A global remote sensing initiative backed by ESA is advancing a growing area of research in the field of satellite data validation.

Hyperspectral sensors mounted on unoccupied aerial vehicles (UAVs) are emerging as an important asset for assessing the accuracy of Earth observation surface reflectance data as they can cover larger areas and a wider range of environments than traditional sampling devices.

However, to unlock this potential, a globally accepted protocol for the collection and use of these observations must be established.

With this objective in mind, the ESA-supported Surface Reflectance Intercomparison eXercise for Vegetation (SRIX4Veg) – which began in 2022 – is working to put forward good-practice procedures agreed through a series of workshops and a dedicated UAV campaign.

The latest SRIX4Veg meeting was held at ESA’s centre for Earth observation (ESRIN) in Italy on 23 and 24 November 2023 as a follow up to the UAV exercise, which took place in summer 2022.

This campaign – implemented over the Las Tiesas experimental farm in the east of Spain – employed a round-robin format in which seven teams from Europe and beyond used UAVs to collect data over the area using two approaches.

First, the teams used their own individually developed methods to gather observations; then the data collection was repeated while following a provisional protocol prepared by SRIX4Veg experts.

At the recent SRIX4Veg workshop – which was organised with the UK’s National Physical Laboratory – participants presented the results of the round-robin exercise before outlining a protocol template for using UAV-mounted hyperspectral instruments for validation purposes.

These topics were discussed in-person by experts from organisations across the world, including ESA, Geoscience Australia, the University of Wageningen, National Research Canada, the German Aerospace Center (DLR), the German Research Centre for Geosciences (GFZ) and Earth observation specialist GEO-K. Several participants also joined remotely.

On day one of the workshop, participants focused on the need for an inter-comparison exercise to assess differences arising from contrasting sampling protocols implemented by each team.

After presenting the results from the drone monitoring campaign, it was shown that employing a standard data collection method was successful in reducing this variation, demonstrating the value of using an agreed protocol.

Based on the outcomes of the workshop, members of the SRIX4Veg community will now start work on preparing a good practice protocol, which is expected to be finalised in the coming months.

Following the meeting, Niall Origo, Senior Research Scientist at the UK’s National Physical Laboratory, commented: “UAV-mounted hyperspectral sensors are an important developing area of research in satellite product validation.

“These sensors overcome challenges posed by traditional validation techniques by sampling larger areas during the satellite overpass time. They can also collect measurements in environments that were previously inaccessible using handheld instrumentation, such as forests.

“By preparing and agreeing on good practice procedures, SRIX4Veg is supporting the provision of high quality and reliable hyperspectral measurements from UAVs to support the validation of data from a variety of space-borne sensors.”

Valentina Boccia, Data Quality and Cal/Val Manager at ESA and coordinator of SRIX4Veg, concluded: “SRIX4Veg is a crucial activity for ESA and is paving the way for high quality validation of both current and future Earth observation satellite products. This is extremely relevant both for ESA and its cooperating international partners involved in the activity.

“In addition, the endorsement of the Committee on Earth Observations (CEOS) is enabling the outcomes of SRIX4Veg to benefit scientists and satellite product validation teams from all around the world.”

SRIX4Veg is funded by ESA as part of the broader Fiducial Reference Measurements for Vegetation (FRM4Veg) activity.

The activity is endorsed by the CEOS and run by ESA in cooperation with the United States Geological Survey (USGS), Geoscience Australia, the Commonwealth Scientific and Industrial Research Organisation (CSIRO), the Italian Space Agency (ASI) and DLR.