hhttp://www.whackstar.net/
https://flightaware.com/adsb/
This is my free raspberry pi 3 model b+ board set up to report the information of overhead aircraft. You can apply and get the product for free. Provided you meet flight aware's conditions.
I'm using this to teach students about transponders and have hosted the receiver on a high building on the edge of the rift valley. It's very simple and cheap raspberry pi projects. Which potentially provide you with a free subscription to Flight Aware.
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