The experiment is designed to measure O2concentration from 150 km upwards with wide geographical coverage. The attenuation of solar ultra-violet light of 1425 Å to 1490 Å is observed as the satellite enters and leaves the Earth's shadow. The ultra-violet sensors used in the experiment are four gas-filled ionization chambers each of which produces about 10"11 amperes in full sunlight above the molecular oxygen layer. These four chambers all feed into a single electrometer valve amplifier whose circuit and performance are outlined. The constraints on the layout and mechanical structure of the experiment are discussed and the final experiment package is described. An outline is given of the performance of the equipment in orbit.