The European Space Agency's satellite COS-B was dedicated to gamma-ray astronomy in the energy range 50 MeV to 5 GeV and carried a single experiment, a spark-chamber telescope: developed in collaboration by six European institutes. The experiment became operational on 1975 August 17 and was switched off on 1982 April 25 when on-board resources were exhausted. During this period, 65 observations, typically of a month duration, were performed. The satellite was spin stabilized with the telescope axis along the spin axis. Circular sky regions of about 40 degrees in diameter were covered in each observation. The majority of the pointings were distributed along the galactic equator, 15 observations were devoted to regions at high (20 degree) galactic latitudes. Several regions of specific interest were repeatedly observed.
The highly eccentric polar orbit of COS-B with an apogee around , chosen to maximize useful observation time while allowing real-time data transmission, exposed the experiment to the solar modulated interplanetary cosmic-ray flux. The unexpectedly long operational life of the experiment, specifically of the spark-chamber, was accompanied by a long-term degradation and by short-term disturbances of its performances and consequently of the experimental sensitivity. The variation and sensitivity of the instrumental background were thoroughly investigated and integrated into the database. The possible impact of their statistical and systematic uncertainties must be considered in any type of analysis.