Airborne Demonstration of High-frequency Airborne Microwave and Millimeter-wave Radiometer (HAMMR) to Improve Spatial Resolution of Wet-Tropospheric Path Delay Corrections for Coastal and Inland Water Altimetry

Past and current precision ocean altimeters, including the Jason series, have co-located nadir-viewing 18-34 GHz microwave radiometers to correct the radar signal for wet-tropospheric path delay. Due to the significant footprint sizes at these microwave frequencies, the accuracy of wet path delay retrievals is significantly degraded within approximately 40 km of the world's coastlines, and retrievals are not provided over land. To improve this capability, the addition of wide-band millimeter-wave window channels in the 90-175 GHz band will improve spatial resolution using an antenna reflector of the same size.

In May 2014 NASA and CNES administrators signed a mutual agreement to move from feasibility studies to implementation of the Surface Water and Ocean Topography (SWOT) mission, now planned for launch in late 2020. The primary objectives of SWOT are to improve the characterization of ocean processes by improving the spatial resolution from 200-km to 15-km spatial scales on a global basis as well as measure global water storage in inland surface water bodies and the flow rate of rivers. Therefore, an important new science objective of SWOT is to transition satellite altimetry from the open ocean into the coastal zone and over inland water.

The addition of 90-175 GHz high-frequency, wide-band millimeter-wave window channels to current Jason-class 18-34 GHz microwave radiometers is expected to provide retrievals of wet-tropospheric delay in coastal areas and to enhance the potential for over-land retrievals. To this end, CSU and JPL have designed, fabricated and demonstrated the HAMMR airborne radiometer instrument, combining microwave channels similar to those aboard OSTM/Jason-2 at 18.7, 23.8 and 34.0 GHz, with high-frequency, wide-band millimeter-wave window channels at 90, 130 and 168 GHz, as well as validation channels for temperature and water vapor sounding near 118 GHz and 183 GHz, respectively. The new airborne HAMMR instrument will (1) provide calibration and validation support for the SWOT, Jason-3 and Jason-CS missions, (2) assess wet-tropospheric path delay variability on 10-km and smaller spatial scales, and (3) increase the technology readiness level (TRL) of high-frequency millimeter-wave radiometry with direct detection and internal calibration for potential future opportunities for space-borne technology demonstration.

In July 2014, CSU and JPL have demonstrated substantial improvement in spatial resolution on the high-frequency millimeter-wave window channels compared to the traditional microwave channels. HAMMR successfully completed three days of test flight measurements aboard a Twin Otter aircraft over Lake Powell in Utah. Additional instrument flights are planned for September or October 2014 to measure the variability of wet-path delay over coastal Southern California, including ocean, land and inland water.