What is Passive Radar

Newbie here. Excited to find out about this mfg/product/site. Want to understand what the Passive Radar function is. Tried to research it on forum but haven’t found a definition or clear description yet. Understand the code has been removed for now due to legal issue, bur just wanting to learn what it can do or intended to be able to accomplish?

Normal radar sends a pulse and measures how long for it to get bounced back - and figuring out those reflections tells you about (often metal) objects.
Passive doesn’t send the pulse - it looks for reflections off a different transmitter such as a FM radio or TV transmission. That can tell you where interesting objects are.

To add just a bit, active radar sends a series of pulses as it rotates. It then receives these pulses after the reflect off of some object, be it airplanes or raindrops. The time it takes the pulse to return is determined by distance. Early radar was incredibly sensitive as it consisted of a CRT with a rotating magnetic field which rotated in sync with the radar antenna. At each pulse, an electron beam was deflected from center to edge of the CRT, the intensity determined by reflected signal. Very little electronics was involved.

Passive radar depends on reflections of broadcast sources, performs lots of math on the reflected signal in order to produce a map of where objects are.

Passive radar technology is deemed a national security interest as the military make use of it in a much, much more sophisticated way than the kraken. Think of it as outlawing pig latin because encryption is a national security interest.