1. Department of Physics, National and Kapodistrian University of Athens, 15784 Athens, Greece
2. Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing (IAASARS), National Observatory of Athens, I. Metaxa & Vas. Pavlou St., 15236 Penteli, Greece
3. The Johns Hopkins University Applied Physics Laboratory, 11101 Johns Hopkins Road, Laurel, MD 20723, USA
4. Hellenic Space Center, 15231 Athens, Greece