Fig. 1
From: Software-defined radio-based HF doppler receiving system
a Locations of the transmitting and receiving stations of the HFD sounding system in Japan. The transmitting stations are indicated by the red triangles and the receiving stations are indicated by the circles. The color of the circles indicates the type of receiving system, i.e., green for the conventional analog receiving system and red for the new digital system. The middle points of the transmitters and the receivers are indicated by the blue cross marks. b An enlarged figure of the Kanto area