Between 25-35% of the Earth's outgoing longwave radiation (OLR) lies in the far-infrared (FIR) spectral region from 0- 500cm<sup>-1</sup> where the emission is primarily due to water vapour located in the upper and mid troposphere. The local maximum in the absorption spectrum of ice means that high, cold cirrus clouds have a large effect on intensity of the OLR here. To date, no FIR measurements of the OLR have been made from space, resulting in a major gap in our understanding of the Earth's radiative energy budget. Such measurements will provide vital information about the spatial and temporal variability of the OLR with relation to upper tropospheric humidity and clouds which will better constrain radiation parameterisations in general circulation models. REFIR (the Radiation Explorer in the Far-Infrared) is a polarising interferometer designed to bridge this knowledge gap by measuring the OLR from 100-1100cm<sup>-1</sup> at a spectral resolution of 0.5cm<sup>-1</sup>. This instrument's performance is critically dependent on the properties (transmittance and reflectance) of the wire grid polarisers it uses as beamsplitters. These properties have been measured at Imperial College and incorporated into a mathematical (Jones' matrix) model of the interferometer's performance to produce simulated interferograms and spectra. When coupled to a model of detectors suitable for the FIR spectral region, potential spectral noise characteristics of the calibrated radiance spectra produced by REFIR have been modelled. So far, cryogenically cooled detector systems are far preferable to ambient temperature detectors, although measurements with un-cooled devices with suitable accuracies are possible with longer integration times. The effects of the changing scene beneath the interferometer during the interferogram acquisition time have been analysed.