And the squawk codes are broadcast as part of the ADS-B system[0]. These packets can be received with the inexpensive USB software-defined radios (e.g., [1]).

And the squawk codes are broadcast as part of the ADS-B system[0]. These packets can be received with the inexpensive USB software-defined radios (e.g., [1]). [0] https://en.wikipedia.org/wiki/Automatic_dependent_surveillance_%E2%80%93_broadcast https://en.wikipedia.org/wiki/Automatic_dependent_surveillan... [1] http://www.rtl-sdr.com/adsb-aircraft-radar-with-rtl-sdr/ http://www.rtl-sdr.com/adsb-aircraft-radar-with-rtl-sdr/