As New Space providers continue to strengthen the worldwide offering of remote sensing data, ESA supports the harmonisation of calibration activities and development of reference calibration and validation (Cal/Val) sites.

For more than four decades, ESA’s Earthnet programme has provided the framework to access Earth Observation (EO) data from non-ESA missions, via ESA’s Third Party mission (TPM) programme.

ESA bolsters the New Space economy by continuing to expand its offering of commercial EO data, offering free data access for research and scientific purposes.

However, as New Space activity ramps up, the arena is challenged by a heightened need for harmonisation in Cal/Val approaches and common Cal/Val reference networks dedicated to very high resolution (VHR) missions.

The Cal/Val of mission data, and in general the data quality, serve to build confidence in the accuracy of EO data. This is achieved not only with sensor calibration, but also with algorithm verification, validation of the data and inter-comparison with other missions to quantify uncertainties.

Cal/Val requires dedicated measurements taken from established reference Cal/Val sites and networks.

There are a plethora of Cal/Val reference centres available and currently being set up - such as RadCalNet, SARCalNet and HYPERNETS – as well as several Cal/Val sites set-up by national space agencies or institutions to fulfil specific needs.

However, each site is dedicated to a specific task with little flexibility for scalability or adaptation to accommodate new requirements from the remote sensing data user community.

ESA is exploring the idea of an open and pioneering reference site, with a recently developed concept known as the Cal/Val Park, dedicated to HR and VHR optical missions.

The Cal/Val Park aims to test new Cal/Val concepts in temporary trials, while building on existing Cal/Val technologies, as well as developing new methods.

It is a multi-agency effort – open for use by both institutional and New Space – thus ensuring a cost effective and synergistic approach.

For over four years, the Radiometric Calibration Network (RadCalNet) has boosted harmonisation of optical imager radiometric data calibration.

RadCalNet includes multiple sites as part of a network and aims to provide automated surface and atmosphere in-situ data for the purpose of calibration of optical imager radiometric data with frequencies in the visible to shortwave infrared.

As part of the automated RadCalNet processing, surface reflectance and atmospheric measurements are converted to Top of Atmosphere (TOA) reflectance data. The first RadCalNet user workshop was held in 2021 and was attended by over a hundred users worldwide.

Synthetic Aperture Radar (SAR) missions require reference calibration targets to ensure long term monitoring of instrument, calibration and data quality stability.

Currently, the Cal/Val of SAR data is composed of a variety of broadly specified tasks, such as radiometric, geometric, polarimetric and interferometric calibration and quality assessments.

Several calibration sites containing different types of target exist to perform these tasks (see examples in the Point and Distributed Targets database). However, the information about the reference targets is not always provided with the required level of details appropriate for their consistent use in a certain Cal/Val exercise.

Differences in the interpretation of the reference data and the lack of standardised analysis procedures can lead to inconsistent results.

The upcoming SAR Calibration Network (SARCalNet) is a CEOS SAR WGCV initiative, which aims to fill this gap by defining requirements on the information to be provided for different Cal/Val targets, and establishing a curation procedure of the provided information. Additionally it recommends standardised image analysis procedures, further promoting consistent calibration/validation and inter-sensor comparisons.

Implementation of the SARCalNet database is ongoing, with a first version open to the public expected by the end of 2023.

There are also reference networks under development for very specific application areas, such as the Hyperspectral Networks for Water and Land Surface Reflectance (HYPERNETS).

HYPERNETS includes a new lower cost hyperspectral radiometer and embedded calibration device for automated measurement of water and land bidirectional reflectance, with associated uncertainties.

Its reference data can be used to validate surface level products from metre-scale optical sensors, providing for example high resolution water products for applications such as port dredging.

As more and more data from commercial and institutional satellites are utilised by worldwide users, there is clearly a move towards harmonisation of Cal/Val approaches.

Strategic use of Cal/Val reference networks, such as RadCalNet, SARCalNet and HYPERNET is key, as well as the creation of innovative harmonisation sites such as the proposed Cal/Val Park.

Community discussions on Cal/Val reference networks and recent changes in the VHR data landscape were presented at the 2022 VH-RODA workshop (Very High-resolution Radar & Optical Data Assessment) held in ESA/ESRIN. VH-RODA presentations are now available online.