National Physical Laboratory

NPL's Snow Report

NPL's Earth Observation scientists led the world's largest intercomparison of satellite sensors by looking at how snow at the South Pole reflects the Sun's rays.


During December 2008 and January 2009, scientists from around the world examined an Antarctic snowfield using a variety of different satellites. The purpose of this exercise was to see how measurements from each satellite differed when viewing the same patch of ground, and then to see how this information could be used to improve the performance of individual satellites and the data they produce. Ultimately this will lead to improvements in the quality of satellite data that is used in climate studies and improve our ability to mitigate appropriately.

The measurements for this intercomparison exercise were taken over a snowfield in Antarctica known as 'DOME C'. These can only be performed in December and January when the Sun is relatively high in the sky during the southern hemisphere summer.

More than 30 sensors from space agencies across the globe (including several built in the UK) with spatial resolutions ranging from 1 metre to hundreds of metres measured how the snow reflects the Sun's rays. The data from each satellite was cross-compared to every other satellite, making use of complimentary data derived from ground measurements at the site.

The results will allow us to fully quantify differences between the measurements made by the satellite instruments in orbit leading to improvements in their calibration. This will ensure that the data we collect from Earth Observation satellites is quality assured and fit for purpose – providing accurate data for climate change, weather systems and monitoring disaster areas. Some of these measurements need to detect changes of a few tenths of a percent per decade, yet current sensors can disagree with each other by many percent.

One of the triggers for this exercise was the recognition that there are global benefits to remotely monitoring the Earth accurately, and these benefits will only be achieved through international coordination and harmonisation, which in turn requires rigorous and unambiguous quality assurance of the data produced. This is the key objective of the intergovernmental body - Group on Earth Observations) (GEO) and its vision to establish a Global Earth Observation System of Systems, to which the European Global Monitoring of Environment and Security programme is a major contribution.

This intercomparison was the first to be carried out following the guidelines of the newly established international Quality Assurance Framework for Earth Observation (QA4EO). NPL played a lead role in the development of this framework, which was developed for GEO by the Committee on Earth Observation Satellites (CEOS). Although only formally endorsed by CEOS in November 2008, it has already built up significant momentum as demonstrated by the interest in this intercomparison exercise.

QA4EO provides guidance on what needs to be done to ensure that all Earth Observation data can be assigned a "quality indicator" to enable the data users to assess its suitability for their application at face value. It also makes possible the linkage of data derived from different sensors to provide continuous operational data and enable long-time base studies (such as those needed by climate studies) to be made. This framework is now being implemented throughout the world's space agencies, with the European Space Agency (ESA) playing a major lead, ensuring that its next generation of satellites, the Sentinels, are fully compliant and able to deliver the data that is needed.

At the start of the intercomparison exercise in December 2008, Nigel Fox, Head of Earth Observation at NPL, said:

"This is the most comprehensive comparison of its kind ever organised and is a direct result of efforts led by NPL to establish improved quality assurance of Earth Observation data. As the data from many of the sensors involved in this comparison are used in studies of climate change, it is essential that we can reliably combine it together and start to use it as a truly global resource and reference for the future. This comparison will provide the information and evidence to allow this to happen."

This comparison was the first in a series led by NPL, the European Space Agency (ESA) and the British National Space Centre) to address key issues in Earth Observation on behalf of the world's Earth Observation community.

Future comparisons include measurements of ocean temperature, and a salt lake in Turkey. The UK has always been admired for its lead in efforts to study essential climate variables, in spite of not having the largest space programme in Europe.

Scientists from the UK have been keen proponents of the importance of calibration and validation, emphasising that measurements for climate studies must have unequivocal data quality, or their value is very limited. Examples include the "Along Track scanning Radiometers" series of instruments, which are making benchmark studies of the Earth's surface temperature and the Geostationary Earth Radiation Budget instruments flying on Meteosat's second generation, which both exhibit unprecedented accuracy.

Another beneficiary of comparisons like this are commercial satellite and data providers such as Surrey Satellite Technologies Ltd (SSTL) and their subsidiary DMC International Imaging Ltd. SSTL have become highly successful in providing innovative low cost satellites and imaging systems to many countries. SSTL have embraced the QA4EO initiative seeing it as a way of helping them to demonstrate the quality of their sensors and data products to an international audience as an equal to the major space agencies.

Looking to the future, it is hoped that the UK can continue to take the lead in this crucial role to underpin the calibration and validation of Earth Observation satellites, in particular those measurements supporting Essential Climate Variables.

One example is the proposed development of a calibration satellite, or an 'NPL in Space', capable of providing benchmark measurements for climate at the uncertainties needed for studies taking place over decades, whilst simultaneously providing reference calibrations of sites which can then be transferred to other satellites, dramatically improving their accuracy. The world's climate and Earth Observation calibration community are now calling for such a mission. The UK, having already developed a solution called Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS), is being looked to for leadership, to make this an early reality. TRUTHS could be fully built in the UK using technologies pioneered by UK industry and its Natural Environment Research Council (NERC) on board a small low cost satellite such as those of SSTL and could potentially be launched by 2012.

For more information, please contact Nigel Fox

More information on NPL's work on climate research

Last Updated: 16 Apr 2012
Created: 7 Jan 2009


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