"Do it yourself" Airborne Proximity Warning Device.
Currently it uses IEEE 802.11 (Wi-Fi) radio for broadcast communication between equipped aircrafts.
There is also ADS-B option available to receive position reports from airlines traffic.
doc - human readable documents related to this project;
system - pre-built binary files of system-level software;
src - source code files of application sofware written in Python programming language.
2) Read "Parts list and price estimate" next.