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Topic 1d – The Sentinel satellites

Why is Earth observation important in climate policy and planning?
in the ’90s to launch satellites– ERS-1, ERS-2, Envisat– and today we have a lot, a fleet of explorers which are scientific satellites, working on precise, scientific objectives. But we also have an operational series of satellites, the Sentinels, and these are in the context of the European Copernicus programme led by the European Commission. The European Space Agency is a scientific, technical, and research organisation. We build satellites. We launch them, and we operate them. But we’re not disconnected from the societal challenges and the real world. In fact, the satellites we build, we try to make sure they are used for real purposes.
And more and more in the definition of our missions, we connect the user requirements to the building of those satellites. So the instruments we put onboard the satellites we try to match with real user requirements from the user communities. We connect to the decision-making world. And we try to respond with the missions we build and we operate to societal challenges which are typically the nexus water, energy, food, or the climate-change issues, or the societal-development issues.
Over the last two or three decades, we’ve learned an awful lot about how to do remote sensing on an experimental basis, on a scientific basis, on a qualitative basis, but also what we call on an operational basis– launching satellites with overlapping programmes that have sensors that are designed to follow on one after the other, to be complementary, and to give us long-term data records. That ability to provide long-term observations of the same thing in the same way is one of the critical requirements for doing remote sensing for climate, for land use change, and for these longer-term, larger-scale global applications, be they environment, agriculture, climate, humanitarian.
So we can see here this is an artist’s impression of one of the latest satellites launched as part of ESA’s Sentinel programme. So the Sentinel programme is designed to be exactly that– a programme that provides continuity, a series of missions that are complementary to one another but also allow us to provide a long-term, gap-free data record. This is Sentinel-2, which has been launched recently and is now providing images over the globe on a weekly, few-daily basis at high resolution, kind of 10 metres, 10 to 40 metre spatial resolution. This is a real step change in our ability to monitor the Earth. That kind of resolution was only really available through high-resolution, limited-coverage, commercial sensors up until recently.
And what ESA decided to do was to launch this series of programmes, of different platforms and sensors which would bridge this gap between high-resolution, temporally-spotty data every few months and every few weeks and lower-resolution tens to hundreds of metres that we can collect on a weekly sort of basis. So the Sentinel programme is designed to provide a kind of continuity in both space and time– so high resolution on days to weekly basis and then lower resolution on multi-day basis, so several acquisitions a day. The Sentinel-2 is collecting data now, and nominally you can see the surface once every few days, about once a week, depending on cloud cover.
There is a second Sentinel-2 sensor that will be launched in the near future which will be complementary. And so the two of them, working in tandem, will provide us with the ability to see the Earth’s surface every few days at this 10-metre spatial resolution. So this kind of operational programme is really a revolution in our ability to understand and monitor the Earth because it allows us to build and develop applications which use these kind of high-resolution data, but in a way that we know that we’re going to get the same data next week, and the week after, and next year, and the year after that, and a decade down the line– so applications which we may not have been able to develop a few years ago because you can’t really do these kind of high-resolution things like agricultural monitoring.
Agricultural monitoring typically requires tens of metres of spatial resolution, and I need to see crops and fields and so on the sort of time frame of, you know, every week or two weeks. That kind of capability was just not really available until recently, until the advent of things like the Sentinel programme. So what kind of detail can we see with these newer instruments? Well, when we look at remote-sensing data, we typically talk about the spatial and temporal resolution of those data products. When we talk about the spatial resolution, this new Sentinel mission, for example, has what we call a 10-metre spatial resolution. What that means, in practise, is that each pixel in the image is around 10 metres in size.
It sees a chunk of the Earth at 10 metres. Now 10 metres might sound like quite a lot, but from a satellite-imaging perspective, this is pretty high resolution. A 10-metre pixel allows us to see roads. It allows us to see buildings. It allows us to see small fields, urban areas. It allows us to see anything larger than about 10 metres. So at kind of 10-metre scale, we can see an awful lot of detail that is very, very important to us in terms of agricultural, climate, environment, land-use change applications.
In terms of time, temporal resolution, what we mean by that is the ability of a satellite and a sensor system to come back and view the same point on the Earth’s surface time and time again. Having two of these instruments in orbit at the same time means that we can pretty much halve that temporal revisit time, so we can see every point on the Earth’s surface every five days or so. Now, of course, the huge advantage of that is not just there’s a shorter gap in between observation. But it means that if one observation is obscured by clouds, then we may get a view five days down the line that’s cloud-free.
We have a lot more opportunity to see the Earth’s surface– not just shorter time frame, but more opportunity because there are more opportunities to pass over when there may not be clouds covering the surface. So these kind of operational programmes are designed to address applications which would not be able to be carried out unless we had these guarantees of these long-term, regular data records. The problem in the past was that either the spatial resolution was very low, not enough to monitor changes because we cannot really see the heterogeneity of the surface, or that the time series were not long enough to really look at changes from one year to another.
There is variability from one year to the next, just because of weather conditions or climatic, environmental conditions. And we don’t want to look at these changes from one year to the next. We want to look at changes over a period of 20 years so that we really see the trends. And the problem in the past was that we don’t have these long-term series at the resolutions we wanted.
But the new Copernicus Sentinel data, which has become available this year, will really represent a unique opportunity because for the first time we will have systematic acquisitions continuously in our resolution in the order of 10, 20 metres and covering the full Earth and guaranteed data for a period of 20, 30 years, which is exactly what we wanted to have in order to monitor these land-cover changes and vegetation dynamics on Earth. With Sentinel-2, we’ll have for the first time a routine, global picture of the Earth in the optical domain of 10-metre resolution. This will provide an incredible insight into the state of our planet and how it’s changing. It’s the beginning of environmental democracy.

The Sentinel satellites are part of the Copernicus programme, which is an initiative headed by the European Commission in partnership with the European Space Agency. Copernicus is designed to observe the Earth for the benefit of the environment and European security – which, in the civilian sense, includes food security, domestic disaster response, protecting and maintaining human rights (migration, IDP and refugees etc) – and includes observations from the ground complementing those from space.

Sentinel-1 is a family, or constellation, of three satellites, the first of which launched in April 2014. Sentinel-1 uses radar imaging, to look at the solid earth (topography, geology, ice sheets and dynamics etc).

Sentinel-2 comprises two satellites operating on opposite sides of the same polar orbit and uses multi-spectral high-resolution imaging from within the optical range. We will look at its data frequently during this course. It is used for land monitoring including imagery of vegetation, soil and water cover, inland waterways and coastal areas. Some of its data are also available to the emergency services.

Sentinel-3 consists of a pair of satellites to measure sea-surface topography, sea and land surface temperature, ocean colour and land colour. The first of these satellites launched on 16th February 2016.

Sentinel-4 will be part of the next generation of geostationary weather satellites from Europe – Meteosat Third Generation, and will collect data on atmospheric composition from 36,000 km away.

Sentinel-5 will be part of the next generation of polar-orbiting weather satellites from Europe – EPS-SG and will monitor the atmosphere. It also has a precursor mission to fill a data gap caused by the end of observations from Envisat.

Sentinel-6 is also known as Jason-CS because it is intended to offer continuing service in the same fields as the Jason satellites, measuring global sea-surface height using a radar altimeter .

Optical EO data products

The basic information collected from the satellites is reflected EM energy in various slices across the reflected (optical) and thermal part of the EM spectrum. However this reflected energy in itself isn’t particularly useful – we can turn it into nice images, which are certainly useful for some applications, mapping and land use for example. But to add value and interpret what the satellites are seeing, we use models that relate this reflected radiation to other more general properties of interest – things like vegetation amount and function, crop yield, ocean productivity and so on. This process forms the basis of how we turn satellite data into more generally useful quantitative ‘information’ .

Featured Educators:

  • Dr Mathias Disney

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  • Dr Pierre-Philippe Mathieu

Optional Further Reading:

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Explore the Imagery, Data and Satellites:

You can explore the imagery, data and EO satellite missions from this topic more fully using the links and downloads on the next step.

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Earth Observation from Space: the Optical View

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