ESA Cal/Val Park

Scientific challenge

Accurate radiometric calibration and validation (Cal/Val) of Earth observation instruments requires a detailed understanding of how radiation interacts with the atmosphere and the Earth’s surface. Conventional one-dimensional (1D) radiative transfer models generally assume horizontally homogeneous conditions. In heterogeneous environments, however, radiation scattered from neighbouring surfaces can contribute to the signal measured over a target. This adjacency effect can influence both satellite and ground-based observations.

Quantifying these three-dimensional (3D) effects is particularly important for demanding radiometric applications, including the Copernicus Hyperspectral Imaging Mission for the Environment (CHIME). Another challenge is determining how local ground measurements can be related reliably to the spatial footprint of a satellite observation, especially where nearby structures, varying surface reflectance and atmospheric conditions influence the measurements.

S2GOS contribution

This proposed use case applies the DTE-S2GOS Synthetic Scene Generator and the Eradiate 3D radiative transfer model to the ESA Cal/Val Park. The objective is to establish a physically consistent simulation environment for assessing adjacency effects and the relationship between ground-based and satellite observations.

The planned 3D scene comprises three complementary areas:

The Cal/Val Park and its extension playground, with the radiometry, geometric/image-quality, atmospheric and laboratory components, together with relevant masts, towers and structures, represented using detailed site information.
The surrounding landscape, generated from available geographical and high-resolution elevation data to represent nearby surface heterogeneity.
A wider buffer area, represented at an appropriate resolution to account for radiative contributions from outside the immediate site.

The atmosphere will be represented through realistic scenarios, including different vertical profiles, broken cloud fields and thin cirrus. The extent of the surrounding and buffer areas will be assessed through sensitivity analyses.

Demonstration

The baseline demonstration proposed for the ESA Cal/Val Park combines satellite and ground observation simulations within a common 3D scene.

Satellite observations

Sentinel-2/MSI and CHIME front-of-telescope top-of-atmosphere radiance fields will be simulated over areas approximately 1–5 km across, centred on the Cal/Val Park. The radiance fields will be spatially oversampled, with the exact sampling to be agreed with ESA, and calculated at 1 nm spectral sampling. Illumination and viewing geometries will be derived from the corresponding satellite overpass conditions.

These high-resolution radiances are intended to support subsequent application of instrument spectral response functions and point spread functions. Detailed instrument-specific simulation is not part of the proposed S2GOS activity. An idealised Level-1C-like product could be generated if requested, but would not reproduce all characteristics of the actual instruments.

For both MSI and CHIME, the simulations will also provide surface hemispherical-directional reflectance factor (HDRF) reference data. Comparing front-of-telescope radiance and surface HDRF will help characterise atmospheric and surface adjacency effects under different summer and winter scenarios.

Ground observations

The same 3D environment will be used to simulate approximately three to five ground-based observation configurations, with the instruments and their characteristics to be agreed with ESA. The simulations will support analysis of local measurement footprints, nearby structures and adjacency effects, and comparisons between ground-based and satellite-scale HDRF.

Expected results

The proposed activity aims to deliver:

A detailed 3D representation of the ESA Cal/Val Park, its surroundings and the atmospheric scenarios relevant to the simulations.
High-resolution Sentinel-2/MSI and CHIME front-of-telescope radiance fields, together with corresponding surface HDRF reference data.
Quantitative analyses of 3D adjacency effects for different atmospheric and seasonal conditions.
Simulated ground-based observations and an assessment of the consistency and spatial representativeness of ground and satellite measurements.
A scientific basis for improving Cal/Val methods and exploring validation protocols for Fiducial Reference Measurements (FRM) of surface reflectance.

Possible follow-up activities, outside the proposed baseline, include propagation of input uncertainties through the 3D simulations and investigation of a proof-of-concept 3D inversion approach that accounts for adjacency effects and broken cloud fields. The feasibility of enhanced all-sky radiance measurements at the site is also identified as a potential improvement.

The intended outcome is a better understanding of how 3D radiative processes affect radiometric measurements, supporting the calibration and validation of Sentinel-2, CHIME and future Earth observation missions.