Fiducial Reference Measurements for Sentinel-3 and beyond

The Copernicus Sentinel-3 Surface Topography Mission (S3 STM) provides critical surface elevation information over sea ice, land ice and inland waters. These observations are made possible by its high-resolution synthetic aperture radar altimeter, a unique orbit reaching the polar regions up to 81.5°N, and a dual-mission constellation (S3A+S3B) providing a repeat cycle of around 15 days. Ensuring the reliability of these measurements requires comparing the mission’s geophysical retrieval methods, processing algorithms and corrections against independent, fully characterised reference measurements known as Fiducial Reference Measurements (FRM).

Building on the earlier St3TART project conducted by NOVELTIS (2021–2023), St3TART-FO establishes an operational framework for FRM, tackling the distinct challenges of three surface types: sea ice, land ice and inland waters. The project identifies and operates super and opportunity sites, acquires, processes and delivers FRM for Cal/Val activities, characterises the uncertainty of each FRM product, and prepares a roadmap for future altimetry missions beyond S3 STM, including CRISTAL and Sentinel-3 Next-Generation Topography.

Following a successful Operational Readiness Review in early 2025, St3TART-FO entered its operational phase, scaling up site coverage, automating FRM workflows and establishing robust routine data delivery to serve Sentinel-3 and future missions. Central to this effort is the FRM Data Hub, a free, accessible online platform providing traceable FRM datasets and open-source processing tools to the wider Cal/Val community.

Means used

• Identification, installation and operation of super and opportunity FRM sites across inland waters, sea ice and land ice, at multiple locations worldwide, including the polar regions.
• Acquisition, processing and delivery of FRM for Cal/Val activities, including dedicated field campaigns.
• Characterisation of the uncertainty associated with each FRM data product and measurand.
• Development and operation of the FRM Data Hub, offering open data access and processing scripts via Jupyter Notebooks.
• Coordination of an international network of universities, research institutes and engineering partners contributing complementary FRM techniques.