The PI's Page
This page records some of the current thoughts and reactions of the PI team in response to recent developments and recently expressed views about the ATSR Programme.
Where have we got to?
This period in the history of the ATSR project is certainly the most successful and productive in the chequered history of the three sensors.
These are some of our reasons for making such an extravagant assertion:
The third along-track scanning radiometer, AATSR, is now flying on ENVISAT and its data are being used to generate global SST fields which are demonstrably of unprecedented accuracy - starting to meet the needs of climate research for which the original ATSR was conceived. The data are now finding their way into the Hadley Centre's definitive SST dataset, HADISST (http://badc.nerc.ac.uk/data/hadisst/) , where initial assessments indicate a positive impact.
For the first time, the entire the data-set is being fully re-processed to generate a new, complete and generally available archive with uniform processing and seamless formatting.
Operational organisations world-wide are now using AATSR 1km resolution SST data in near-real time as the result of ESA's Data Exploitation Programme Medspiration (http://www.medspiration.org/ ) which comprises the European element of the GODAE/GHRSST Pilot Project. As a result of this development, AATSR data, incorporated into OSTIA (http://ghrsst-pp.metoffice.com/pages/latest_analysis/ostia.html), the UK Met Office's new SST daily analysis scheme - not to be confused with the harbour city of Ancient Rome! We are told that AATSR data are now being evaluated for possible inclusion in the Met Office's daily analysis which goes into our weather forecasts.
The long-term future continuity of AATSR's climate-standard SST data-set is now virtually assured, since ESA are now planning to build a fourth along-track scanning radiometer called, in its development phase, the Sea and Land Surface Temperature Radiometer (SLSTR); will this become ATSR-4? Whatever its name will be flying on the Sentinel-3 satellite in the second decade of this century, with the same thermal channels and viewing angles as the three predecessors had. An overview presentation of the currently defined Sentinel 3 payload can be seen at:- http://esamultimedia.esa.int/docs/GMES/ESA/5_coloc_GMES_S3_GSC_day_7_8_March_2007_ESRIN_I1.pdf
Finally, it is not my intention to give the impression that AATSR has reached its final goals - there are still plenty of outstanding issues of data quality which must be addressed before the quality and accuracy will fully satisfy the needs of climate science and for that reason the (A)ARC Project, A)ATSR Re-processing for Climate, has been set up under the leadersip of Chris Merchant at the University of Edinburgh. For the first time there is a funded consortium from all the principle centres of expertise in ATSR SST retrievals, working towards a new revision and reprocessing of the complete ATSR SST data set.
It is over 25 years since the original ATSR proposal was submitted to ESA and the data have been flowing with little interruption since the summer of 1991. To have risen from the status of an experimental sensor and now to have reached to that of an operational sensor providing a benchmark for accuracy in the measurement of an Essential Climate Variable (ECV) is a major achievement, one which reflects extremely well on all those who have contributed to he the programme, not forgetting those involved in the early days of ATSR's technical and programmatic development, whose efforts have led to something which has really lasted!.
The AATSR visible channels and Land Applications
The three Visible channels of the ATSR system were introduced on the ATSR-2 sensor in 1995. They are perhaps more accurately designated, in combination with the reflected infrared channels at 1.6 microns wavelength, the Visible and Near Infrared, or Vis/NIR channels. Their objective was to allow the development of improved atmospheric corrections for land applications. This was an easily stated objective but one which was extremely challenging to realise.
In fact it is only now, over ten years later, that we have access to the kind of computing power that makes this important objective of the ATSR programme achievable on a useful scale. Peter North, then at a NERC Institute in Monk's Wood, Cambridgeshire, has developed a scheme which used the visible-channels dual view to extract the aerosol signal over land to forma the basis of an atmospheric correction and a means of quantifying atmospheric aerosol content over land surfaces. Peter North is now at the University College of Swansea, where he and William Grey are perfecting the technique and preparing to carry out a long-term re-processing operation, perhaps using a GRID network. work has the potential to move land remote sensing, especially studies on vegetation cover, into new levels of quantitative accuracy and effectiveness. Therefore it is gratifying to note that ESA, in their presentations at the recent ESA/ENVISAT Symposium in Montreux, stated their clear intention to intend to continue with dual-view visible channels on the SLSTR sensors.
Until recently I we have not seen global 1km land data from the ATSR instruments, William Grey has just produced this global composite from the system he and his colleagues are setting up on a grid processing system.
This composite of TOA reflectance over land was produced on the Swansea AARDVARC processor derived from approximately 1200 AATSR strip-lines for June, July and August 2005. The composite has been produced on the basis of maximum NDVI, which is intended to eliminate cloud. But even over some regions (particularly tropical equatorial regions) cloud persists even after 3 months of compositing. The red, green and blue of the CRT correspond to the 1.6, 0.87, 0.55 micron nadir channels, respectively. Once this system is running on a GRID processor it will be possible to produce these kind of composites for the life time of the (A)ATSR missions. This work is leading to a data-set of atmospherically corrected global land reflectances at ikm resolution, which will have the potential to move land remote sensing, especially studies on vegetation cover, into new levels of quantitative accuracy and effectiveness. Therefore it is gratifying to note that ESA, in their presentations at the recent ESA/ENVISAT Symposium in Montreux, stated their clear intention to intend to continue with dual-view visible channels on the SLSTR sensors.
Aerosols, Clouds and Dust
It ahs always been an important secondary objective of the ATSR programme to develop new ways of detecting and quantifying the presence of particulate matter in the atmosphere.
There are numerous efforts to develop effective ways of, firstly, compensating for the degradation of AATSR’s SST accuracy when dust and aerosols or clouds are present I the atmosphere; and, secondly, making useful quantitative estimates of particle size and density, together with cloud parameters. The GRAPE project (http://badc.nerc.ac.uk/data/grape/ )is one such effort, GRAPE is led by Don Grainger at Oxford University and the GRAPE team are continuing to develop ATSR’s ability to develop the techniques further, using the dual view and the thermal channels as well as the dual view..
The work at Swansea, mentioned above, has also successfully led to the possibility of an ATSR aerosol product over land.
This is an important and difficult area for ATSR. There are other semsors I space generating aerosol products from the same wavelengths, e.g. MODIS, SEVIRI and MERIS but none of these have the benefit of the dual-view.
Merged Products which use AATSR data
With its relatively narrow swath and three-day repeat, combines with very stringent and conservative cloud-clearing algorithms, AATSR provides data-coverage which adequate for analysis schemes, model ingestion and applications which use spatial averages of tens of kilometres, but for Mesoscale or smaller-scale process studies, for example, following the detailed evolution of a current system such as the Gulf-Stream the small number of SST pixels can make process studies ineffective, despite the high levels of SST accuracy of the valid pixels. Therefore, merged products which combine the accuracy of AATSR data with the wide-swath coverage of other sensors have great potential for quantitative process studies.
For scientific uses of ATSR data, the great benefit of the GHRSST Pilot Project is that users now have access on a daily basis to data from virtually all of the current active SST space sensors now collecting data, namely AATSR, AVHRR, MODIS, Meteosat/SEVIRI and the GOES satellites, as well as the microwave sensors, TMI and AMSR-E,. The important point is each of these data streams has been re-formatted on to GHRSST Level 2p, which is a common easy-to-read version of NetCDF. Co-registration and inter-comparison of any number of these data-streams, hitherto a nightmarish and tedious project for Earth Observation data; it has become a near-trivial task to carry out inter-sensor comparisons and to merge products
What should we now call the ATSR sensors?
We already have ATSR-1.ATSR-2, AATSR and, In the future, a new along-track scanning radiometry sensor still known by its functional developmental name, SLSTR, which does not roll easily off the tongue, does say exactly what the sensor aims to do.
Currently, data from the first three sensors are referred to as (A)ATSR data – agin not easily rolling off the tongue.
What should the new sensor and its successors be called? Some have suggested “ATSR-4”, continuing with ATSR-n and perhaps re-designating AATSR “ATSR-3”. This has obvious simplicity, but not everyone agrees.
Does anyone have any suggestions for future naming system?
It needs to be simple, self-explanatory and smoothly pronounceable. (Rude or disparaging suggestions will, of course, not be considered).