> AMBIIENT’s goal was biological imaging.. sensors near a body for medical/neuroscience applications.

As with all DARPA projects, there is a civilian use-case and a military use-case.

That demo was at a conference, in a city, surrounded by electronics and RF noise. The fact that it worked at all in that environment is surprising. As the subject got closer to the apparatus, the signal became larger than the background noise. So, I think the distance is limited mostly by background noise. The press release did state that it was an optimal environment for locating their target, i.e., an isolated person with only geomagnetic noise and known signatures of the aircraft.