MSU

Also: AMSU,

Microwave Sounding Unit

An instrument used to measure the estimated temperature of a vertical layer of the atmosphere from a satellite above. These instruments have been included in Weather satellites since the 1970's and data from them is also  used in climate science to estimate long term changes in air temperatures at different altitudes.

They work by measuring the strength of the signal of microwaves radiated by Oxgen molecules in the atmosphere. This was chosen as the way of measuring atmospheric temperatures because Oxygen concentrations in the atmosphere are very stable, and other gases in the atmosphere are largely transparent to these microwaves. So any changes in the signal strength are due to temperature changes.

Using multiple sensors working at very close wavelengths these instruments can sample multiple layers of the atmospphere.

In later generations of satellite, including all satellites currently in orbit, Advanced Microwave Sounding Units (AMSU) are used. These work on exactly the same principle but are able to measure a finer range of wavelengths, and thus 'divide' the atmosphere up into more layers.

See here for more details:

Definition based on (revised from) IPCC AR4.