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Integration, characterization, and calibration of the high-frequency airborne microwave and millimeter-wave radiometer (HAMMR) instrument


Current satellite ocean altimeters include nadir-viewing, co-located 18-34 GHz microwave radiometers to measure wet-tropospheric path delay. Due to the large antenna footprint sizes at these frequencies, the accuracy of wet path retrievals is substantially degraded within 40 km of coastlines, and retrievals are not provided over land. A viable approach to improve their capability is to add wide-band millimeter-wave window channels in the 90-183 GHz band, thereby achieving finer spatial resolution for a fixed antenna size. In this context, the upcoming Surface Water and Ocean Topography (SWOT) mission is in formulation and planned for launch in late 2020 to improve satellite altimetry to meet the science needs of both oceanography and hydrology and to transition satellite altimetry from the open ocean into the coastal zone and over inland water. To address wet-path delay in these regions, the addition of 90-183 GHz millimeter-wave window-channel radiometers to current Jason-class 18-34 GHz radiometers, is expected to improve retrievals of wet-tropospheric delay in coastal areas and to enhance the potential for over-land retrievals. To this end, an internally-calibrated, wide-band, cross-track scanning airborne microwave and millimeter-wave radiometer is being developed in collaboration between Colorado State University (CSU) and Caltech/NASA's Jet Propulsion Laboratory (JPL). This airborne radiometer includes microwave channels at 18.7, 23.8, and 34.0 GHz at both H and V polarizations; millimeter-wave window channels at 90, 130, 168 GHz; and temperature and water vapor sounding channels adjacent to the 118 and 183 GHz absorption lines, respectively. Since this instrument is demonstrating this technology for the potential use in future Earth science missions, substantial effort has been put into ensuring the instrument has a minimal mass and volume and is robust and well characterized. To this end the optical alignment has been extensively tested and characterized and a novel blackbody calibration target has been designed and integrated into the system. All supporting sub-systems such as power distribution and data acquisition have been integrated into the chassis allowing the instrument to be easily run by a single operator. Preliminary test flights have been done that demonstrate the reliability and robustness of this instrument as well as demonstrating the increased special resolution of the millimeter-wave window and sounding channels over that of the Jason-class 18-34 GHz radiometers.


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