Climate Change Profile

Sentinel Satellites Initiate New Era in Earth Observation

The European Space Agency’s Josef Aschbacher talks with Eos about the new Sentinel fleet of Earth observing satellites.

By and

With the European Space Agency’s (ESA) Sentinel-1A satellite expected to enter into its operational phase later this summer, it ushers in a new series of Earth observation satellites. The first generation of the fleet of satellites, which will total 20 when all are launched, will comprise “the most comprehensive Earth observation system worldwide,” according to Josef Aschbacher, head of program planning and coordination for ESA’s Earth Observation Programs directorate. The satellites are a key part of Copernicus, the European Earth Observation Program that from 1998 to 2012 was called the Global Monitoring for Environment and Security (GMES) program.

Aschbacher recently spoke with Eos about Copernicus, which is a joint initiative of the European Commission and ESA; the Sentinel fleet of satellites; free and open data access; and the relationship between the Sentinels and some other Earth-observing satellites.

He told Eos that because of the resolution scale and the way the Sentinels are built, they will be used mostly for environmental applications. “Literally, it is for the benefit of mankind,” he said of the Sentinel fleet. “The biggest hope is to have an everlasting observation system based on Earth observation data that provides all these observations, ad infinitum, eternally, in order to serve many disciplines on our planet, from agriculture to marine science to climate change to forestry to security to disaster management and so on.”

The Copernicus program includes space, services, and in situ components, with the space component comprising about 80% of the budget, he explained. Within the space component are the Sentinels and the use of other data from contributing missions available on the market for free or on a commercial basis.

The Sentinel Satellites

Sentinel-1A, which was launched on 3 April, is a polar-orbiting radar mission with a 5-meter ground range resolution and a 12-day orbit repeat cycle at the equator. Its applications comprise land and ocean monitoring, including sea ice detection, oil spill monitoring, ship detection and tracking, and flood mapping. Although the satellite is still in its commissioning phase, it already has been useful for some important monitoring, including providing data that helped map floods in Bosnia and Herzegovina in May. When its twin, Sentinel-1B, is launched in 2016, the pair will have a 6-day repeat cycle.

The Sentinel-1 family helps ensure continuity with ESA’s Envisat satellite, which had a 35-day repeat cycle and stopped working in April 2012. Envisat’s failure at the end of 10 years—twice its nominal life—“was a bit of a shock for us. It certainly put urgency on the Sentinel development,” in particular Sentinel-1 with its critical radar capability, Aschbacher said.

Other members of the fleet will include Sentinel-2A and -2B, a high-resolution optical mission for land monitoring; Sentinel-3A and -3B, a medium-resolution imaging and altimetry mission for marine observation; Sentinel-4A and -4B, a geostationary atmospheric chemistry mission­; Sentinel-5A, -5B, and -5C, a low Earth orbit atmospheric chemistry mission; and Sentinel-6A and -6B, an altimetry mission. Also, Sentinel-5p is a low Earth orbit atmospheric chemistry precursor mission. In addition, several Sentinels will have recurrent units.

“This really is a suite of sensors in one programmatic context—Copernicus—which doesn’t exist anywhere else in that magnitude covering the major domains in terms of instruments and sensors,” Aschbacher said. “We are starting to develop a long-term operational program with satellites that are providing measurements for decades from now, up to 2030.”

Sentinels and Landsat

This first image from Sentinel-1A, acquired on 12 April 2014 just 9 days after launch, captures Brussels and its surroundings in Belgium. The image also shows a more detailed view of the city in the inset. Green corresponds to vegetation, red-blue to urban areas, white to high-density urban areas, and black to waterways and low-reflective areas such as airport runways. Credit: ESA
This first image from Sentinel-1A, acquired on 12 April 2014 just 9 days after launch, captures Brussels and its surroundings in Belgium. The image also shows a more detailed view of the city in the inset. Green corresponds to vegetation, red-blue to urban areas, white to high-density urban areas, and black to waterways and low-reflective areas such as airport runways. Credit: ESA

Aschbacher discussed the Copernicus program and the Sentinels in the context of other Earth observation systems. He said the program has “extremely good cooperation” with U.S. federal agencies involved with distributing Landsat data. “The plan is that we have both science groups and technical groups to work on the complementarity of Landsat and Sentinel-2 data, for example—because Sentinel-2 and Landsat are quite similar in terms of sensors, resolutions, and purpose.”

Aschbacher said that with Landsat and the Sentinel-2 pair working together, global coverage would be more frequent. Landsat along with Sentinel-2 “is a great combination for a number of applications, including for natural resources, agriculture, and forestry,” he said.

ESA and U.S. federal agencies involved with Landsat have cooperated for a number of years already, and “we are working now more intensely on how this can be better exploited, because this would be fantastic to better use Landsat together with Sentinel-2,” Aschbacher said.

The United States is interested in access to data from other Sentinel satellites as well, he added, noting that there are discussions regarding how to better exchange data and allow ESA and U.S. federal agencies involved with Landsat each to have access to the other’s respective data.

“We are very open and very constructive in finding the best ways of giving the best access to [Sentinel] data,” Aschbacher said. “Europe has been the beneficiary of using Landsat. I would hope that in the future, also, America can be a beneficiary of using our Sentinels.”

Free and Open Access to Data

A policy of free and open access to environmental data—with some security and other exceptions—from the Copernicus program, including from Sentinel satellites, in some ways follows on the Landsat open data policy, Aschbacher said. The Landsat policy “was one of our main influences or inspirations for the data policy,” he stated.

When the Sentinel-1 calibration validation is completed later this summer, “we expect to widely open distribution to everyone,” he said. “I expect that the free and open data will be a big change for the community, and this is not only the specialized Earth observation community, but really the IT [information technology] community at large [that] is combining different sources of information.”

A European Commission regulation on free and open Sentinel data, which came into force in December 2013, noted that the data could help users “to capitalize on the social benefits arising from an increased use” of the data.

Aschbacher said that the adoption of the data policy “was quite a debate.” The debate “was mostly between some commercial data suppliers who are in the domain which is complementary to the Sentinels but maybe close enough in terms of resolution so that they feared that free data from the Sentinels would impact on their market,” he said.

Widening the Market and the Access to Data

“I think what convinced them, at the end, that a free and open data policy may, first of all, not hinder their market but maybe even be supportive to their market is, first of all, with free data—free Sentinel-2 data, for example—you really promote the use of Earth observation at large to many users [who] don’t exist today and which they may not even have access to. It’s also a campaign to widen the market and widen the access to these data,” he said.

“Secondly, if they could offer their commercial data in combination with free data from the Sentinels, they have a product which is unique and which may be a good market entry from many segments,” he noted.

Although some companies have expressed concern about the data policy, Aschbacher stressed that commercial and Sentinel systems can work complementarily. He said that commercial systems often have a resolution of a half meter or 1 meter—much better than the Sentinel satellites—that can focus on details for city planning, security, or other applications. Sentinel and commercial systems could, he said, be used “in combination: one for the large overview and the other one really to focus on a certain area [to] zoom in and get information on the particular area of interest that you need and a much better geographic resolution.”

The European Association of Remote Sensing Companies has supported the adoption of a free and open data policy for the Sentinel data as the “most effective way to develop the downstream market (and generate pull on the upstream),” among other reasons, association secretary general Geoff Sawyer noted in a March 2014 presentation about industry’s perspective on Copernicus and big data.

These satellites will be providing an enormous amount of data: Whereas Envisat provided 0.3 terabyte (TB) per day, each Sentinel-1 will provide 1.8 TB/day, with Sentinel-2s providing 1.6 TB and Sentinel-3s providing 0.6 TB, Aschbacher said. Access to the large volume of Sentinel data “will be a bit of a hurdle at this point in time, but with the advancement of technology, this will be figured out,” he said, acknowledging an initial potential bottleneck.

Aschbacher compared the availability and amount of satellite data today with what he worked with as a doctoral student. He used data from NASA’s Nimbus satellite, launched in 1978. Accessing that data on huge magnetic tapes—for which he received special access through his professor and for which he remains grateful to NASA—was cumbersome, he said, adding, “Data were really handled like gold in my age as a student. There, you could only dream of getting free and open access to data.”

Financial Challenges

Another challenge that needed to be overcome was finances for the Copernicus program. The program currently is fully funded through 2020, he said.

Aschbacher said financial discussions were complicated by the number of parties involved, including member states of ESA and the European Union, ESA itself, the European Commission, and the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT), the latter of which is a partner for Sentinel-3, -4, and -5 operations. He added, “To get the financing organized in times of financial crisis [was] not an easy thing.” However, he said that Copernicus’s usefulness to society and cost-benefit analyses that indicated the program’s economic value helped convince parties to fund the program.

With various hurdles to the program overcome, Aschbacher admitted that “when Sentinel-1A was launched, I had an emotional moment. I thought, goodness me, 16 years ago”—since the 1998 Baveno Manifesto that marked the start of GMES—“when we [began] putting this together, and working hard over all these years to make it happen, and despite arguments against, and despite resistance of people [who] needed to be convinced, now it flies. Of course, one has to see it through [to] the end and the whole thing has to fly as a constellation, but yes, I think it is pretty much an achievement.”

For more information, see https://sentinel.esa.int.

—Randy Showstack, Staff Writer

© 2014. American Geophysical Union. All rights reserved.