The stability of the absolute calibration of Level 1 brightness temperatures (TB) is critical to the success of the Aquarius mission. It is necessary in order for the noise present in independent measurements of a particular region of the ocean to be reduced most effectively by averaging. It is necessary in order to reliably detect small but significant trends in global and regional salinity distributions. And it is extremely useful as a means of diagnosing and “bootstrapping” corrections for potential drifts in instrument hardware parameters used by the ground calibration algorithms. A vicarious cold reference method can be used to validate and trend the absolute calibration. This method has been successfully applied to the TOPEX Microwave Radiometer to characterize its long term performance and to correct for small drifts in its on-board calibration hardware [Ruf 2000]. Simulations of measurements to be expected by Aquarius have been performed that include the effects of instrument noise and variable environmental factors such as the global water vapor and ocean surface temperature, salinity and wind distributions. The vicarious cold reference is found to be insensitive to instrument effects and most environmental factors.
In the vicarious cold reference method, an effective TB calibration target is extracted from a large ensemble of Earth viewing measurements by statistical analysis. The analysis consists of determining the lower bound on the TB inverse cumulative distribution function (ICDF) of the ensemble by polynomial extrapolation of the measured ICDF in its low TB range. The lower bound on TB for a downward looking microwave radiometer operating in the atmospheric windows occurs over calm wind oceans in clear and low humidity skies. This combination of conditions produces an extremely stable reference TB against which instrument calibration can be verified with tenths of Kelvin stability. The procedure relies solely on the final, main-beam referenced TBs and so tests the complete end-to-end system calibration, including the TA (receiver) calibration algorithm, the stability of hot and warm TB calibration standards, side lobe and cross-pol antenna pattern corrections, and spacecraft attitude corrections. The technique has been extensively used with the TOPEX Microwave Radiometer at 18-37 GHz at nadir incidence. Its application at oblique angles has been confirmed using SSM/I measurements at 19-37 GHz. Simulations performed at 1.4 GHz indicate that it will be at least as successful for use by Aquarius.
RSG is currently implementing the vicarious cold calibration algorithm on the Aquarius brightness temperature data as part of the calibration/validation stage for the Aquarius radiometer.