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Comment by mikewarot | Hacker News Reader

This sounds like an acoustic analog of the radio tracking device that was disclosed by Snowden a decade ago involving a quarter wave antenna that was selectively grounded. It was trackable at 20 miles, and totally passive.

It could run for months on a single coin cell battery.

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I guess I was so enthralled with the rest, that I have missed this device. If it's totally passive, what's the battery for?

Observing the parameter being monitored and deciding whether to ground or not ground the resonant element.

"Passive" in this case meaning that none of the battery power was turned into RF. It did not produce any RF. It merely absorbed more or less RF to encode information.

The trove of information released around the time of the Snowden leaks are not all necessarily from Snowden. There was a second leak that everyone likes to attribute to Snowden. Most of the info on the gadgets that I had seen seem to all be from the second source. But, yeah, they had some pretty cool toys.

"Passive" in this case meant it didn't emit its own signals per se; active sonar emits an audible noise (which can easily be traced back to an origin), passive sonar just listens.

Now this is really really cool (technologically; let's not talk about the implications for people's freedom).

Do you happen to have some pointer were read more about this? Thanks

It's probably one of these:

https://www.wikiwand.com/en/ANT_catalog

Likely TAWDRYYARD:

https://www.wikiwand.com/en/File:NSA_TAWDRYYARD.jpg