Use of Allan Deviation for Characterizing Satellite Microwave Sounder Noise Equivalent Differential Temperature (NEDT)

Currently, the instrument sensitivity of sensors onboard weather satellites is quantified by computing the standard deviation of the measurements taken from their calibration targets. The standard deviation is valid for describing the spread of a statistical distribution of the measured values around its mean that is stable. However, the actual measurements of a calibration target can exhibit considerable variations in time as shown from the Suomi National Polar-orbiting Partnership Advanced Technology Microwave Sounder (ATMS) blackbody data. In this letter, the Allan deviation is proposed as an alternative to the standard deviation for characterizing the instrument sensitivity. It is found that, in the overlapping Allan deviation formula, the averaging window size has to be set to one in order to accurately assess the noise magnitudes for both stationary and nonstationary time series. Furthermore, from the ATMS on-orbit data, the estimates of the noise magnitudes at several channels show a large discrepancy between the Allan deviation and the standard deviation. Finally applying the Allan deviation, the sensitivity of the NOAA-18 Advanced Microwave Sounding Unit-A is also derived and compared against the traditional algorithm results. From this comparison, significant improvements can be seen in the Allan deviation-based noise-equivalent-differential-temperature estimation.