Bistatic 0. one transmitter + two receivers. Very fun topic as I worked on a few and sync them up as the transmitter radar needs to connect to the second receiver radar (usually a full radar for redundancy). Multistatic is fun also as they employ many covert receivers. Had many war game scenario conversations about the setups.

Maybe they’re getting a bistatic return with multiple microphones separated in distance enough to be significant.

Ultrasonic frequencies generally start at around 20,000 Hz (20 kHz) and can extend up to several gigahertz (GHz). 1

0. https://en.m.wikipedia.org/wiki/Bistatic_radar

1. https://www.rfcafe.com/references/general/ultrasonic-spectru...