Thanks - very interesting.

There's another approach they may be using - passive radar. That uses illumination from non-traditional radar sources such as broadcast stations, and even satellites (especially broadcast satellites).

Because there is no radar transmitter, they are not detectable. But that also limits the ability of a single station to get range information.. However, with multiple receivers, you can use time difference of arrival to calculate location (this is what GPS receivers do, BTW).

There are inexpensive commercial systems that can do passive radar - KerberosSDR and KrakenSDR.

The first test of this that I am aware of used an early broadcast satellite to track aircraft in the Los Angeles, California area.