Fig. 5From: Ionospheric electron density modelling using B-splines and constraint optimizationNumber of electron density observations \(N_e = N_e({\varvec{x}}_{\mathrm {GRACE}})\) from GRACE, \(N_e = N_e({\varvec{x}}_{\mathrm {F3C}})\) from the F3C mission and \(N_e = N_e({\varvec{x}}_{\mathrm {SA}})\) from the separability approach during the time interval March 12, 00:00 UT to 24:00 UT, 2015Back to article page