| Abstract: |
The Global Precipitation Measurement (GPM) Mission incorporates observations from a diverse constellation of research and operational microwave radiometers built and launched by various space agencies. Calibration differences on the order of several Kelvin between “well-calibrated” radiometers must be accounted for in order to achieve the GPM goal of producing unified precipitation retrievals from the various constellation members. This involves identifying calibration issues, developing corrections, and applying adjustments for residual calibration differences in order to ensure that the observed brightness temperatures are physically consistent after accounting for differences in channel frequencies, polarizations, and view angles etc. Subsequent efforts to extend the data record back in time to include previous generations of spaceborne radiometers have benefitted from improved intercalibration techniques and knowledge gained from the GMI calibration. Dealing with older, no longer operational instruments with limited documentation and sensor characterization, however, presents additional challenges including linking the GMI calibration back over several radiometer generations. Finally, new technology being developed for upcoming CubeSat missions seeks to achieve the requirements of current operational sensors with constellations of small, low-cost instruments. This involves additional issues relating to manufacturing tolerances for miniaturized components as well as the low-cost nature of these instruments.
This talk will discuss many of the issues that can impact the relative calibration between sensors, even for constellations of “identical” radiometers. This includes differences in the instrument spectral response and antenna patterns, pointing errors resulting from slight offsets in the instrument mounting angles, and imperfections in reflector manufacture/coatings resulting in slightly emissive surfaces to name just a few. For applications using data from radiometers with different designs and/or heritage there are many other issues to consider. An overview of the approaches used by the GPM intercalibration working group, or XCAL team, to intercalibrate radiometers in the GPM and TRMM constellations will be discussed along with several of the calibration corrections that were developed and associated lessons learned in terms of instrument design and on-orbit calibration characterization. Considerations for future instruments, including CubeSat/SmallSat missions will also be discussed.
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