The Moon as a Tool for the Calibration of HIRS

Observations of the Earth’s atmosphere with sensors on a polar orbit employ a two-point calibration using a cold and a hot reference point. The hot reference point is an on-board blackbody target. The cold reference point is the deep-space view. In case of the moon being in the direction of the deep-space view, it provides an additional calibration target.

The infrared-relevant surface properties of the moon are well known. This allows to calculate the temperatures of the sun-illuminated parts for the different helio-centric distances (0.981 ... 1.019 au). The integration over the satellite-centric visible parts of the surface gives the total lunar flux at each HIRS observing epoch. The HIRS instruments have seen the moon under very different phase angles (typically between half moon and full moon) which makes it possible to monitor their performance over a very large range of flux values.

Analysis of calibration scans of High-resolution Infrared Radiation Sounder (HIRS) on various satellites, which are “contaminated” by the moon, helps to characterize and intercompare the performance of the sensor. We investigated both geometric and radiometric aspects. Among the former we focused on the instantaneous field of view of the various channels, which is sometimes misrepresented in documents and web pages, and compared the values from ground tests to those obtained in flight. We found that the field of view varies slightly with wavelength, and as well between short-wave and long-wave channels. Then we measured the disk-integrated flux at a variety of phase angles to provide observational constraints on a radiative model of the moon (Müller et al. 2020). Such a model is needed to compare directly the flux calibration of future HIRS-like sensors to those that were operational decades ago.

The HIRS instrument has been mounted on various satellites since 1975. By establishing the Moon as an absolute flux standard, it will become possible to clear climate data records from artificial, non-climatic effects that are common to all instruments of a certain type and that can therefore not be identified by postlaunch matchups. The long time series of HIRS is very valuable for climate research, in particular the search for trends in tropospheric humidity.