(a) Temporal variation of battery voltage (charge). The lines in gray show abnormal voltages during daytime hours when the panel was also charging the battery. (b) S ₄ index for all the 1.6 GHz GNSS signals transmitted by satellites with 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. Note that 1 TECU corresponds to 1 × 10¹⁶ electrons/m² or 0.16 m delay error in the GPS L1 = 1.575 GHz signal.