The computation of the satellite/buoy match-ups is performed differently for altimeters and SAR (which provides directional information). In any case, for a given buoy, there can be one and only one matchup retrieved between this buoy and a single satellite orbit, even if several measurements may match the proximity criteria defined for the colocation process : only the
closest in distance is kept. Each match-up come in the final dataset with the neighboring measurements (about +/-50 km from the crossing point) of each satellite. All parameters measured by the buoy and the satellite are included too.
Measurements from a buoy and an altimeter match up when they are closer than 60km and 60 minutes.
For SAR, where the number of measurements is very low compared to altimeter (about one SAR spectrum every 100km for ENVISAT), using the same method as for altimeter would result in a very sparse dataset. In order to increase the number of match-ups, a more dynamic colocation process is performed, using the directional information provided by SAR measurements to propagate forward (up to 1000 km) virtual SAR observations along the great circle defined by the measured direction at the source SAR observation. This improved satellite sampling allows to dramatically increase the possible number of match-ups. It is always possible for users to filter out these virtual observations (and then keeping only the "true" SAR observations) by discarding all the measurements whose distance to the source observation is different from 0.
Data are available on ftp in netCDF3 format.Currently match-ups files are orbit-based (one file contains all the buoy - from a given buoy network - match-ups of a given altimeter or SAR L2P file). Buoy-based files (all match-ups with a given satellite for a given buoy) will be soon provided.