Observations of terrestrial albedo gamma-rays from the COS-B orbit.

It is suggested that a cross check of the sensitivities of different satellite-borne gamma ray detectors might be achieved by using the earth's atmosphere as a calibration source. An intensity of gamma rays above 100 MeV at an altitude of 500 km is computed which is consistent with the observed counting rate of OSO-3. If these results can be extrapolated to greater distances and lower energies, they may be folded with the response of the COS-B experiment (as measured at accelerators) to give an estimate of the counting rate to be expected as a function of distance from the earth and the angle between the experiment axis and the earth-satellite vector. The results of such extrapolations are discussed in terms of their implications for a possible orbital calibration of COS-B and for the observation of sources in directions close to the earth.