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Photon counting is useful in space-based imagers wherever quantitative light- intensity evaluation is necessary. Various types of radiation, from cosmic rays to high-energy proton beams to gamma radiation, have an effect on the functionality and accuracy of imagers and the literature is extensive. Techniques to maximize sensor tolerance have also been developed for a number of years and several imagers resistant to up to 30Mrad (Si) of gamma radiation have been reported. These sensors have several shortcomings: either significant noise performance degradation, up to several orders of magnitude, or unacceptably high pre-radiation noise levels. In addition, many radiation-tolerant sensors use dedicated processes, thus possibly limiting their suitability for mass-market applications. This paper describes a CMOS photon-counting imager designed to detect the Earth's airglow, which is the atmospheric oxygen emission at 762 nm due to oxygen recombination. Airglow occurs day and night and enables geostationary and orbiting satellites to infer their position referred to the Earth's center for attitude determination. The goal is to develop a sensor that reduces the requirements on weight and size of navigational telescope optics mounted on an ultra- low-cost micro-satellite.