A short history of geophysical radar at Arecibo Observatory

As Arecibo Observatory (AO) approaches its 50th anniversary, it is appropriate to review the many radars and ionospheric heaters that have been deployed on or near the 305 m dish and to summarize some of the innovative radar-based geophysical research that has resulted. The reasons William E. (Bill) Gordon developed the 305 m Arecibo dish are well known but are briefly reviewed. The early and then more recent radar/feed designs are reviewed as geophysical uses of Arecibo have evolved and as the full potential of the dish and nearby facilities was and is being realized from HF through S-band frequencies. This history surely has some gaps and there are a few mysteries. The community is encouraged to fill these gaps and to help complete the history.