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Overview of the ASELSU project: Aims and Metrological Background
Overview of the ASELSU project: Aims and Metrological Background
The Sentinel-6 Michael Freilich Mission and the ASELSU project
The Sentinel-6 Michael Freilich Mission and the ASELSU project
The Copernicus Sentinel-6 Michael Freilich (S6-MF) satellite is the reference mission with responsibility to continue the long-term record of sea-surface height measurements started in 1992 by the Topex-Poseidon satellite and then by the Jason series. The role of S6-MF is both to extend the record for climate studies and also to monitor the changing height of the sea surface with greater accuracy than before. S6-MF, launched November 2020, is the first of two identical satellites that will ensure continuity of sea-surface observations into a fourth decade. The second of these satellites, Sentinel-6B, is scheduled for launch November 2025.
The Assessment Sea Level Rise Stability Uncertainty (ASELSU) project aims to:
Establish an end-to-end sea level stability uncertainty budget for S6-MF,
Identify possible improvements for the Sentinel-6 Next Generation (S6NG) mission in order to reach the scientific uncertainty requirements needed for climate studies.
The uncertainty requirements are assessed for calculation of global mean sea level (GMSL) and regional mean sea level (MSL). The GMSL calculation is based on averaging observations along the reference orbit, providing a global (up to 66°N/S) mean of all observations of mean sea level (MSL) over the ~10 day repeat cycle for the reference orbit. MSL values are regional spatiotemporal averages of sea-level anomaly (SLA), made within 1° latitude by 3° longitude boxes over 10 days. The choice of 1° by 3° is detailed in Henry et al. (2014), and Scharffenberg and Stammer (2019). SLA is calculated from satellite observations at 1 Hz (that is, every second and averaged over that second) and filtered to remove invalid observations.
To evaluate the uncertainty associated with GMSL, metrological aspects must be considered. A key aim of the ASELSU project is to have an uncertainty budget that is well-established and documented, following metrological principles through adoption of the theoretical framework established in the Fidelity and Uncertainty in Climate Data Records from Earth Observation (FIDUCEO) project.
Metrological Background
Metrological Background
Metrology, the science of measurement, is the discipline responsible for maintaining the International System of Units (SI) and the associated system of measurement. It is imperative to the SI that measurements are stable over several centuries and that measurement standards are equivalent worldwide. These properties are realised through two key principles of metrological traceability, uncertainty analysis and comparison, respectively.
In Earth Observation (EO), we refer to the metrological guidelines provided by the Quality Assurance Framework for Earth Observation (QA4EO), which defines the following principle regarding EO data quality:
"It is critical that data and derived products are easily accessible in an open manner and have an associated indicator of quality traceable to reference standards (preferably SI) so users can assess suitability for their applications, i.e., 'fitness for purpose'."
QA4EO defines high level processes to help achieve this, including documented procedures, participation in comparisons, and uncertainty assessments. Traceability demands that this quality indicator be based on “a documented and quantifiable assessment of evidence demonstrating the level of traceability to internationally agreed (where possible SI) reference standards.” The QA4EO principle stops short of requiring SI-traceability in all circumstances, recognising that the full rigour of linkage to SI may not be viable for all applications and measurements; however, the accompanying guidelines are based on concepts adapted from guidelines of the international metrology community and a metrological approach is strongly implied. A full summary of the guidance for a metrological approach to fundamental data records (FDR), thematic data products (TDP), and fiducial reference measurements (FRM) can be found on the QA4EO website.
The ASELSU project follows the guidelines on the QA4EO, which details the 'QA4EO Five Steps' towards developing an uncertainty budget. See the graphic below for more details.
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