A Cross-Scan, Along-Track NEDT Requirement for Polar-Orbiting Microwave Sounders Performance Improvement
Numerical weather prediction models and data assimilation systems that directly assimilate polar- orbiting microwave sounder temperature measurements typically assume the errors in the brightness temperatures are uncorrelated white noise. The current on-orbit polar-orbiting microwave sounder design employs shared amplifiers that lead to correlated noise between channels. In addition, spatially-correlated noise within the sensor image is also being seen in brightness temperature measurements, manifested as cross-track (along-scan) striping. Current microwave radiometric sensitivity, or noise-equivalent delta temperature (NEDT), requirements do not fully address or constrain these excess noise characteristics in the sensor measurements. The scanning geometry along with the calibration scheme of the current radiometer colludes with the amplifier noise to introduce cross-track, spatially-correlated noise. While any design change necessary to reduce this excess correlated noise may not survive a cost-benefit assessment, the current design should accommodate a more-complete requirement on noise characteristics of the measured brightness temperature. This paper proposes augmenting the current instrument NEDT requirement with an additional, along-track NEDT requirement. The additional requirement, if met, could lead to improved overall sensor performance impact in weather models through testable criteria in sensor component acquisition. While it is expected that the along-track NEDT requirement would effectively govern the allowable level of low-frequency gain fluctuations of the instrument amplifiers, it is feasible the NEDT requirement could be met by constraining other aspects of the sensor system. An along-track NEDT requirement at the sensor system level is verifiable using data collected with existing sensor test procedures and would not overly constrain hardware implementation. This paper provides the necessary background and quantifies the proposed new requirement.