Long-term availability of Earth observation data from a given instrument is crucial to many data users and their research—but just how does it work?

ESA has been gathering Earth observation data for a long time: the Agency began systematically archiving data from other agencies’ satellites, so-called Third Party Missions, in the early 1980s before its own Earth observation programme kicked off in 1991 with the launch of its first satellite, ERS-1.

ERS-1’s mission ended in 2000 and its successor, ERS-2, was active until 2011, but the foundation Earth Science measurements that the Agency initiated with their launch in 1991, continue to be acquired to this day. These measurements include high-resolution C-band Synthetic Aperture Radar imagery over land and ocean, land and sea surface temperatures and height and scatterometer-based recordings of global wind fields.

These datasets were extended first through ERS-2 and then through ESA's follow-up environmental satellite, Envisat, with scatterometer measurements being taken over by EUMETSAT’s MetOp satellites. Along the way, new measurements were also started: ERS-2 began recording atmospheric ozone concentrations while Envisat started measuring ocean colour, low-resolution multispectral images over land and levels of greenhouse gases in the atmosphere.

Most of the measurements initiated by the ERS missions and then expanded by Envisat, are now continued by the Copernicus Sentinel missions, in particular by Sentinel-1, Sentinel-3 and Sentinel-5 Precursor.

Earth observation missions ? ESA
Earth observation missions


The importance of mission continuity

The main reason Earth observation satellites are so powerful as information tools is their capacity for regular revisits. An aerial mapping aircraft, subject to turbulence, can seldom reproduce its previous trajectory, but a satellite in orbit does so effortlessly, time after time. As a consequence, consecutive images or data of a given location can be compared on a precise basis.

This ability adds a once-unattainable time dimension to remote sensing: differences between images become as illuminating as the images themselves, capturing the complex, dynamic nature of the Earth system and humankind’s increasingly disruptive role within it.

With 10, 20 or even 30 years of data available within the archives, trends can easily be observed within areas, such as urban growth, desertification, shrinking polar ice and the warming and rising height of oceans.

The spectacular progress in information and communication technology has played a facilitating role, argues Henri Laur, Head of the Earth Observation Mission Management Division at ESA, "For all that today's satellites are better and more robust than their predecessors, the most significant technical progress has been on the ground. Enhanced computer power lets us take fresh looks into the archive in a way that was quite impossible 20 or 25 years ago. In those days researchers were only able to use a few images at a time, today the internet provides access to as many images as they want."

Engaging user organisations

ESA, as a research and development organisation, does not have the mandate from its Member States to use its own budget to acquire the same remote sensing data parameters indefinitely, but needs to find partner organisations willing to take up responsibility for continuing and funding sustained measurements.

The Agency does this by demonstrating the usefulness of new types of instruments and data products, as with the Earth Explorer missions. Once this has been achieved and a community of users has sprung up around it, the hope is that other institutions will be inspired to take up the torch. One example of this is how control of Europe’s MeteoSat weather satellites was transferred from ESA to the European Organisation for the Exploitation of Meteorological Satellites, EUMETSAT, which was specifically set up for this purpose.

In the process, the satellites themselves shifted from a research to an operational role, providing near-real time data for weather forecasts. ESA retains a role in the design and procurement of future satellite systems, such as the forthcoming MeteoSat Third Generation satellites.

A similar shift occurred with the European Copernicus programme in which the Sentinel series provide data for operational Global Monitoring for Environment and Security services. ESA financed only the first set of Sentinels, whose operations and future replacements are funded by the European Commission.

Looking beyond individual missions

Mission continuity is good for most satellite sensors, as Laur explains, "If you are measuring something new, you should go on doing it for as long as possible. Take one example from ESA’s Earth Explorer missions each of which focus on a particular parameter of Earth's environment: CryoSat’s nominal design lifetime was three years, but with such a useful parameter as polar ice thickness its operational lifetime is being extended for as long as possible."

He adds that the issue of mission continuity is high on the agenda of all Mission Managers: “Firstly, that means ensuring the mission stays healthy to go on gathering useable data for as long as possible. It also means the Mission Manager adopts something of an ambassadorial role with the data being gathered, to raise awareness of their usefulness to a level that other institutions consider supporting a similar satellite in their future plans.”

Such follow-up missions do not necessarily have to be European, as long as continued and easy data access is assured to the user community. The ideal situation is to have multiple satellites observing the same parameters, enabling cross calibration and validation and making the final measurements that much more robust.

Third Party Missions offer alternative data sources

ESA Third Party Missions ? ESA
ESA Third Party Missions

The potential of data gaps arising is a general concern across the wider Earth observation community. The Third Party Missions are non-ESA missions in which the Agency invests through its Earthnet programme in order to provide data access to European users for research and development purposes. A similar approach exists for European operational services within the Copernicus Contributing Missions scheme. These data are used not only for mission continuity, but also as alternative data sources in case one of ESA’s own satellites stops working.

As an example, the use of Third Party Missions was crucial when the Envisat mission ended in 2012 before the Sentinel missions were ready to be launched. The first Sentinel was launched only in April 2014, creating a two-year data gap.

The fall-out for climate research and other scientific and operational applications when such a data gap occurs can be dramatic, depending on how long it continues.

"If measurements of sea surface height are interrupted for a year then that's survivable, researchers can extrapolate through the gap, but if it goes on longer then the problem grows, and the reliability of the dataset is put in doubt,” explains Laur. "The problem is much more acute for activities dependent on near-real time data, such as oil spill monitoring."

Up to today, ESA has been very successful at ensuring that comparable data products remain available and accessible, not just for the life of one individual mission, but beyond it.