It’s all declassified today so you can read all about it. During the Vietnam war the ongoing problem was the NVA had the ability to supply all the units and troops via the Ho Chi Minh trail in Laos.
In order to try to target the trail and stop the free movement of troops and supplies many things were deployed. From SOG groups in Laos using PRC-74 transceivers and code burst systems that would transmit CW at 300 WPM that was recorded and played back slow to decode to the many type of seismic transmitters.
An entire network of radios that were dropped by helicopters and airplanes would be monitored by orbiting Batcat EC-121 that flew endless orbits above the trail that would pick up the VHF signal and relay it back to Thailand were the signals were analyzed to develop air strikes or other coordinated activities.
This was a huge undertaking that involved hundreds of men in deploying, analyzing and generating intelligence.
The radios were spear shaped cylinders with antennas and sound sensors that appear like a plant.
We also had a lot of smaller systems for local use that includes things like the turd radio.
It’s a testament to all of that generation that security was never compromised and no word got out at the time or afterwards.
Igloo White is a great example of what can be done with nineteen seventies technology. And just one example of the many clandestine and incredibly large, expensive and somewhat successful projects that were conducted in the sixties, seventies and later.
For more information on Igloo White see:
https://www.vietnamwar50th.com/assets/1/7/VW50th_SensorTech_12-3-19.pdf