Today we are interviewing ESA's Dr Nuno Miranda, who is responsible for the quality of the data from the Sentinel-1 mission.

Sentinel-1 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.

Both Sentinel-1A and -1B satellites are in orbit. They each have a life-span of at least seven years and fly at an altitude of 693 km in a polar, Sun-synchronous orbit so that each has a revisit time of twelve days (at the equator). The repeat cycle of the two-satellite constellation is six days.

They carry an advanced C-band synthetic aperture radar (SAR) at 5.405 GHz. Over land it operates mainly in interferometric wide-swath mode at 250 km and 20 m spatial resolution (for the Ground Range Detected (GRD) products, 5 looks).

The radar images Earth's surface through cloud and rain, regardless of whether it is day or night.

Both Sentinels deliver radar scans for an array of operational services and scientific research in the areas of oceans, ice, land and emergency response.

Nuno Miranda has been at ESA since 2007. He received his Master's Degree in Telecommunications and Signal Processing from the University of Brest in 1998 and went on to specialise in image processing at the University of Bordeaux III in France.

He started collaborating with the Centre D'Etudes Spatiales de la BIOsphere (CESBIO) and with the CNES French space agency in Toulouse where he focused on SAR image segmentation and multitemporal filtering. In 2002, he worked with ESA on the quality control and calibration of the ERS and Envisat (ASAR) missions. In 2005, he teamed up with Altamira on SAR interferometry.

Since 2007 he has been responsible for the performance and algorithm of ESA's and Third Party SAR missions with the Sensor Performance, Product and Algorithm section in the Ground Segment and Mission Operations Department in ESA's directorate of Earth Observation Programmes. As such, he leads the development of the Sentinel-1 processor, responsible for generating the product distributed to the users and he manages industrial contracts linked to the Sentinel-1 Mission Performance Center activities.

ESA: Dr Miranda, could you please explain what your role of Data Quality Manager entails?

Miranda:

Regarding users, the most important thing is that the products are acquired over their area of interest and that these products are consistently good - such that they can be used for their application almost blindly. My role is basically to take care of the product content.

Products are, to me, the most important elements in missions because they are the result of a long acquisition and processing chain and they are our main interface with the users.

Achieving products of constant quality is not something easy. At the end of a satellite commissioning phase, most of the performance requirements impacting the end-products (and thus the end-users) are in general met at 90%. During the routine phase, it is often a challenge to reach the last 10%.

However, the effort produced in this work allows us to increase our understanding and knowledge of the instruments and associated data. This allows the state of the art Sentinel-1 instruments to be exploited to the maximum, and in some cases improve the performance beyond the original specifications.

This experience gained then yields innovative solutions when the satellites show signs of degradation so that the data have seamless performance as an instrument ages.

Thus, on one side we are very close to the machinery whether it is in space or on-ground, on the other side we interface with the users to understand the way they want the data to evolve.

Sentinel-1 is an operational mission, providing data to users for many years. However, our products will need to evolve in order to constantly match users' and operational services' evolving needs. So we have to be close to them and understand their expectations.

The Data Quality Manager therefore interfaces with different people over different domains of expertise, from the satellite manufacturer to the oceanographer who benefits from the beauty of that function.

ESA: What work do you and the Mission Performance Centre (MPC) carry out and which contracts are under your supervision?

Miranda:

In order to perform these activities, I am not alone. I rely on experts in various domains related to SAR in general and Sentinel-1 in particular. They come from different places such as public institutes, universities and industries that are distributed across five different countries.

Each of them is given a clear responsibility, such as radiometric calibration, geometric calibration, SAR processing or the retrieval of geophysical parameters from SAR data.

They are responsible for monitoring the performance evolution in the long run and for proposing corrections or improvements. Being part of the same group, we regularly exchange information that can be of interest to the other, creating coordination and strength.

They are all renowned people in their area of expertise. Therefore, they can report to different SAR communities, creating a direct link with end-users or working groups and providing feedback that eventually creates a new evolution or improvement loop.

ESA: How do you ensure that there is a constant improvement of Sentinel-1 data and what are routine calibration tasks, etc.?

Miranda:

Curiosity and motivation!

In order to improve, it is necessary to increase the knowledge we have on the overall system, and a SAR mission is a complex remote sensing system. It therefore means that you have to be curious and to look at the data always with a fresh eye. It can be any data, like orbital or attitude information, instrument metadata (temperature, power), in-situ measurements and of course the images generated. It is then possible to analyse and identify unexpected or weird characteristics, which require further investigation.

The findings are exposed to the external users in order to be reviewed, commented on and criticised. It is important to be open to new ideas and not to stay fixed on what we believe is true. Exchanges with other colleagues and experts in or out of the MPC are extremely important as it is often the only way to go beyond our own barriers.

This work requires being extremely motivated, open-minded and sustained in the long-term. In the best case, this leads to the discovery of new phenomena or anomalies that can (or cannot) be fixed, or that can lead to lessons-learnt and eventually to improvement for future satellites.

During the first months of the instrument's life, several activities are undertaken to define the initial calibration parameters. This allows us to characterise, for instance, the radiometric and geometric performance of the system. We do this by measuring the response of the instrument over areas of known characteristics, which we refer to as calibration sites.

These sites can be purpose built dedicated targets, forests, oceans or deserts. Each of them has a particular signature that allows us to retrieve characteristics at a certain time. During the routine phase, we collect time series of information from which we can assess the instrument's health, the product degradation and to define counter-measures.

ESA: How do users benefit from the constant control of the data quality?

Miranda:

Sentinel-1 is an operational mission, especially designed to serve Copernicus Services, which allows others to build their own services thanks to the open and free data policy and to the systematic data processing concept.

On the other hand, we are leaving the golden age of SAR with so many missions currently flying, so why would a service provider use Sentinel-1 data rather than others? Partly because the data are free, open and available, but also because the data are good. Users are assured that ESA and its partners, being responsible for the product performance, are constantly looking at data, so these can be used blindly in their services or value-added products.

Users also help ESA in the task of performance assessment. As a matter of fact, we are always tuning our monitoring system to spot any unexpected behaviour, but, unfortunately, we can't spot everything. The users also contribute to this task by providing early feedback on anomalies they find. In most cases, these are false alarms, however, in other cases it can be really constructive feedback that allows us to improve.

The Quality Manager bridges several communities. It requires understanding the users' concerns and eventually transforming these into investigation lines at ground or spacecraft level, interfacing in the back office with experts from the ground segment, spacecraft operations and satellite manufacturers.

ESA: What are the difficulties or challenges you face in monitoring the quality of satellite data?

Miranda:

There are of course many technical challenges. However, I am convinced that there are no issues that cannot be solved, thanks, in particular, to the great ESA experience acquired operating SAR missions (Sentinel-1 is built on the heritage of ERS-1/2 and Envisat's ASAR), and to the support we have from scientists.

In these complex activities, the human factor is very important. Considering that they require coordination with many people collaborating in different domains, it is fundamental to all be working in the same direction.

Since Sentinel-1 is an operational mission, Quality Control and calibration monitoring are crucial tasks that mean having to be permanently vigilant. After more than 10 years on the project, it is not always easy to be fresh and to keep the team motivated and reactive. As I mentioned earlier, if we stop being curious then we will fall into a routine way of working that doesn't allow us to progress and to try to do better.

However, seeing all the good results generated by the scientists and operational service providers keeps me enthusiastic. Sentinel-1 is a game changer and the positive feedback we get from the users is a fabulous reward, giving energy to the whole team.

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.

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.