Automatic dependent surveillance—broadcast (ADS–B) is a surveillance technology in which an aircraft determines its position 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 dongle will provide very high sensitivity and greater range. These dongles are inexpensive and can be purchased with a 1090 MHz filter built in.
Finally, a good antenna will enhance the system capability to receive ADS-B signals. A home-brewed inexpensive collinear antenna can be constructed for just a few dollars and in a very short time. My 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. Instructions for building the antenna can be downloaded from here.