A harmonised satellite series is one where all the calibrations of the sensors have been made consistent with (a) reference dataset(s) which can be traced back to known reference sources, in an ideal case back to SI. Each sensor is calibrated to the reference in a way that maintains the characteristics of that individual sensor such that the calibration radiances represent the unique nature of each sensor. This means that two sensors which have been harmonised may see different signals when looking at the same location at the same time where the difference is related to known differences in the responses of each sensor such as differences in the sensors spectral response functions. Harmonisation can be achieved to within an uncertainty that should be estimated, and the uncertainty contributes to the component of uncertainty that is common across the whole record of a given sensor.