Many users employing Earth observation data from a given instrument would like to be sure they can continue to rely on its availability further into the future. How does this work in practice?
ESA has been gathering Earth observation data for a long time: the Agency began systematic archiving of data from other agencies' satellites in the early 1980s - known today as 'Third Party Missions' - while its own Earth observation programme commenced in 1991 with Europe's first Earth Resources Satellite (ERS) satellite.
The foundation Earth science measurements that the Agency initiated in 1991 continue to be acquired to this day, despite ERS-1's mission ending in 1995 and its successor ERS-2 being shut down in 2011. 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.
The task of extending these datasets was transferred first from ERS-1 to ERS-2 and then to ESA's follow-on Envisat environmental satellite, with scatterometer duties passed to EUMETSAT's operational MetOp series of satellites. And along the way new measurements were started: ERS-2 began recording atmospheric ozone concentrations, for example, while Envisat commenced ocean colour measurements, low-resolution multispectral images over land and levels of greenhouse gases in the atmosphere.
So when it comes to 'mission continuity' - ensuring that comparable data products remain available not just for the life of one individual mission but beyond it - ESA has managed a more or less 100% success rate, up to the present day.
The current anomaly with the Envisat satellite, which began on 08 April, highlights the importance of maintaining this data continuity.
After Envisat, the Sentinels
The ageing Envisat satellite was funded to continue operations until the end of 2013, after which the Sentinels would be needed in orbit to ensure mission continuity. Should Envisat prove unrecoverable, there would be, at least, a year-long gap in the data Envisat acquires until the launch and start of operations of the first Sentinel missions.
The Sentinel series are families of satellites providing data for operational Global Monitoring for Environment and Security (GMES) services, supported by the European Commission. ESA is financing only the first set of Sentinels, whose operations and future replacements are to be funded by the EC.
The fall-out for climate research and other scientific and operational applications could be dramatic, depending on how long the data gap continued. "Crucial long-term observations would be interupted, and we'd have some very upset users!" says Henri Laur, Envisat Mission Manager. "Some might not come back to us, or to space at all.
"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. The problem is much more acute for activities dependent on near-real time data, such as oil spill monitoring."
Laur remains an optimist on the issue, but he cautions that ESA cannot always go on building recurrent versions of satellites if their measurements are not judged essential by others.
ESA's Directorate of Earth Observation is helping to foster broader take-up of remote sensing through various activities including the Data User Element and Earth Observation Market Development programme. These involve working with new groups of end-users and businesses to develop and demonstrate novel uses of Earth observation.
But all this comes down to having the satellites in space - and maintaining the collective will to keep them there.
Why is mission continuity so important?
The main reason why Earth observation satellites are so powerful an information tool is their capacity for regular revisits. An aerial mapping aircraft, buffeted by turbulence, can seldom reproduce its previous trajectory but a satellite in orbit does so effortlessly, time after time. 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 increasing disruptive role within it. And with 10, 20 or even 30 years of data available within the archives, subtle trends can be teased out - such as urban growth rates and desertification, shrinking polar ice and the slight but steady warming of the oceans, as well as their interannual circa 3 mm height rise.
The spectacular progress in information and communication technology has played a facilitating role, argues Henri Laur: "For all that today's satellites are better and more robust than their predecessors, the most significant technical progress has been in on the ground. Enhanced computer power lets us take fresh looks into the archive in a way that was quite impossible 10 or 15 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."
User organisations taking responsibility
The only problem is that ESA, as a research and development organisation, does not have the mandate from its Member States to continue acquiring the same remote sensing data parameters indefinitely without outside financial support. Eventually the Agency needs to concentrate on demonstrating new types of instruments and measurements, let user organisations take responsibility for continuing existing measurements.
Once the usefulness of a particular data product has been demonstrated and a community of users has sprung up around it, the hope is that other institutions will be inspired to take up the torch. The textbook case is how control of Europe's MeteoSat weather satellites was transferred from ESA to the European Organisation for the Exploitation of Meteorological Satellites, EUMETSAT, specifically set up for this purpose.
In the process, the satellites themselves shifted from a research to 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 and MetOp Second Generation).
Looking beyond individual missions
But mission continuity is good in principle for virtually all satellite sensors, explains Laur: "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 one focused on a particular parameter of the Earth environment: CryoSat-2's nominal lifetime is three years, but with such a useful parameter as polar ice thickness it will be useful to extend those measurements for as long as possible.
"The same is true in principle for most of the Earth Explorers, with the exception of GOCE which is measuring a more accurate steady-state geoid map of Earth's gravity field - in that case the same data can be used for a long time, perhaps 10 or 15 years, before it is improved upon."
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."
An 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.
"Different missions rarely measure exactly the same thing, so it's good to have two instead of one" comments Laur. "And if one stops working there is an alternative data source remaining."
Third Party Missions offer alternative data sources
That is the thinking behind ESA's Third Party Missions, non-ESA missions for which the Agency makes some investment in providing data access to European users. The US Landsat series is the oldest example, with data acquired over Europe on a systematic basis since 1984. And Japan's ALOS mission gathered L-band SAR radar imagery, complementing the shorter wavelength C-band SAR imagery acquired by the ERS missions and Envisat, until it was deactivated in May 2011: "ALOS is an example of a data gap arising from a lack of mission continuity," says Laur. "Because now there is no more radar imagery of that type available."
The potential of data gaps arising is a general concern across the wider Earth observation community. Take Landsat, the longest-running remote sensing satellite programme: Landsat-7, which reached orbit in 1999 has a scanning fault which creates blind spots across a quarter of each image acquisition, leaving Landsat-5, launched back in 1984, to acquire most of the usable imagery with its antique Thematic Mapper. The ageing Landsat-5 has been experiencing it's own problems, of late, raising doubts to it's continued operation. The next in the series, Landsat-8 (otherwise known as the Landsat Data Continuity Mission), is due to launch no earlier than December 2012.