ESA corner reflectors viewed from the NASA Operation IceBridge P-3 aircraft. The NASA IceBridge teams participated in ESA's campaign to validate CryoSat data by collecting measurements over experiment sites. ESA teams on the ground had installed corner reflectors and GPS buoys. Including the ESA airborne activities, there were a total of four aircraft taking part along the same survey line over the Arctic sea ice, while CryoSat passed overhead and teams on the ground gathered in situ ice measurements.