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Cocoa1090 is a Mode S (ADS-B) aircraft transponder decoder for Mac OS X, written for use with RTL-SDR devices (RTL based USB tuner dongles). These transmissions are at 1090 MHz, and can be received at a distance of a few hundred miles, often even with a modest antenna.
Cocoa 1090 uses rtl_tcp, part of the free RTL-SDR package, to control the RTL device, and produce I/Q data.
Some tips for installing RTL-SDR are here: http://www.hfunderground.com/wiki/RTL-SDR
Cocoa1090 displays all received aircraft, including the altitude, heading, speed, and flight number, as well as the tail number and aircraft model (if it is part of the included database, you can add your own tail numbers). Some transponders include position information, this is also displayed if received, and used to map those planes (Click on the image below to see it full size):
Cocoa 1090 is free, a copy can be downloaded from the website: http://www.blackcatsystems.com/software/cocoa1090.html