They detect microwave emissions from oxygen molecules that vary in frequency with temperature. (bishophill.squarespace.com)
All these sensors make measurements at critical frequencies at and above 85 gigahertz (GHz); sensors measure microwave emissions at 183 GHz, the signature frequency band emitted by water vapor, making it feasible to detect frozen hydrometeors (snow, ice, and the like) in the atmosphere. (eos.org)
Ocean surface temperatures are determined from ships and bouys, tied into satellite infrared measurements of ocean surface skin temperature, while MSU measurements use microwave emissions from the atmosphere, so that these measurements can be considered to be essentially independent. (climateaudit.org)