During 1981 - 82, the European Space Agency oversaw an intensive design study for a satellite to observe the submillimetre spectrum and the millimetre anisotropy of the cosmic microwave background. Technically the spectral instrument was based on the use of ESA 's space technology cryostat, CRHESUS, capable of sustaining a cryogenic experiment for a lifetime compatible with precision measurements of the background spectrum. Technically the anisotropy experiment is based on relatively conventional millimetre wave instrumentation. The design goal of the spectral instrument is to measure absolute flux from a 3 K source with an uncertainty in the millikelvin range, between 0.3 mm and 3 mm wavelength, at a spectral resolution of at least 20. The design goal of the anisotropy instrument is to detect large-scale anisotropies, on all angular scales greater than 2°, with a relative sensitivity of 3 parts in 106. In addition, the cyrogenic instrument will be able to map the low-temperature dust over large reaches of the Galaxy. A comprehensive description of the project is given in this report.