1. National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK
2. National Centre for Earth Observation, University of Reading, Earley Gate, Reading RG6 6BB, UK
3. Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center, Dorfstrasse 33, 7260 Davos Dorf, Switzerland
4. NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
5. NASA Langley Research Center, Hampton, VA 23681-0001, USA