Design and performance of an ambient calibration target for an airborne microwave and millimeter-wave radiometer

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 90–180 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 90–180 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.