**Preface to "Fiducial Reference Measurements for Satellite Ocean Colour"**

Ocean colour's fundamental measure is water-leaving radiance, with chlorophyll estimates derived therefrom providing a proxy for phytoplankton occurrence. This is why ocean colour measured by satellite-mounted optical sensors is an essential climate variable that is routinely used as a central element in assessing the health and productivity of marine ecosystems and the role of oceans in the global carbon cycle. For satellite ocean colour to be trustworthy and used in these and other important environmental applications, its data must be reliable and of the highest quality.

Pre-flight and on-board calibration is conducted for satellite ocean colour sensors; however, once in orbit, their data quality can only be fully assessed via independent calibration and validation activities using surface measurements. These measurements therefore need to be at least as high quality as the satellite data, which necessitates SI-traceability and an uncertainty budget. This is the basis of fiducial reference measurements (FRM) that are: a suite of independent ground measurements that provide the maximum return on investment for a satellite mission by delivering to users the required confidence in data products. This is in the form of independent validation results and satellite measurement uncertainty estimation, over the entire end-to-end duration of a satellite mission. The FRM must: have documented traceability to SI units (via an unbroken chain of calibrations and comparisons); be independent from the satellite retrieval process; be accompanied by a complete estimate of uncertainty, including contributions from all FRM instruments and all data acquisition and processing steps; follow well-defined protocols/community-wide management practices and; be openly available for independent scrutiny.

Within this context, the European Space Agency (ESA) funded a series of projects targeting the validation of satellite data products (atmosphere, land, and ocean) and set up the framework, standards, and protocols for future satellite validation efforts. The Fiducial Reference Measurements for Satellite Ocean Colour (FRM4SOC) project was structured to provide support for evaluating and improving the state of the art in ocean colour radiometry (OCR) through a series of comparisons under the auspices of the Committee on Earth Observation Satellites (CEOS) working group on calibration and validation and in support of the CEOS ocean colour virtual constellation.

The objectives of FRM4SOC were to establish and maintain SI-traceable ground-based FRM for satellite OCR with the relevant protocols and uncertainty budgets for an ongoing international reference measurement system supporting the validation of satellite ocean colour. This was undertaken to ensure the high quality and accuracy of Copernicus satellite mission data and, in particular, the Sentinel-2 MSI and Sentinel-3 OLCI ocean colour products, providing and continuing to provide a fundamental contribution to the European system for monitoring the Earth (Copernicus).

This Special Issue of MDPI Remote Sensing was designed to showcase this essential Earth observation work through the publication of the project's main achievements and results, accompanied by other select relevant articles. It covers the following topics:

• FRM4SOC project overview and scientific roadmap for the future of ocean colour validation (Article 1);

• Measurement requirements and protocols when operating FRM OCR for satellite validation (Articles 2 and 3);

• Improvements in ocean colour radiometers (Article 4);

• OCR calibration source inter-comparisons (Article 5);


• Improvements in satellite ocean colour application to lakes and coastal areas (Articles 15 and 16).

We sincerely thank everyone that contributed to FRM4SOC, both to the project and this Special Issue.

## **Andrew Clive Banks, Christophe Lerebourg, Kevin Ruddick, Gavin Tilstone, Riho Vendt** *Editors*
