[1] Delft Institute for Earth-Oriented Space Research (DEOS),
[2] Delft University of Technology,undefined
[3] Thijsseweg 11,undefined
[4] NL-2629 JA Delft,undefined
[5] The Netherlands,undefined
[6] e-mail: klees@geo.tudelft.nl; Tel.:+31 15 2785100; Fax: +31 15 2783711,undefined