Next Generation Spaceborne Instrument for Monitoring Ocean Salinity with Application to the Coastal Zone and Cryosphere
[29-Aug-2018] Dinnat, E.P., Le Vine, D.M., De Amici, G., and Piepmeier, J.
Presented at the 2018 Ocean Salinity Science Team and Salinity Continuity Processing Meeting
We report on the science requirements and technical definition of a next-generation spaceborne instrument for ocean salinity remote sensing. The new sensor will be designed to improve retrievals in cold water and enhance applications closer to the shores where there are important interactions between land, ocean and ice. With the advent of the SMOS and Aquarius instruments in 2010, it has become possible to monitor global sea surface salinity (SSS) on a weekly to monthly basis. These sensors have also shown the increased complexity of retrieving SSS in the cold water of the high latitudes and close to land and ice boundaries. These limitations have hindered the application of space-borne SSS observations to the study of important processes such as ocean freshening due to ice melt and river outflow, especially in the high latitude and in narrow costal currents.
We report on our design of a multi band microwave radiometer coupled to a large reflector antenna with the goals of:
- Enhancing spatial resolution to provide observations closer to coasts and the sea ice edge;
- Increasing radiometric sensitivity to salinity in cold waters.
Our objective is to be able to resolve surface features as small as 20 km in order to monitor changes in SSS in coastal currents such the West and the East Greenland currents and the Benguela coastal current near the Congo River mouth. The increase in sensitivity to SSS will come from using low frequencies of about 600-800 MHz (P-band). The radiometer will operate over a wide range of frequencies between about 600 MHz and 5000 MHz, spanning the P-,L-,S- and C-bands. The choice of center frequencies and bandwidths is one of the aspects being studied. The upper range of frequencies will be used to infer information about other environmental parameters necessary to retrieve SSS, such as surface roughness and sea water temperature. It is also expected that the increased spatial resolution and frequency coverage will allow for enhanced applications for the cryosphere. For example, SMOS observations at 1413 MHz have been used to infer the thickness of the thin sea ice (< 0.5 m), which is challenging with more traditional techniques which either have high uncertainty for ice thinner than 1 m (e.g. altimeters) or cannot operate through clouds (e.g. thermal IR). Our design with improved spatial resolution and more wavelengths will offer advanced capabilities for such applications.
Because radio-frequency interference (RFI) is likely to be encountered over significant parts of the bandwidth, at least in coastal areas, advanced RFI mitigation techniques derived from the Soil Moisture Active Passive (SMAP) mission will be employed. In particular, the radiometer could use a 16,000-channel wideband spectrometer with 275-kHz resolution. The spatial resolution and low frequency needed to achieve the scientific goals dictate a large 15-meter class reflector. Early findings of our tradeoff study are that the revisit time, sensitivity and swath coverage suggest the use of multiple simultaneous footprints. One approach we are investigating is the use of a single-dish-multiple-feed interferometer in order to synthesize high resolution electronically steerable beams.