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. Every month, John Christy and I update global temperature datasets (see here that represent the piecing together of the temperature data from a total of fourteen instruments flying on different satellites over the years.
As of June 2013, the Advanced Microwave Sounding Unit (AMSU-A) flying on NASA’s Aqua satellite has been removed from the processing due to spurious warming and replaced by the average of the NOAA-15, NOAA-18, NOAA-19, and Metop-A AMSUs. The graph above represents the latest update; updates are usually made within the first week of every month. Contrary to some reports, the satellite measurements are not calibrated in any way with the global surface-based thermometer records of temperature. They instead use their own on-board precision redundant platinum resistance thermometers (PRTs) calibrated to a laboratory reference standard before launch.