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The Global Precipitation Measuring Mission requires the ability to compare the calibrations of similar, but not identical, orbiting microwave radiometers. A fitting algorithm has been developed which adjusts a set of geophysical parameters to match the radiances of a source sensor. The adjusted parameters are then used to compute the radiances for a target sensor. For comparison purposes, a simple (weather forecast) analysis-based algorithm has also been implemented. The algorithms have been tested on two pairs of sensors, TMI/Windsat and TMI/AMSR-E. The differences in the results between the two algorithms are generally small. The contribution of various error sources has also been evaluated. The error analysis suggests similar quality for both algorithms. A comparison of the observed variability in the differences between the two sensors in each pair shows very similar variability for the TMI/AMSR-E pair as for the TMI/Windsat pair. It is also shown that the fitting algorithm partially compensates for shortcomings in the radiative transfer models by introducing spurious correlations among the retrieved parameters.