Enter SI-traceability. It is fundamental to climate research that any atmospheric temperature measurement purporting to track climate change must (a) have a precision of a few tenths of a deg C or better, (b) have an absolute bias smaller than a few tenths of a deg C for all time, and (c) be directly traceable to an absolute standard that documents its performance. Without such rigorous documentation no remote atmospheric temperature measurement can be relied upon at the required level. The universal standards for measurements are codified in the Système Internationale (SI), commonly known as the metric system. For documenting atmospheric temperature change, sensors must possess the required accuracy and precision buttressed by "SI-traceability."  Radiometers tend to fall short on all three.