrumentation, working at room temperature, with an estimated overall mass of 70 kg and a power consumption of 80 W, including electronics. The overall data rate toward the ground station is foreseen to be of 170 kbps before on-board data compression. This work highlights the main technical results at the end of phase-B0 study. The technical solutions adopted for the instrument are outlined and an accurate analysis of performances is shown.