Despite containing up to half of the Earth's thermal emission to space, the far-infrared spectral region (FIR, defined here as 100–667 cm<sup>-1</sup> or 15–100 µm) has seldom been observed from satellites. This has contributed to substantial uncertainties in the spectroscopy of water vapour, radiative properties of clouds, and surface spectral emissivity; these in turn limit confidence in modelled FIR energy flows. With the advent of the Polar Radiant Energy in the Far-InfraRed Experiment (PREFIRE), a step is taken towards new, systematic, spectral observations of the Earth in the FIR. Launched in 2024, the two PREFIRE CubeSats offer a new perspective of Earth's outgoing longwave energy with moderate spectral resolution. However, the observational uncertainty budget requires further consolidation.</p> <p>In this study we assess the accuracy of PREFIRE spectral measurements through a ‘ground-to-space’ closure experiment. Using zenith-viewing observations from the ground-based Far INfrarEd Spectrometer for Surface Emissivity (FINESSE), we gauge the representativity of atmospheric data from in situ sensors and reanalysis. With these data we simulate PREFIRE-observed radiances for an overflight of our field site in eastern Canada. Simulations of the FINESSE radiances are in very good agreement with observations, while those from PREFIRE indicate some bias beyond the calculated uncertainties. We find that atmospheric water vapour specification, uncertain surface properties, and instrument noise dominate the uncertainties. Based on these findings, we highlight proposed techniques for closure experiments using terrestrial and satellite instruments alike. Such experiments will provide a ground-truth for validation of future FIR satellite missions.