Intercalibration of the GPM Constellation Using the GPM Microwave Imager (GMI)
Abstract:The Global Precipitation Measurement (GPM) mission relies on a constellation of satellites with microwave radiometers to achieve reasonable sampling. The GPM Core satellite, which was launched in March 2014, is intended to tie the constellation of disparate satellite instruments into a coherent whole. A first aspect of the constellation that needs to be made consistent is the calibration of the microwave radiometers. The GPM Intersatellite Calibration Working Group (aka X-CAL) is charged with deriving transformations that put the various constellation radiometers on a common basis with proper allowance for the differences in viewing parameters such as frequency, polarization and view angle. The low inclination (65°) prograde orbit of GPM Core allows frequent observations by the GPM Microwave Imager (GMI) that are coincident in space and nearly so in time with the various constellation radiometers. The comparisons are done via a variety of methods. The similarity of the results allows estimates of the uncertainty and adds credibility.
Early simulations of the cross calibration role of GMI have been done with the TRMM Microwave Imager (TMI), and as a temporary measure, X-CAL has provided transformations to map all the constellation radiometers to the calibration of TMI. As of July 2014, TRMM was nearly out of fuel and the orbit will decay over the next year or so.
As with any new sensor, there are several small problems with GMI that must be addressed before it can be used as a transfer standard. The largest appears to be an interaction with the Earth’s magnetic field that produces errors in the 1 to 2 K neighborhood.
Some of the algorithms derive surface and atmospheric parameters from the observed radiances. These algorithms provide, as a by-product, a measure of the consistency of the radiances with models for the atmospheric absorption and ocean surface emissivity . Comparisons of GMI radiances with TMI, Windsat, MHS and other sensors will be presented.