Development and Scientific Achievements of the Ionospheric Doppler Sounder at Peking University: a Forty-Year Retrospective

Since 1985, Professor XIAO Zuo has led the development of the ionospheric Doppler sounder at Peking University. The initial system implemented a computer-based digital acquisition system capable of continuous real-time monitoring of ionospheric disturbances. During the Chinese Meridian Project Phase I, the instrument was upgraded using a GPS-disciplined frequency standard and direct digital synthesis technology, achieving improved performance and hardware miniaturization. In Phase II, further advances were made through the incorporation of software-defined radio techniques and an array configuration of one transmitter and three receivers, enabling a monitoring network that spans multiple spatial scales. Based on the long-term observational datasets obtained from these instruments, Professor Xiao’s team conducted pioneering studies on ionospheric responses to solar flares, geomagnetic storms, typhoons, and earthquakes, yielding seminal insights into multi-sphere coupling in geospace. This retrospective provides a comprehensive review of the technological evolution of the instrument and the associated scientific achievements, commemorating Professor Xiao’s academic legacy and his enduring contributions to space physics research.