Fig. (1) Locations of (triangles) ionosondes and (dots) GPS receivers. Chillán (CH, 36°38’S; 72°00’W), Jicamarca (JI, 11°57’S; 78°52’W), La Serena (LS, 29°52'S; 71°15’W) and Tucumán (TU, 26°50’; 65°14’W).