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We present a new high-fidelity method of calibrating a cross-track scanning microwave radiometer using Global Positioning System (GPS) radio occultation (GPSRO) measurements. The radiometer and GPSRO receiver periodically observe the same volume of atmosphere near the Earth's limb, and these overlapping measurements are used to calibrate the radiometer. Performance analyses show that absolute calibration accuracy better than 0.25 K is achievable for temperature sounding channels in the 50-60-GHz band for a total-power radiometer using a weakly coupled noise diode for frequent calibration and proximal GPSRO measurements for infrequent (approximately daily) calibration. The method requires GPSRO penetration depth only down to the stratosphere, thus permitting the use of a relatively small GPS antenna. Furthermore, only coarse spacecraft angular knowledge (approximately one degree rms) is required for the technique, as more precise angular knowledge can be retrieved directly from the combined radiometer and GPSRO data, assuming that the radiometer angular sampling is uniform. These features make the technique particularly well suited for implementation on a low-cost CubeSat hosting both radiometer and GPSRO receiver systems on the same spacecraft. We describe a validation platform for this calibration method, the Microwave Radiometer Technology Acceleration (MiRaTA) CubeSat, currently in development for the National Aeronautics and Space Administration (NASA) Earth Science Technology Office. MiRaTA will fly a multiband radiometer and the Compact TEC/Atmosphere GPS Sensor in 2015. [ABSTRACT FROM PUBLISHER]

Ten underflights of the Special Sensor Microwave Imager/Sounder (SSMIS) with Conical Scanning Millimeter-wave Imaging Radiometer (CoSMIR) onboard the NASA ER-2 aircraft were conducted over the coastal region of California between March 2004 and March 2005. The measured brightness temperature (Tb) values from both sensors are collocated and compared at frequencies of 50.3, 52.8, 53.6, 91.655, 150, 183.3 ± 1,183.3 ± 3, and 183.3 ± 6.6 GHz. The more transparent channels at 50.3, 91.655, and 150 GHz are strongly affected by the changes in surface emission and low-level liquid clouds. Thus, the average differences in Tb values (δTb), measured by the two sensors, and their changes from flight to flight are difficult to assess. For the remaining opaque channels, using the CoSMIR measurements as reference, the lowest SSMIS Tb values occur when the SSMIS is completely under the Earth's shadow. As the satellite moves out of the Earth's shadow in the ascending passes, the SSMIS Tb values are found to gradually increase with more exposure to the sun. The magnitudes of these Tb changes are about 4-5 K for the three 183.3-GHz channels and about 2 K for the 52.8- and 53.6-GHz channels. [ABSTRACT FROM AUTHOR]