My understanding is that before the development of observation satellites in the 1960s, radio echos from the Moon were used to detect nuclear explosions (testin

My understanding is that before the development of observation satellites in the 1960s, radio echos from the Moon were used to detect nuclear explosions (testing or military use) from the far side of the Earth. Even if direct observations were not possible / were limited, so long as the Moon was observable both over the test site and a US-friendly (not necessarily US-based) receiving station, such signals could be detected. The Arecibo’s site origins trace back to the 1950s, when Cornell University proposed its construction to the Department of Defense’s Advanced Research Projects Agency (ARPA), which is today known as the Defense Advanced Research Projects Agency (DARPA). A desire to better understand the composition of the ionosphere and how it might impact objects passing through, including ballistic missile reentry vehicles carrying nuclear warheads, was a key reason for its construction. At the time, ARPA was in charge of a broad ballistic missile defense program known as Project Defender. It was believed that nuclear warheads would produce a distinct signature when reentering the atmosphere, making it possible to distinguish them from decoys, so long as that signature could be quickly identified and categorized. The plan was to use the Arecibo Telescope to help gather general, but still valuable information about the ionosphere in support of this effort. <https://www.twz.com/37898/collapsed-arecibo-radio-telescope-was-first-built-for-ballistic-missile-defense-research https://www.twz.com/37898/collapsed-arecibo-radio-telescope-...>