Séminaire

In May 2014, NASA and CNES administrators signed a bilateral agreement to move from feasibility studies to implementation of the Surface Water and Ocean Topography (SWOT) mission, now planned for launch in late 2020. An important science objective of SWOT is to transition satellite altimetry from the open ocean into the coastal zone and over inland water. Past and current precision ocean altimeters, including the Jason series, have nadir-viewing, co-located low-frequency 18-34 GHz microwave radiometers to correct the radar signal for wet-tropospheric path delay. Since surface footprints are substantial at these frequencies, the accuracy of wet path delay retrievals is significantly degraded within approximately 30-40 km of the world’s coastlines, and retrievals are not provided over land. To improve this capability, high-frequency millimeter-wave window channels in the 90-180 GHz band may be added to achieve finer spatial resolution for a fixed reflector size. These higher-frequency channels are expected to provide retrievals of wet-tropospheric delay in coastal areas and to enhance the potential for over-land retrievals.

To address these needs, Colorado State University (CSU) and NASA/Caltech Jet Propulsion Laboratory (JPL) have designed, fabricated and demonstrated the HAMMR airborne radiometer instrument with a total of 25 channels, combining low-frequency microwave channels similar to Jason-2/3 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 temperature and water vapor sounding channels near 118 GHz and 183 GHz, respectively. The new HAMMR instrument (1) provides calibration and validation support for the SWOT, Jason-3 and Jason-CS missions that is complementary to JPL’s AirSWOT, (2) assesses wet-tropospheric path delay variability on 1-km and smaller spatial scales, and (3) provides high-frequency millimeter-wave radiometers with direct detection and internal calibration that can be integrated into future space missions, including nanosatellite constellations.

The HAMMR instrument was deployed on a Twin Otter aircraft for the West Coast Flight Campaign (WCFC) between November 4 and 17, 2014. HAMMR successfully collected more than 53.5 hours of data under diverse atmospheric conditions and over nearly the entire U.S. West coast from Camarillo, CA, to the Strait of Juan de Fuca, WA. Both coastal and inland water were overflown at different times of day to measure diurnal variations in wet-path delay under a variety of atmospheric conditions, including clear sky, clouds and fog. The HAMMR WCFC antenna temperatures were accurately calibrated and geolocated to retrieve wet-tropospheric path delay with approximately 1-mm precision and 150-m spatial resolution. Next, the spatial spectral variability of wet-path delay over coastal areas will be analyzed.