Methods for improving microwave radiometer calibration and data quality for geophysical applications

Research on methods to improve microwave radiometer calibration and data quality is presented. The investigations chosen were those relevant to the Airborne Imaging Microwave Radiometer (AIMR) at the National Center for Atmospheric Research, Atmospheric Technology Division, but are applicable to other passive microwave instruments. Instrument calibration was determined through comparison of output brightness temperatures to a microwave radiative transfer model, through analysis of data obtained from the application of different calibration methods, and through an inter-comparison between AIMR and the Surface Based Radiometer. AIMR data shows good agreement with the model and a comparison between a tipping calibration and calibration from AIMR internal calibration targets yields similar results. The magnitude of the brightness temperature uncertainty for typical scene temperatures (152--200 K) is estimated at 2 K. The uncertainty for colder scene temperatures is higher. Data utility was improved through several investigations and the modification of calibration algorithms to reduce data uncertainty and improve image quality. Coupling of the cold scene temperature into the measurement of the hot calibration load is identified as a contributor to reduced image quality. Gradients in the AIMR calibration targets are investigated through the use of infrared imaging. A gradient of about 1.3 K across the face of the AIMR target is measured. Ground work is laid for future investigations into the infrared properties of passive microwave calibration targets. A calibration sample averaging scheme that is consistent with the stability of the calibration system is proposed. Modifications of AIMR post-processing software to correct the application of the calibration algorithm are tested and result in a reduction in the brightness temperature uncertainty by 0.3 K. The utility of airborne passive microwave sensors is enhanced through the development of a novel geolocation device to aid in precise alignment of sensor imagery with ground locations. This technique has applications for validation efforts, where airborne measurements need to be aligned with in-situ measurements on the Earth's surface, as well as for other field projects.