The Orbiting Carbon Observatory-2 Spectrometer (OCO-3) platform aboard the International Space Station carries a single instrument consisting of three high-resolution grating spectrometers (instruments that measure properties of light within the electromagnetic spectrum). The grating separates incoming sunlight into a spectrum of multiple component colors. The instrument measures the intensity of three relatively small wavelength bands—weak carbon dioxide (CO₂), strong CO₂, and oxygen (O₂). Each wavelength band is specific to one of the three spectrometers. The absorption levels indicates the presence of the different gases.

The OCO-3 instrument measures sunlight reflected off Earth's surface. Sunlight rays entering the instrument pass through the atmosphere twice—once as they travel from the Sun to Earth and then again as they bounce off Earth's surface and return to the OCO-3 instrument. Carbon dioxide and molecular oxygen molecules in the atmosphere absorb light energy at very specific colors or wavelengths.