Over land, sea and air, users give MERIS high marks
More than 150 researchers from across Europe, Canada, the United States, China and as far away as Chile have come together to recount their many and varied uses of a single instrument – a desk-sized camera called MERIS, hurtling through space aboard Envisat at more than seven kilometres per second.
The Medium Resolution Imaging Spectrometer (MERIS) is one of ten sensors on Envisat, looking down from the earthward face of the spacecraft. It works by recording visible and near-infrared radiation reflecting from our planet across a range of up to 15 programmable wavebands, to a resolution of either 1200 or 300 metres.
This flexibility and the excellent radiometric and spectral performance designed into the instrument means that scientists can make use of MERIS data across many different fields, variously obtaining information on Earth's oceans, coastal zones, land surfaces and atmosphere – and potentially combining it with results from other Envisat instruments.
Since MERIS was launched as part of Envisat in March 2002 there have been several gatherings concerned with complex calibration and validation processes needed to 'fine-tune' instrument performance, but last week's MERIS User's Workshop 2003 was the first time that users of MERIS products had a chance to meet up, share initial results and exchange their views of MERIS with ESA and each other.
"The MERIS data users attending included principal investigators, students and value-adding companies," said Workshop organiser Peter Regner. "The feeling among attendees seems to be that such activities are an excellent communication tool and will be repeated in the future."
The User Workshop started on 10 November and was hosted by ESA at its ESRIN centre in Frascati, near Rome. Attendees were in for a busy four days, with more than 50 presentations and round tables packed into the schedule, together with poster sessions, training sessions in the BEAM (Basic ERS, Envisat (A)ATSR and MERIS Toolbox) software used for data processing and special sessions on key themes including water, land and atmosphere applications.
Olivier Arino, Head of Projects in ESA's Earth Observation Applications Department gave the Workshop an overview of projects being run by the Agency that use MERIS or similar instruments to attempt to develop operational services to the global community, responding to the requirements of key users. Work is already being done across a variety of fields such as carbon modelling, food security, marine pollution, fishery and snow mapping and desertification.
Ocean colour scene
As a pioneer in the field of ocean colour, Professor André Morel of the University of Pierre and Marie Curie in Paris has had a long association with MERIS. During the Workshop he co-chaired a splinter session on MERIS water products. Accurate measurements along signature wavebands means water contents can be known to a high degree of accuracy – from suspended sediments to living phytoplankton.
"Our expectations were very high for this instrument," said Morel. "The scientific community was waiting a long time. Now at last the products are routinely available for use by scientists. Overall the results we are getting seem very positive and reliable. The performance of the MERIS instrument is living up to the promise of its original design."
Accurately measuring ocean colour with MERIS presents a real technical challenge, particularly because only 1% of the top-of-atmosphere signal actually comes direct from ocean colour – the 99% comes from the atmosphere. To banish the effects of atmospheric distortion for ocean colour study meant extensive study of signal alteration – and almost by default this knowledge also enabled improved observation of atmospheric constituents.
Something in the air
"MERIS was not originally envisaged as an atmospheric sensor, but we have found it works very well as one," said Prof. Jan-Peter Muller of University College London. "For instance, we can detect surface and cloud top pressure up to an accuracy of 100 metres, the 