Automatic Dependent Surveillance—Broadcast (ADS–B) is a surveillance technology WHERE an aircraft determines its position and periodically broadcasts it over a 1090 MHz signal data stream, allowing it to be tracked. This broadcast information is used by the air traffic control ground stations as a replacement for secondary surveillance radar, as no interrogation signal is needed from the ground.
Better air space coverage is a achieved through crowd-sourced distributed networks of ADS-B receivers to track air traffic and report it to a number of websites such as Flight Aware and Flightradar 24. Both Flight Aware and Flightradar 24 offer a free subscription plan for individuals who feed ADS-B data to their system using either a commercial or home-brewed receiver.
Both the Flight Aware and Flightradar 24 websites have instructions on how to build Raspberry PI based hub to stream data 
to their respective data system. While an RTL-SDR USB dongle will receive the ADS-B signal, purchasing an ADS-B USB (with with built in 1090 MHz filter) dongle will provide very high sensitivity and greater range.
Finally, a good antenna will enhance the system capability to receive ADS-B signals. Using RG-6 coaxial cable, a home-brewed inexpensive collinear antenna can be constructed in a very short time for just a few dollars . My home brew ADS-B collinear antenna works very well and I feed data to the Flight Aware ADS-B network. Most of the items to build the antenna were either in my electronics parts box or from the plumbing items in my garage. My antenna is housed in a section of 1/2″ CPVC pipe.
Instructions for building the antenna are available in the reference library project notes.
