[1] Univ Oslo, Dept Phys, POB 1048, N-0316 Oslo, Norway
[2] Univ Warmia & Mazury, Fac Geoengn, Ul Oczapowskiego 2, PL-10719 Olsztyn, Poland
[3] German Aerosp Ctr DLR, Inst Solar Terr Phys, Kalkhostweg 53, D-17235 Neustrelitz, Germany
[4] Wuhan Univ, GNSS Res Ctr, Luoyu Rd 129, Wuhan, Hubei, Peoples R China
[5] Univ Politecn Cataluna, IonSAT, Dept Math, Barcelona 08034, Spain