Jakowski, N., Wehrenpfennig, A., Heise, S., Reigber, C., Lühr, H., Grunwaldt, L., Meehan, T. K. (2002): GPS radio occultation measurements of the ionosphere from CHAMP: Early results. - Geophysical Research Letters, 29, 10.https://doi.org/10.1029/2001GL014364