Building the Calibration and Validation Roadmap for CRISTAL’s Hydro-Cryo Products

In the frame of the ESA CRISTAL IN-PROVA project, NOVELTIS leads an 8-partner consortium establishing the calibration and validation roadmap for CRISTAL’s Level-2 Hydro-Cryo data products, combining direct comparison with reference measurements and cross-validation against other altimetry missions.

The CRISTAL mission, run by ESA within the European Union’s Copernicus programme, represents a major step forward in cryosphere observation. As the first high-resolution, dual-band, interferometric, operational radar altimeter mission, CRISTAL will deliver invaluable data on sea ice, icebergs and land ice, as well as on oceans and inland waters.

CRISTAL IN-PROVA contributes to the mission’s success by establishing a calibration and validation roadmap for CRISTAL’s Level-2 Hydro-Cryo products, covering the cryosphere (sea ice, icebergs and land ice) and inland waters (rivers, lakes). The project combines two complementary approaches: direct comparison with Fiducial Reference Measurements (FRM) – independent, tailored and fully characterised reference data with traceable uncertainty – and cross-validation against other satellite missions such as Copernicus Expansion Missions, including Sentinel-6, CryoSat-2, ICESat-2, SWOT, Sentinel-3 NG Topography, CIMR and ROSE-L.

To strengthen this validation framework, the project includes the development of two dedicated tools: a Validation Performance Simulator, which assesses calibration and validation methods through simulated and co-located datasets aligned with the CRISTAL Mission Requirements Document, and a Match-Up System, which co-locates CRISTAL data with external measurements to enable precise accuracy comparisons.

The project has also grown to include INSIGHT, a platform under development by NOVELTIS. INSIGHT is a European initiative aimed at providing open access to in-situ measurements through a centralized and standardized data hub. The platform’s primary objective is to support calibration and validation (Cal/Val) activities for satellite altimetry, while also serving a broad range of end-users involved in environmental services and studies, as well as hydrological and polar monitoring at both European and international levels.

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Space

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Cal/Val, Copernicus, CRISTAL, Cryosphere, FRM, Hydrology

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Building the Calibration and Validation Roadmap for CRISTAL’s Hydro-Cryo Products

In the frame of the ESA-TROPOSIF project, NOVELTIS is in charge of elaborating and validating a new SIF product from Sentinel-5p data and of its exploitation to improve global scale simulations of carbon and water cycles with a terrestrial biosphere model (data assimilation).

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Space

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Cal/Val, Copernicus, CRISTAL, Cryosphere, FRM, Hydrology

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The terrestrial biosphere plays a key role for climate by offsetting about one fourth of carbon dioxide (CO2) emissions released by anthropogenic activities into the atmosphere. Terrestrial ecosystems assimilate CO2 in leaf chloroplasts by photosynthesis, but most of the assimilated carbon is released back into the atmosphere (mainly as CO2) through ecosystem respiration. Our ability to characterize the spatial and temporal variations of carbon uptakes by land surfaces remains largely uncertain, and hence our ability to anticipate possible evolutions of terrestrial ecosystem functioning under a changing climate.
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During the past decade, space-borne estimates of solar-induced chlorophyll fluorescence (SIF) have opened-up new prospects to improve our knowledge of the global carbon cycles. SIF is an electromagnetic signal emitted by the chlorophyll a of assimilating plants: part of the energy absorbed by chlorophyll a which is not used for photosynthesis emitted at longer wavelengths as a two-peak spectrum roughly covering the 650–850 nm spectral range. SIF responds instantaneously to perturbations in environmental conditions, which makes it a direct proxy for photosynthetic activity. However, SIF emission in only a slight fraction of the radiation at the top of the canopy, which is mostly made of reflected light.
Estimation of SIF from space-borne instruments is made possible by spectrometers initially dedicated to atmosphere characterization (green house gazes and trace gazes) because they provide the necessary spectral and radiometric sensitivity (e.g. GOSAT, OCO-2, GOME-2, SCIAMACHY).
Despite the great success of these satellites, the exploitation of SIF measurements for most applications is however limited by their coarse spatial resolution and low number of observations.
These limitations could be largely alleviated by the data provided by the TROPOMI instrument on-board Sentinel-5p: this instrument combines a high spatial resolution (3.5 km x 7 km at nadir), a large swath, and a daily revisit frequency, which enables a quasi-continuous sampling of the spatial and temporal variations of SIF at the global scale. The measurements also have a high signal-to-noise ratio and cover a large spectral range. These characteristics constitute a significant improvement in terms of accuracy and number of data available daily compared to those provided by previous missions.

Results

  • Calibration and validation roadmap for CRISTAL’s Level-2 Hydro-Cryo products.
  • Dedicated protocols for direct comparison with Fiducial Reference Measurements (FRM) and cross-validation against satellite altimetry missions.
  • Validation Performance Simulator ensuring alignment with the CRISTAL Mission Requirements Document.
  • Match-Up System enabling precise co-location and comparison of CRISTAL and reference data.
  • INSIGHT platform providing open, standardised access to in-situ measurements.

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