Map of six Rx stations (Sarobetsu, Sugadaira, Oarai, Chofu, Kure and Onna), VOR Tx stations (black dots) and the Es propagation zone centered at Chofu, Tokyo. All the possible reflection points (i.e., mid-points between the Tx and Rx stations) are shown by the colored crosses (purple = Sarobetsu, light blue = Oarai, green = Chofu, blue = Sugadaira, orange = Kure, and red = Onna). The locations of the four ionosonde stations in Japan are also shown.