Inter-satellite radiometer calibrations between WindSat, TMI and AMSR

NASA´s Global Precipitation Mission will rely on a constellation of cooperative satellites with a variety of microwave radiometers to make global rainfall measurements. Thus, it is crucial to achieve brightness temperature (T_b) measurement consistency among the constellation as well as to maintain sustained calibration accuracy over the lifetime of each satellite sensor. In-orbit inter-satellite radiometric calibration techniques provide a long term, group-wise solution; however, since radiometers operate at different frequencies and viewing angles,T_b normalizations are made before making intermediate comparisons of their near-simultaneous measurements. In this paper, we investigate two approaches to perform these normalizations from one satellite´s measurements to another. One approach is uses Taylor´s series expansion around source frequency to predict T_b´s of destination frequency. The relationship between T_b´s and frequencies are derived from simulations using an oceanic radiative transfer model (RTM) over a wide variety of environmental conditions. The other approach uses a multi-variable linear regression built from RTMT_b simulations among all frequency and V- and H-polarizations. These two approaches are applied in comparing external measurements between WindSat (on Coriolis),TMI (on TRMM) and AMSR (on ADEOS-II). Accuracy prediction and error source analysis are discussed along with calibration results.