The SAGE III instrument operates by measuring the amount of solar light as it passes through the limb or edge of the Earths atmosphere as the spacecraft views the rising and setting of the sun during each orbit. The instrument measures light at wavelenghts in the part of the electromagnetic spectrum visible to the human eye, allowing it to make very accurate measurements of aerosols, ozone, water vapor and other trace gases. The advanced SAGE III instrument will also make measurements using moonlight that will provide new observations of trace gas species that affect ozone distibution. SAGE III willl also make essential temperature and pressure measurements for helping scientists better understand climate change.
A limb measurement is taken as a satellite instrument views sunlight through the atmosphere as it ascends or descends from behind the Earth.