Title: Development of Low-Mass, Low-Power, High-Frequency Microwave Radiometers with Internal Calibration to Provide High-Resolution Wet-Tropospheric Path Delay Measurements for the SWOT Mission
Author: Steven Reising
Organization: Colorado State University
Co-Authors: Darrin Albers, Alexander Lee, Pekka Kangaslahti, Shannon T. Brown, Douglas E. Dawson, Todd C. Gaier, Oliver Montes, Daniel J. Hoppe, Behrouz Khayatian, Alan B. Tanner, Sharmila Padmanabhan

Critical microwave component and receiver technologies are under development to reduce the risk, cost, volume, mass, and development time for a high-frequency microwave radiometer needed to enable wet-tropospheric correction in the coastal zone and over land as part of the Surface Water and Ocean Topography (SWOT) Mission. Since June 2010, SWOT has been one of two missions selected for acceleration from Tier 2 of the National Research Councilís Earth Science Decadal Survey. Current satellite ocean altimeters include nadir-viewing, co-located 18-37 GHz multi-channel microwave radiometers to measure wet-tropospheric path delay. However, due to the area of the instantaneous fields of view on the surface at these frequencies, the accuracy of wet path retrievals begins to degrade at approximately 40 km from the coasts. Addition of higher-frequency microwave channels to the Jason-class radiometers on the SWOT mission will improve retrievals in coastal regions. New high-sensitivity, wide-bandwidth millimeter-wave radiometers using both window and sounding channels show good potential for over-land retrievals. In addition, the 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.

The nadir viewing geometry and lack of moving parts require the radiometers to be internally calibrated. In addition, a single, integrated feed horn and triplexer are needed to minimize the mass and volume of the high-frequency radiometer channels to meet the needs of the SWOT mission. During the first two years of this Advanced Component Technology project, we have developed and fabricated MMIC PIN-diode switches for integrated internal calibration (discussed in a related ESTF 2011 paper by Montes et al.), high-Excess Noise Ratio (ENR) noise sources and a single, tri-frequency feed horn. These new components are currently being tested and integrated into a MMIC-based low-mass, low-power, small-volume radiometer with channels at 92, 130 and 166 GHz, optimum mm-wave window channels for wet-path delay retrievals in coastal regions. This radiometer will serve as a breadboard demonstration by providing realistic mass, volume and power estimates to feed into SWOT mission trade studies.

In a new project under the 2010 Instrument Incubator Program, we will develop, build and flight test an internally-calibrated High-Frequency Airborne Microwave Radiometer (HFAMR) to reduce the risks associated with wet-tropospheric path delay correction over coastal areas and fresh water bodies. This instrument is designed to (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.