(a) Temporal variation of battery voltage (charge). (b) S ₄ index for all the 1.6 GHz GNSS signals transmitted by satellites with an elevation greater than 25°. (c) Absolute vertical TEC derived from code and phase measurements provided by the platform and from the MIT Madrigal TEC maps. (d) Linear correlation analyses between phase TEC values measured by the platform and those provided by the MIT Madrigal maps. Note that 1 TECU corresponds to 1 × 10¹⁶ electrons/m² or 0.16 m delay error in the GPS L1 = 1.575 GHz signal.