My old school buddy, the now retired former CIA Deputy Director, sent me this link and I thought I would pass it along (don't worry, it been declassified ;-)
It talks about a particular cold-war incident that took place and what the technology of the day was like (this all occurred years before my friend joined the agency):
https://www.cia.gov/library/publications/historical-collection-publications/underwater-ice-station-zebra/Hexagon%20Booklet%20web%20FINAL.pdf
Note that because the above link is to an 'https://' site you'll need to first copy and paste it into your browser before it can be opened.
OCU
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