“I have a little software defined radio dongle that I use to pick up aircraft transponder pings. I log up to one ping per second per aircraft in a database. You can see more information about the setup and information I can receive at this page: Tracking Aircraft Over Los Angeles. Since I had two months worth of transponder pings at this point, including transponder squawk codes and callsigns, I checked to see if there was anything to it. Of the 15,000 or so aircraft I had tracked, I found 8 that had used 4414/4415, and one with a JENNA callsign—that also had squawked 4414. And they were all registered to generically named companies. I felt like there might be something to this.”
http://arstechnica.com/tech-policy/2015/06/how-i-tracked-fbi-aerial-surveillance/