Earth Observation (EO) from space has demonstrated to be a formidable technique to support a wide range of real-world applications, from agricultural management to international security and from climate change to water or air quality. Nowadays, information derived from EO platforms has become an essential ingredient of evidence-based policy making.
Delivering products and services is far from trivial. Space instruments can only measure spatiotemporal changes in gravity or in electromagnetic fields, from which other geophysical parameters and pertinent products must be inferred.
The interpretation of these raw measurements and their translation into information useful to users and stakeholders require a wide range of skills and procedures that must be explored, evaluated and operationalized.
Today there is a lot of EO data and products available from Copernicus services and international sources. Any user wonders which data/products best meet their needs? Or in other words are there EO products fit for purpose?
The motivation to create new EO missions can start primarily from the community (users) or from key stakeholders (policy). Baseline technical solution for building new satellites is frequently proposed to meet requirements, schedules and specifications. These outcomes are normally estimated to evaluate the validity of the requirements and the feasibility of the proposed space design. Calibration infrastructure and validation procedures are also essential to demonstrate the adequacy and the performance of the satellite mission.
Fitness for purpose of EO products can be addressed from different perspectives, which may entail systematically assessing aspects such as requirements, specifications, performance, physical meaning and consistency of measurements in space and time, calibration and validation procedures.
To kick-off the fitness for purpose topic, we develop here initial ideas on specific focus areas or view angles that KCEO can develop around the fitness for purpose of EO products.
The examples described concern global warming and ocean related datasets, but the concepts are clearly applicable to other thematic areas.
Requirements and specifications
Formulation of users’ requirements is a key element to translate them into technical prerequisite for building EO systems.
For example, within climate change studies, the requirements are taken from Global Climate Observing System (GCOS) that guides the next generation of global climate observations through the Essential Climate Variables (ECVs) for “the development of in-situ and satellite global climate observations by all national meteorological services and space agencies and other oceanic and terrestrial networks.” Implementation report includes also various foreseen actions to reach target EO requirements also to fit the adaptation and mitigation aspects.
The IOCCG (International Ocean Colour Coordinating Group) Report #18 (2019) addresses “Uncertainties in Ocean Colour Remote Sensing”. This report presented the state of the art in the field and provides recommendations to the space agencies and the user community.
Long-term EO products Corner
Assessing the fitness for purpose of long-term EO products, implies addressing issues such as the consistency of the products over time, for example to properly support the analysis of long-term trends, or the consistency of the physical measures across multiple products or satellite missions.
Concerning the use of long-term EO products for assessing interannual variation of physical properties over land surface studies looked for example into the inconsistencies of interannual variability and trends in long‐term satellite leaf area index products or into the feasibility of reprocessing NOAA terrestrial observations (available since 1980s) using the latest available techniques to derive harmonised long-term time series, and the assessment of its fitness for purpose for the analysis of global terrestrial change. Regarding the assessment of physical consistencies between various products, studies investigated the consistency between global products of surface albedo and burned area. This type of assessment is particularly important when products are intended to be used in Earth System Modelling.
Also, differences between ocean missions may create artefacts in multi-mission data records preventing their use to quantify long-term trends. In this respect a protocol has been proposed to assess the fitness for purpose of multi-mission climate data records and applied to global ocean chlorophyll data.
Reference Measurements Corner
A fundamental approach in the assessment of the fitness for purpose of EO products aims at the in-depth scrutiny of the actual variables being remotely measured against reference measurements or models, to find the possible presence of artifacts, perturbing factors or interactions, the sources of variability or uncertainties that may affect the reliability or even the ultimate physical meaning of the intended measurements.
Results may provide key evidence to inform future developments of EO products or suggest strategies to improve products calibration.
For example, a study has compared the Sentinel-3 Ocean Land Color Instrument (OLCI) ocean radiometric products with reference field measurements. This type of analysis makes it possible to assess the performance of Sentinel-3 ocean products and fosters current developments to improve these products.
The evaluation of adjacency effects around an island such as Lampedusa has been performed with radiative transfer modelling. Adjacency effects for remote sensing over sea pixels are due to radiance contributions from the nearby land surface and are thus perturbing factors. This study supports the selection of a European vicarious calibration site by providing recommendations about its location with respect to the island.
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