Centralizing in-situ Hydrological measurements for satellite altimetry validation: the INSIGHT platform 

Reliable validation of satellite altimetry over inland waters relies on long-term, high-quality in-situ water height measurements over different types of waterbodies. The strategy implemented in the St3TART Follow-On (FO) project relies on controlled super sites to produce high quality Fiducial Reference Measurements (FRMs) and on a high number of data provided by public national hydrological networks considered as opportunity sites.However, these measurements from national hydrological networks remain highly heterogeneous in terms of formats, units, and metadata description, limiting their direct large-scale use for Cal/Val activities. The first step of data uniformization has been performed by vorteX-io team during St3TART-FO project. As an adaptation of the validation strategy for Sentinel-3 is considered for CRISTAL inland waters products, this uniformization work should be extended to cover more virtual stations for other altimetry missions.This contribution presents the hydrological component of the Hydro-Cryo in-situ platform, INSIGHT, an ESA-funded project, extension of CRISTAL IN-PROVA project, aiming at the centralization and harmonization of publicly available in-situ water surface height data across Europe. This work participates in the preparation for the Cal/Val phase of the future CRISTAL mission and in support of ongoing Sentinel-3 validation activities, with support from the European Environment Agency (EEA) as coordinator of the Copernicus In-Situ component.In this first phase, the platform will integrate data from twelve national hydrological networks covering France, Switzerland, Belgium (Wallonia), Ireland, Portugal, Norway, Poland, Italy, Slovenia, Croatia, the Netherlands and Germany. The data from fixed in-situ sensors deployed on Cal/Val super sites for Sentinel-3 will also be integrated in the platform. The back-end architecture is designed to easily integrate additional networks in Europe and all over the world. Native temporal resolutions provided by in situ sensors are preserved without aggregation or resampling, and up to ten years of historical observations are considered when available.The harmonized hydrological datasets will be disseminated on a dedicated Data Hub developed by NOVELTIS together with reference Cryosphere data for satellite altimetry validation. This open-access platform is designed to serve the Cal/Val community by providing a unified entry point for inland water and cryosphere reference measurements relevant to multiple altimetry missions.The core objective of the hydrological processing chain is the harmonization of in-situ water height measurements by standardizing measurement units and metadata across heterogeneous national public datasets. Attention is given to the consistency of the altimetric reference of the in-situ sensors. This harmonization is essential for the use of in situ stations as FRMs for the validation of both Sentinel-3 and CRISTAL, as well as for others satellite altimetry missions.Beyond the altimetry community, this platform addresses the broader hydrological community by providing access to a standardized water height dataset from public national networks. By lowering technical barriers to data use, the infrastructure supports cross-border hydrological studies and contributes to the reuse of public hydrological observations.This project, currently under development, establishes the data infrastructure for the needs of inland water altimetry validation, while simultaneously enabling wider scientific exploitation of harmonized in-situ water level observations at the European scale.