Earth limb calibration of scanning spaceborne microwave radiometers

We introduce a new technique for absolute “through-theantenna” calibration of cross-track-scanning passive microwave radiometers viewing earth from a low-earth orbit. This method offers significant advantages, in that neither internal calibration targets nor noise diodes are needed to calibrate the radiometer. The algorithm does require periodic updates of the atmospheric state, which can be readily provided by GPS radio occultation observations, for example. An iterative algorithm retrieves the radiometer gain given a sequence of observations of the earth´s limb. The algorithm uses a parameterized radiative transfer model of a spherically-stratified atmosphere. The algorithm works best for opaque temperature sounding channels. This method, when used on idealized radiometer measurements (impulse response functions in frequency and space), yields calibration accuracies similar to those that could be obtained with ideal internal calibration targets. This analysis is based on global Monte Carlo simulations using the NOAA88b profile set. An analysis will also be presented showing how calibration performance degrades as the radiometer characteristics deviate from the ideal case. Among the factors considered are: 1) antenna pattern, 2) spectral passband, 3) pointing errors, 4) atmospheric state variability, 5) the number of limb observations required, and 6) sensitivity to sensor noise.