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Day four and still the slightly hazy conditions at the measurement site were there to greet us. Once again we had the 3.5 km walk from the drop-off point to the measurement site, although our bags were taken ahead of us by the quad-bike and trailor combination.

Arriving on site

Some teams were continuing with the site characterisation part of the comparison while others were concentrating on satellite calibrations. Some were attempting to do both. There were eight different groups characterising the site today, measuring seven of the nine individual measurement sites.

Measuring downwelling radiance	Automated sunphotometer

Further measurements were made with the GRASS instrument and an analysis made of the downwelling radiance measured. This is a new capability and is due to a new radiometer that had recently be used with GRASS. It was developed by the University of Edinburgh and enables GRASS to make upwelling radiance measurements from the ground almost simultaneously with the downwelling radiance measurements from the sky. This is important as site characterisation measurements should really be made under stable conditions. Early results show that the conditions, particularly close to local noon are quite stable. If the conditions at near noon tomorrow are similar then the first of the on-site cross-comparison measurements will be made. This is where we will replicate the laboratory comparison measurements but in the field. 

The sunphotometer on site is part of the Aeronet network and among others things helps determine atmospheric optical depth. This is an important parameter in satellite calibration.

The team from VITO (Sindy Sterckx and Els Knaeps), are leaving tomorrow, this was their last day on site. A photograph of the current particpants is shown above. Also Françoise Viallefont from ONERA has joined the campaign. She is replacing Yannick Boucher who is leaving early on Sunday morning.