Use Cases | S2GOS

Benchmarking of land parameter upscaling methods

Mon, 01 Jan 0001 00:00:00 +0000

Scientific challenge

Satellite land-product validation requires reference measurements representative of areas much larger than those directly sampled by individual ground instruments. Upscaling ground-based observations to satellite-pixel scales therefore introduces assumptions about vegetation structure, spatial heterogeneity and the representativeness of measurement locations.

This issue is central to the Ground-Based Observations for Validation (GBOV) service, which supports the validation of Copernicus land products using ground reference measurements and derived land products. Upscaling uncertainties are particularly difficult to quantify when independent reference values at the satellite scale are unavailable.

S2GOS contribution

DTE-S2GOS provides a physically based framework in which ground measurements, satellite observations and reference land parameters are simulated from the same three-dimensional scene. This allows the effects of vegetation structure, illumination, atmospheric conditions and measurement geometry to be explored in a controlled setting.

The use case focuses on uncertainties in the upscaling of land parameters such as surface albedo, leaf area index (LAI), fraction of absorbed photosynthetically active radiation (FAPAR) and fraction of vegetation cover (fCover). The scenes will partially incorporate terrestrial laser scanning (TLS) data to represent terrain and vegetation structure in three dimensions.

The planned observation configurations include:

Ground observations: a tower-mounted broadband shortwave radiometer (pyranometer) for albedo and a network of below-canopy photosynthetically active radiation (PAR) sensors.
Satellite observations: synthetic Sentinel-2/MSI and Sentinel-3/OLCI measurements.
Reference quantities: pixel-level top-of-canopy bidirectional reflectance factor (BRF), bihemispherical reflectance (BHR), FAPAR, LAI and fCover.

Demonstration

Two contrasting sites are considered: San Rossore near Pisa, Italy, and Patagonia National Park, Chile. Simulated ground and satellite observations will be combined to test the spatial representativeness of local measurements and compare ground-to-satellite upscaling approaches.

The technical design includes seasonal configurations, daily observation cycles and vegetation perturbations. FAPAR is planned to be simulated hourly under clear skies, shallow cumulus clouds and stratus cloud conditions, allowing the role of changing illumination and atmospheric conditions to be investigated.

An illustrative Patagonia configuration includes a HyperNet land tower equipped with a Hypstar radiometer. These simulations demonstrate the level of geometric detail available for modelling an actual ground sensor and its field of view.

Simulated HyperNet land tower and Hypstar instrument in the Patagonia scene.

Simulated hemispherical-conical reflectance factor (HCRF) observed by Hypstar; the instrument field of view is indicated in the plot.

Expected results

The use case aims to provide a quantitative assessment of uncertainties associated with upscaling ground observations by comparing simulated ground measurements, upscaled estimates and satellite-scale reference quantities.

In particular, the analysis will examine how vegetation heterogeneity, sensor placement, observation geometry, seasonal changes and atmospheric conditions influence the representativeness of ground-based measurements. The resulting framework is intended to support more robust validation of satellite-derived albedo, FAPAR, LAI and fCover.

The interpretation will also account for limitations in the 3D scene representation, including uncertainties in tree geometry and foliage properties derived from TLS and other input data. Specific upscaling algorithms and evaluation metrics will be finalised as part of the demonstration protocol. Land surface temperature and soil moisture are outside the scope of this phase.

CHIME radiance simulation

Mon, 01 Jan 0001 00:00:00 +0000

Scientific challenge

The Copernicus Hyperspectral Imaging Mission for the Environment (CHIME) will acquire hyperspectral observations in the visible to shortwave infrared spectral range. Preparing and evaluating its instrument and processing chain requires realistic radiance scenes that reproduce the spectral and spatial variability of the Earth as viewed from space.

A key challenge is to represent surface heterogeneity, atmospheric effects and three-dimensional radiative transfer processes at a resolution sufficiently fine to provide meaningful input to an instrument performance simulator. Such input must be distinguished from the simulated measurements and Level-1 products produced after modelling the instrument response.

S2GOS contribution

DTE-S2GOS generates high-spectral- and high-spatial-resolution front-of-telescope (FoT) radiance using detailed three-dimensional Earth scenes and the Eradiate radiative transfer model. These radiances describe the incoming signal before it is modified by the CHIME optical system, detectors and onboard acquisition chain.

The simulation accounts for the interaction of solar radiation with the surface and atmosphere, including spatially varying surface properties and three-dimensional effects such as adjacency. It draws on the scene generation and atmospheric simulation capabilities also used for the ACIX-IV use case.

S2GOS does not develop the CHIME instrument simulator. Instead, its intended contribution is to supply physically consistent FoT radiance scenes that can be used as input to the separately developed CHIME instrument performance simulation chain. Instrument-specific effects, including the detailed spectral and spatial response, detector behaviour and onboard processing, are outside the scope of this S2GOS use case.

Demonstration

The demonstration uses the Furioso region of interest in Patagonia National Park, Chile, with contrasting seasonal scene conditions. High-resolution radiance simulations illustrate the variability of the incoming signal that a hyperspectral instrument would observe.

The two figures below show RGB visualisations derived from Eradiate simulations for December and June. They illustrate seasonal differences in the simulated scene; they should not be interpreted as final CHIME Level-1 products or as outputs of the CHIME instrument simulator.

RGB visualisation of an Eradiate simulation over the Furioso region of interest in December.

RGB visualisation of an Eradiate simulation over the Furioso region of interest in June.

The exact radiance sampling, data format and interface to the external simulator must be agreed with its developers before end-to-end interoperability can be demonstrated.

Expected results

This use case aims to provide:

Physically consistent FoT radiance datasets with fine spatial and spectral sampling, suitable for subsequent instrument simulation.
Realistic variability arising from three-dimensional surface structure, atmospheric conditions, observation geometry and seasonal changes.
Reference input scenes for assessing the effects of instrument characteristics and processing on the observed signal.

By separating the simulation of the Earth–atmosphere radiance field from the simulation of the instrument response, this approach supports independent assessment of both components and facilitates reuse of S2GOS scenes for different observation scenarios.

ESA Cal/Val Park

Mon, 01 Jan 0001 00:00:00 +0000

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.

Satellite image simulation over bright desert PICS

Mon, 01 Jan 0001 00:00:00 +0000

Scientific challenge

Bright desert Pseudo-Invariant Calibration Sites (PICS) play an important role in monitoring the radiometric stability and consistency of Earth observation instruments. Their relatively stable surface reflectance makes them suitable for long-term calibration and validation activities.

Conventional PICS-based approaches generally average satellite observations over a selected area to estimate a mean top-of-atmosphere (TOA) radiance or bidirectional reflectance factor (BRF). These spatial averages are then compared with reference values, often calculated using one-dimensional (1D) radiative transfer models. While effective for monitoring mean radiometric behaviour, averaging reduces the information available about spatial variability and individual detector pixels.

The challenge is to establish whether realistic, spatially resolved simulations can support pixel-by-pixel radiometric validation, while accounting for the uncertainties associated with both synthetic and actual observations.

S2GOS contribution

DTE-S2GOS proposes an alternative approach in which each simulated satellite pixel is retained rather than averaged over the calibration site. The Synthetic Scene Generator produces a detailed three-dimensional representation of the terrain and surface properties, while the Observation Simulator uses the Eradiate 3D radiative transfer model to simulate the radiometric signal observed by satellite and ground-based instruments.

The simulations are designed to account for:

Surface heterogeneity and topography, represented using high-spatial-resolution data (approximately 1 m or better where available) and a digital elevation model.
Local infrastructure, including the GHNA and GONA measurement masts and their surrounding fences, which can influence the measured radiometric signal.
Observation conditions, including solar and viewing geometry, atmospheric properties and instrument spectral response.
Uncertainty propagation, from the relevant simulation inputs through to the simulated observations.

The resulting synthetic observations can be compared with actual satellite measurements at the individual-pixel level, with uncertainty estimates accompanying the comparison.

Demonstration

The demonstration focuses on the Gobabeb region in Namibia, a well-characterised bright desert environment. It uses information from the Gobabeb HYPERNETS Namibia station (GHNA), equipped with a HYPSTAR radiometer, and the nearby Gobabeb calibration site (GONA), which is part of the RadCalNet network. The two stations are approximately 650 m apart.

The planned simulation area extends up to approximately 1.5 km around GHNA station. The demonstration will combine detailed surface characterisation with simulations of Sentinel-2/MSI and Sentinel-3/OLCI observations, as well as the ground-based HYPSTAR measurements at GHNA.

For comparisons with satellite acquisitions, simulations will reproduce the acquisition conditions as closely as possible, including illumination and viewing geometry and atmospheric conditions. Appropriate spectral and spatial instrument characteristics will be considered to make the simulated and measured quantities comparable.

Simulated 10 x 10 km Sentinel-2/MSI quicklook over the Gobabeb area.

Expected results

This use case aims to establish a framework for pixel-by-pixel radiometric assessment at bright desert calibration sites, extending conventional approaches based on spatially averaged observations.

The expected outcomes include:

A detailed comparison between simulated and actual satellite observations, preserving spatial variability within the calibration area.
Pixel-level radiometric differences and uncertainty estimates, allowing the significance of discrepancies to be assessed.
An evaluation of the accuracy achievable with S2GOS when realistic surface structure, observation geometry and atmospheric conditions are represented.
An assessment of the potential for characterising individual detector-pixel behaviour, which cannot be investigated using only spatially averaged calibration values.

The comparison methodology will account for uncertainties in both the simulated and observed quantities, as well as uncertainty introduced by differences in their spatial, spectral or temporal representation. The results are intended to inform future calibration and validation activities, including those supported by RadCalNet, HYPERNETS and ESA’s vicarious calibration services.

Synthetic satellite observations for ACIX-IV

Mon, 01 Jan 0001 00:00:00 +0000

Scientific challenge

The Atmospheric Correction Inter-comparison eXercise (ACIX) compares the performance of atmospheric correction processors, primarily through their surface reflectance products. Aerosol optical depth (AOD) and total-column water vapour are also relevant outputs. The fourth phase, ACIX-IV, offers an opportunity to evaluate processors against controlled, physically consistent synthetic observations.

With real satellite images, reference surface reflectance and atmospheric properties cannot be measured accurately for every pixel at the satellite’s spatial resolution. This complicates the attribution of differences between processors. A synthetic benchmark can instead provide a known scene and a consistent set of reference quantities, while retaining realistic three-dimensional radiative effects, including adjacency effects.

S2GOS contribution

DTE-S2GOS is designed to generate synthetic top-of-atmosphere (TOA) radiance for both multispectral Sentinel-2/MSI (approximately 10 m) and hyperspectral CHIME (approximately 30 m). The observations are intended to be distributed in an orthorectified, Level-1C-like form, with instrument-related characteristics and radiometric noise represented in the simulation.

The same 3D radiative transfer calculations provide pixel-level reference surface hemispherical-directional reflectance factor (HDRF). Reference atmospheric fields, including AOD and total-column water vapour, are derived from the atmospheric scene configuration informed by the Copernicus Atmosphere Monitoring Service (CAMS).

An important feature of the ACIX-IV workflow is the separation of data access: participants receive the synthetic satellite observations and associated public metadata, while the reference HDRF and atmospheric quantities are retained for the benchmark evaluation. This prevents the reference values from influencing the processors being tested.

Demonstration

The planned exercise covers selected regions of interest spanning different land covers, including forest, cropland, urban and arid environments. Winter and summer configurations are foreseen, with atmospheric variations and cloud conditions, including cumulus clouds, considered in the simulation design.

Each synthetic image is intended to include appropriate ancillary information, such as geographic coordinates, solar and viewing geometry, terrain elevation, cloud or cloud-shadow masks where applicable, spectral band definitions, solar irradiance and radiometric-noise information.

Synthetic Sentinel-2/MSI RGB quicklook over the Furioso region in December, simulated with Eradiate.

Corresponding synthetic Sentinel-2/MSI RGB quicklook in June, illustrating seasonal changes in the simulated scene.

Expected results

ACIX-IV participants will process the synthetic TOA observations using their atmospheric correction algorithms. The retrieved surface reflectance will subsequently be compared with the simulated pixel-by-pixel HDRF reference, allowing a quantitative assessment across land-cover types, seasons and atmospheric conditions.

The exercise is expected to clarify the sensitivity of atmospheric correction algorithms to surface heterogeneity, atmospheric variability and three-dimensional radiative effects. It will also demonstrate how physically based synthetic images can support the development and validation of retrieval algorithms for future missions, including CHIME. Detailed comparison metrics remain subject to the ACIX-IV evaluation protocol.