Today we are interviewing ESA's Dr Ferran Gascon who is responsible for the quality of the data from the Sentinel-2 mission.
Sentinel-2 is part of the family of EU-owned satellites, which are developed and operated in the context of Copernicus, the European Union's Earth Observation and Monitoring Programme.
Sentinel-2A has been in orbit since June 2015 while Sentinel-2B is scheduled to join its twin in March 2017. They each have a lifespan of at least seven years and fly at an altitude of 786 km in a polar, Sun-synchronous orbit.
They both carry a multispectral imager (MSI) covering 13 spectral bands (443–2190 nm) with a swath width of 290 km and spatial resolutions of 10 m (4 visible and near-infrared bands), 20 m (6 red-edge/shortwave-infrared bands) and 60 m (3 atmospheric correction bands).
As a two-satellite mission coverage and global revisit times are optimised: revisit at the equator is reduced to five days with both satellites in orbit and an even faster revisit for higher latitudes.
The mission's main applications range from monitoring agriculture, forests, land-use change, land-cover change, mapping biophysical variables such as leaf chlorophyll content, leaf water content and leaf area index, to monitoring coastal and inland waters, risk mapping and disaster mapping.
Born in Girona, Spain, Ferran Gascon has been at ESA since 2002. In 1998 he received his Master's Degree in Telecommunications Engineering from the Polytechnic University of Catalonia. He later spent three years at the Centre d'Études Spatiales de la Biosphère and obtained his PhD in remote sensing from the University of Toulouse III.
In 2002, he was recruited as Research Fellow at ESTEC, ESA's research and technology centre in the Netherlands. His research focused on the retrieval of biophysical variables using optical imaging sensors. In 2004, he joined ESA's Directorate of Technical and Quality Management to work on the development of the Sentinel-2 mission for aspects related to data processing, calibration and validation.
In 2009, he moved to ESRIN, ESA's centre in Italy. Here he works on sensor performance, products and algorithms of optical high-resolution missions in the Ground Segment and Mission Operations Department of the Directorate of Earth Observation Programmes. As the Sentinel-2 data quality manager, he is in charge of the development and operations of the mission for all aspects related to the sensor calibration, products validation, products quality control and the definition of products and algorithms evolutions.
ESA: Dr Gascon, could you please explain what your role of Data Quality Manager for Sentinel-2 entails?
The main goal of my job is to ensure that products distributed to users are of good quality, in the sense that they meet mission requirements and user needs.
Keeping this background objective in mind, the activities of a data quality manager evolve over the life of the mission.
During the development phase, work involves the definition of the products, the implementation of the associated algorithms in operational processors, the development of the calibration, the validation and the quality control methods.
Once actual products start flowing during the operational phase, the data quality manager and his team ensure that the distributed products meet a set of quality criteria, including the mission requirements. Activities include regular fine-tuning of the Sentinel-2 multi-spectral instrument and the ground processing chains in the procedure called calibration. In parallel, the content of the products generated is compared to independent references in a process called validation. This includes checks on both the radiometry, for instance the sensor modulation transfer function or the signal-to-noise ratios, and the geometry, verifying features such as the geolocation of the pixels and to see if images acquired on different dates can be superimposed.
Finally, my tasks also include the management of product and algorithm evolutions, either for existing ones that are improved, or defining and prototyping new ones that are proposed to the Mission Manager. This based on recommendations from the Sentinel-2 Quality Working Group and users such as the ones involved in the Sentinel-2 Validation Team. This requires bridging with the users to understand their expectations and evolving needs.
ESA: What work do you and the Mission Performance Centre carry out and which contracts are under your supervision?
For performing all the activities, I rely on a team of experts covering all data quality domains. The team includes experts from satellite manufacturing companies, national space agencies, public research laboratories and small/medium enterprises that are distributed across five different countries in Europe.
Each institution is assigned a responsibility, such as sensor calibration, product radiometry validation, product geometry validation or algorithms development. All together, they are responsible for monitoring the performance evolution in the long run and for proposing and implementing corrections or improvements. Being part of the same group, we regularly exchange information that can be of interest to the other, to ensure coordination and to find synergies.
In terms of contracts, most of the experts are included in the consortium called Mission Performance Centre. Additionally, we have a contract with the French space agency CNES, where they provide valuable support for the validation of the generated products and recommendations on how to improve them. This contribution brings to the team CNES' long-lasting experience in optical imaging sensors which has been built for more than 30 years with missions such as SPOT or Pléiades.
ESA: How will the upcoming launch of Sentinel-2B change your work and what differences will it bring?
The arrival of the second satellite unit will be like the arrival of a second child in a household. During the first months much attention will go to the newcomer, but at the same time the older brother will not be forgotten. Just after launch, the data quality team will focus on Sentinel-2B to ensure a successful commissioning. In parallel, attention remains dedicated to Sentinel-2A, performing routine calibration and quality control activities to ensure its nominal operations.
Once Sentinel-2B commissioning has been completed, team activities will progressively evolve to have a more equal share between the two satellites. Some activities will benefit both satellites at the same time, such as the improvement of the data processing algorithms and products.
During routine operations of the full constellation, the data volume generated by the mission will double and this will significantly impact the efforts required for controlling the quality of the production.
The two satellites, like two children, come with their own specific needs. Some of these are already known, such as their different instrument spectral responses, but others will only be discovered in-flight. New methods or algorithms might need to be developed ad-hoc for each spacecraft.
ESA: How do users benefit from the constant control of the data quality?
To answer this question we should imagine a mission without constant control of the data quality... anomalies in the behaviour of the space and ground segments might translate into varying-quality products being distributed to users. On the users' side, the products received would soon be considered as unreliable or inaccurate for operational applications such as smart farming or ecosystems monitoring. Users could then switch to other data sources, resulting in a failure for the mission.
Ensuring good quality in terms of products format and content, for both radiometry and geometry, is an essential component of the mission. Anomalous products are systematically filtered before they reach the user community.
During the first year and a half of operations, Sentinel-2A has been a great success, with largely positive feedback from users on the quality of the products. It has gone beyond initial expectations for some product features such as the signal-to-noise ratio. Operational applications can therefore find an excellent input for their development.
At the same time, we should also say that we benefit from the users through their feedback. The Sentinel-2 user community scrutinises a large number of products, which complements our quality control activities and sometimes triggers earlier detection of unexpected anomalies, or provides recommendations on how products could be improved.
ESA: What are the difficulties or challenges you face in monitoring the quality of satellite data?
During my daily work I have to interface with people who play different roles in the mission such as specialists covering the different elements of the spacecraft and the Payload Data Ground Segment, as well as the user community providing feedback on the products. This requires a significant amount of coordination as often anomalies or evolutions involve many people.
Another challenge comes from the fact that Sentinel-2 is a mission that generates a large and continuous flow of data. At times it creates overflows of the team's capacity to deal with several parallel anomalies that have to be solved. This means prioritising tasks so that issues that could have a larger impact on users are tackled and solved first. It is a continuous effort because anomalies can appear at any time but quality has to be ensured all year round.
About the Sentinels
The Sentinels are a fleet of dedicated EU-owned satellites, designed to deliver the wealth of data and imagery that are central to Europe's Copernicus environmental programme.
In partnership with EU Member States, the European Commission leads and coordinates this programme, to improve the management of the environment, safeguarding lives every day. ESA is in charge of the space component, responsible for developing the family of Copernicus Sentinel satellites and ensuring the flow of data for the Copernicus services, while the operations of the Sentinels have been entrusted to ESA and EUMETSAT.