Since 1979, NOAA satellites have been carrying instruments which measure the natural microwave thermal emissions from oxygen in the atmosphere. The intensity of the signals these microwave radiometers measure at different microwave frequencies is directly proportional to the temperature of different, deep layers of the atmosphere.
The temperatures in the four layers presented here were once calculated using a hodgepodge of code that had been written by different scientists over years. Eventually a new version (6.0) was written by a single scientist resulting in 9000 lines of FORTRAN code. In 2015 Roy W. Spencer et al. of the University of Alabama in Huntsville used this to reanalyse the available raw atmospheric data since 1978. He writes:
Version 6 of the UAH MSU/AMSU global satellite temperature dataset is by far the most extensive revision of the procedures and computer code we have ever produced in over 25 years of global temperature monitoring. The two most significant changes from an end-user perspective are (1) a decrease in the global-average lower tropospheric (LT) temperature trend from +0.140 C/decade to +0.114 C/decade (Dec. 1978 through Mar. 2015); and (2) the geographic distribution of the LT trends, including higher spatial resolution.Roy W. Spencer et al. 2015