Existing sea-surface altimeter missions rely on nadir-viewing, co-located 18-37 GHz microwave radiometers to correct wet-tropospheric path delay errors over oceans. However, realizable space-borne antennas have inherently large surface footprints at these frequencies, leading to large wet path retrieval errors closer than approximately 40 km from the coasts. In this context, the Surface Water and Ocean Topography (SWOT) mission recommended by the National Research Council's Earth Science Decadal Survey has been accelerated in preparation for a 2020 launch. The SWOT radar interferometer will for the first time both improve spatial resolution and broaden the field of view to enable mesoscale ocean measurements, including coastal areas and inland surface water. Therefore, variability of atmospheric water vapor across the swath will adversely affect the accuracy of sea surface altimetry. To reduce these errors, future sea surface altimeters may include high-frequency radiometers along with the current 18-34 GHz Jason-class radiometers to improve retrievals of wet-tropospheric delay in coastal areas and to increase the potential for over-land retrievals. Specifically, high-frequency window channels at 92, 130 and 166 GHz are optimum for wet path delay retrievals in coastal regions. New, high-sensitivity, wide-bandwidth mm-wave radiometers using both window and sounding channels show good potential for over-land wet-path delay retrievals.

For the past several years, we have developed critical microwave component and receiver technologies to reduce the risk, cost, volume, mass, and development time for high-frequency microwave radiometers. First, we are designing and fabricating low-power, low-mass, small-volume high-frequency (90-170 GHz) radiometers with integrated calibration sources. Three key component technologies under development to achieve these objectives are PIN-diode switches for internal calibration that can be integrated into the receiver front end, high-Excess Noise Ratio (ENR) noise sources and a single, tri-frequency feed horn. These new components are being integrated into a MMIC-based laboratory demonstration radiometer with channels centered at 92, 130 and 166 GHz. This radiometer will serve as a breadboard demonstration by providing realistic mass, volume and power estimates to feed into studies of future altimetry missions, including SWOT.

To demonstrate higher technological readiness for space, we are producing an airborne instrument that combines high-frequency, high-sensitivity window and sounding channel radiometers with low-frequency Jason-class radiometers to provide substantially improved spatial resolution and the potential for multiple fields of view across the SWOT airborne radar's swath. The current airborne instrument development and flight demonstration will (1) assess wet-tropospheric path delay variability on 10-km and smaller spatial scales, (2) demonstrate high-frequency 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. We will discuss the current status of these component and receiver technologies enabling the transition from research to operations.