The Earth Observing System (EOS) Microwave Limb Sounder (MLS) is one of four instruments on the NASA's EOS Aura satellite, launched on July 15th 2004.

MLS makes measurements of atmospheric composition, temperature, humidity and cloud ice that are needed to (1) track stability of the stratospheric ozone layer, (2) help improve predictions of climate change and variability, and (3) help improve understanding of global air quality. MLS observes thermal microwave emission from Earth's 'limb' (the edge of the atmosphere edge) viewing forward along the Aura spacecraft flight direction, scanning its view from the ground to ~90 km every ~25 seconds.

Aura is in a near-polar 705 km altitude orbit. As Earth rotates underneath it, the Aura orbit stays fixed relative to the sun; to give daily global coverage with ~13 orbits per day. Aura is part of NASA's A-train group of Earth observing satellites. These satellites fly in formation with the different satellites making measurements within a short time of each other.

The MLS measurements are made globally day and night. A feature of the MLS technique is that its measurements can be obtained in the presence of ice clouds and aerosol that prevent measurements by shorter-wavelength infrared, visible and ultraviolet techniques.