This very good article by Mac McClellan talks about 1975 as being the earliest public knowledge of the system.
However, the Apollo programme (whose technology was almost entirely in the public domain) used the correlation principle in the 1960s, as a highly precise “DME” which yielded the distance to the spacecraft (from the earth) to an accuracy of something like 10m. Together with the known equations governing the trajectory (gravity, etc), grabbing thousands of such readings gives you the complete trajectory. Here under “Ranging”.
