How the NSA Triangulates HF Number Stations

[Mork]

We've talked a fair amount about HF radio recently, in the context of OTH
radar. Recall that an extremely useful property of HF radio here is that HF
frequencies can reflect off of the upper layers of the atmosphere, causing them
to rebound towards earth at a range much further than the horizon. This
behavior, called ionospheric propagation or (more commonly among radio
operators) skywave propagation, is the best way to achieve radio communication
over ranges much longer than line of sight, without the use of satellite
relays. Since satellites didn't come onto the scene as an option until the
'60s, for much of the 20th century HF radio was the major method used for
long-range communications when no supporting infrastructure (such as microwave
relays or a subsea cable) was available along the way. This included
applications like long-distance phone calls and all contact with ships and
aircraft at sea.

https://computer.rip/2023-01-16-huff-duff.html