Speaker
Description
Microwave radiometers have a long heritage of providing a dedicated wet tropospheric correction to the path delay of radar altimeter pulses and echo backscatter due to water vapour content within the troposphere. With the introduction of synthetic aperture processing in radar altimetry, more accurate altimetry data is anticipated for coastal and inland waters. Therefore, the objective of this study was to address MWR instrument design for future operational radar altimetry missions such as next Generation Sentinel-3 and Jason-CS+.. Such a design was to be compact and to include the classical, current MWR channels for ensuring observation continuity, augmented by a set of high frequency channels for enabling accurate altimetry over coastal and inland waters. As a secondary objective, the selection of the high frequency channels was also to consider the retrieval of wet tropospheric delay over sea ice and ice-sheets.
In order to meet the objectives, the study was divided into three major tasks. Each task generated a task report, the final versions of which are contained within the Final Report.
• Task 1 to be concerned with the selection of an optimum set of high frequency channels based upon a scientific review and analysis.
• Task 2 to be concerned with the derivation of an instrument suite of requirements, from which alternative MWR design concepts were to be developed using the frequency sets established in Task 1, followed by a trade-off of the concepts with a baseline MWR instrument concept to be proposed for the Agency’s approval.
• Task 3 was concerned with the preliminary design of the MWR instrument and assessment of the accuracy of the wet tropospheric delay retrieval under representative atmospheric conditions and over different surface types (water, sea ice, ice-sheets) for the baseline design.
The study has undertaken a fully consistent approach in an iterative process relating instrument design and retrieval, such that an instrument design has been proposed addressing the most up-to-date requirements on sensitivity, spatial resolution, spatial discrimination, stability and by physical considerations (radiative transfer) assessed in other studies. The instrument concept is based upon a multi-frequency quasi-optic feed and draws upon experience gained from MetOp-SG MWS developments, whilst addressing the additional challenges of operating an accurate and stable radiometer in non sun-synchronous orbits.
Two dedicated retrieval algorithms, (i) a classical robust Empirical Neural Network and (ii) a promising flexible and performing 1DVAR have been developed, and have demonstrated that high frequency channels do improve the performances: over open ocean, over coastal regions, over ice/sea ice. It was not possible to ensure obtaining the goal of 1cm rm. error everywhere, due to limitations in the data processing capabilities and to unknown issues (clouds, rain, etc.) as the retrieval algorithms do not yet include this and further development of the 1DVAR algorithm is recommended.
| ESA Technical Officer | Manuel Martin-Neira |
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