quinta-feira, 12 de abril de 2012

Envisat's mysteries

I read in the news today that connections have been lost with the Envisat satellite. ESA has already confirmed it too, but reading the latest Mission Operations News, it seems it would be predicted for a satellite that had only been planned for a five year mission.

So I ran to see how the sea level graphs had finished, and to my biggest surprise, the graph from AVISO had changed dramatically! I recall seeing it about a week ago, with totally different values! From an historical perspective, several older graphs can be seen in a post 9 months ago (in Portuguese), or compared with other satellite measurements in this WUWT post. Please compare the graph 9 months ago on the left, and the more recent one on the right (click to zoom):

Notice that the slope has gone up from 0.76 mm/year to 2.33 mm/year! This manipulation, which has no other name, has been justified by Aviso with the following notes:
  • Envisat time series extended before 2004 starting from Mai 2002.
  • Envisat V2.1 GDR reprocessed data used. The new standards are also detailled in the table "Processing and corrections".
  • Instrumental correction sign corrected (impact of around +2mm/year). The error detection and impact on data is detailled in:
    • Envisat 2011 yearly report, A. Ollivier & M. Guibbaud, soon on the Aviso website
    • Envisat Reprocessing impact on ocean data, A.Ollivier & M. Guibbaud, soon on the Aviso website
    • A.Ollivier et al. 2012, Envisat ocean altimeter becoming relevant for mean sea level long term studies? (submitted in Marine Geodesy)
  • new NetCDF CF format in the products and images selection interface

Now, this looks like a small part of the Envisat mystery. Please check that the older graph starts in 2004, but the newer graph starts in mid 2002! Notice that in the newer graph, the 2002 and 2003 values were much higher that those of 2004, and that the highest values of 2003 were not surprassed till late 2008. Now imagine why they were not there in the older graphs, and how being there would create a trend probably very near to ZERO!

The last image, the above one on the right, that's on the AVISO site is dated "Tue, 10 Apr 2012 09:14:03 GMT", so clearly has been put there after the satellite failed, which occurred last Sunday. No doubt that the hiding the decline was already planned, but probably was executed swiftly after the fail. Strangely, the last color image taken by the satellite was above Portugal, which is obviously a coincidence. But it looks like its mysteries have only started...