It would seem that the best way to get an open source radar (short of engineering from one from scratch) would be to first determine an "open radar display and control protocol" , then develop either code or reprogrammable black boxes that convert the proprietary protocol to the open protocol. This would open the way for software developers to freely implement radar. It sounds like most already know how, it's just the licensing and multiple protocols holding them back.
I would also think that after the first few radar hacks, the radar code converting gurus would get very good at it and would already have all the necessary test equipment
to analyze new protocols and rapidly put out new conversion code. The radar manufacturers would eventually give up evolving new protocols and agree to a standard.
Has anyone heard of a community doing this? This is an issue whose time has come. Some great work has been done by individuals for individual programs. Now is the time to put together a community that can get all ethernet radars speaking a common language.
Just look at the rapid progress and innovation since open protocols like NMEA 2000
have finally taken hold with most manufacturers. An open radar protocol is the last big obstacle to true competition in the marine navigation