I was going to say, it would make more sense if they got the data digitally by knowing pre-registered flight plans and plotting that in 3D. Alternatively, they

I was going to say, it would make more sense if they got the data digitally by knowing pre-registered flight plans and plotting that in 3D. Alternatively, they could point a camera at the sky and look for "plane-like" objects. Either would be cheaper than ADS-B hardware. Then I learned you could actually scan radio signals with software... http://www.rtl-sdr.com/about-rtl-sdr/ http://www.rtl-sdr.com/about-rtl-sdr/ Mind blown. :)