Development of Wide-Band Airborne Microwave and Millimeter-Wave Radiometers to Provide High-Resolution Wet-Tropospheric Path Delay Measurements for Coastal and Inland Water Altimetry

Current satellite ocean altimeters include nadir-viewing, co-located 18-34 GHz microwave radiometers to measure wet-tropospheric path delay. Due to the area of the surface instantaneous fields of view (IFOV) at these frequencies, the accuracy of wet path retrievals is substantially degraded near coastlines. In fact, Jason-1 and Jason-2 retrievals are flagged as contaminated within 50 km and 25 km of the coasts, respectively. In addition, they do not provide wet path delay estimates over land. A viable approach to meet these needs is the addition of wide-band millimeter-wave window channels at 90-170 GHz with internal calibration, yielding finer spatial resolution for a given antenna size. The addition of millimeter-wave channels near 90, 130 and 166 GHz to current Jason-class radiometers is expected to improve retrievals of wet-tropospheric delay in coastal areas and to enhance the potential for over-land retrievals. The principal objective of this research is to assess the ability of higher-frequency radiometers to meet the needs of the Surface Water and Ocean Topography (SWOT) mission recommended by the U.S. National Research Council's Earth Science Decadal Survey, accelerated in 2010 and planned for launch in 2020. The primary objectives of SWOT are to characterize ocean sub-mesoscale processes on 10-km and larger scales in the global oceans, and to measure the global water storage in inland surface water bodies, including rivers, lakes, reservoirs, and wetlands. Therefore, an important new science objective of SWOT is to transition satellite radar altimetry into the coastal zone, indicating the need for a novel microwave radiometer to provide fine-scale spatial resolution wet-tropospheric path delay corrections near land. In addition, the Ka-band SWOT radar interferometer will for the first time broaden the altimeter field of view and improve spatial resolution to make coastal and inland surface water measurements, so the variability of atmospheric water vapor across the swath will affect altimeter accuracy. To reduce the risks associated with wet-tropospheric path delay correction over coastal areas and fresh water bodies, we will develop, build and flight test an airborne radiometer with the Advanced Microwave Radiometer (18.7, 23.8 and 34.0 GHz) channels, millimeter-wave (90, 130 and 166 GHz) window channels, and millimeter-wave sounders near 118 and 183 GHz to validate over-land retrievals of wet-path delay. In addition, a high spectral resolution ASIC is under development to substantially reduce the mass and power of millimeter-wave spectrometers. The millimeter-wave radiometer channels will have substantially improved spatial resolution and the potential for multiple fields of view across the radar's swath. This instrument development and airborne flight demonstration will (1) assess wet-tropospheric path delay variability on 10-km and smaller spatial scales, (2) demonstrate millimeter-wave radiometry using both window and sounding channels to improve both coastal and over-land retrievals of wet-tropospheric path delay, and (3) provide an instrument for calibration and validation in support of the SWOT mission.