GOME sees these small traces by differential absorption spectroscopy. Decoding the words reveals the simple idea behind the forbidding name. Spectroscopy measures the intensities of light at different wavelengths. Seen from space, the Earth's surface shines by reflected sunlight. Four gratings then decompose segments for a second time, and silicon light- detectors register 1024 different wavelengths in each segment.
Absorption means the interception of some of the earthshine by atmospheric molecules.
The wavelengths of missing earthshine identify the molecules responsible, while the extent of the absorption measures their abundance. Ozone has bands of strong absorption in three of the four segments of GOME's spectrum. Differential refers to the fact that trace gases appear as small additions, or blips, on the large absorptions by the atmosphere as a whole. GOME spots them by the differences in the absorption between closely adjacent wavelengths. To ensure its accuracy, GOME also compares the earthshine spectrum with light coming directly from the Sun to the satellite in space.