I was looking for software to use in conjunction with the DDF. I found it and another technology update of a "Roanoke" DDF. Created by PA8W it is based on an Arduino. I am doing what he did but with a nicer switching unit and a bigger display. He was nice enough to post some detailed information and other useful tips. I think the Teensy should have much better bandpass filters. PA8W claims 2.2 degrees accuracy. Sadly, I would need to build one of his units to try one out.
The PA8W DDF works with "RF Mapper" from Jonathan Musther. Several devices from twenty years ago used Agrelo DF jr serial format. I need to track down more info.
I also have bumped onto direction finding in the APRS standard. There is a standard method for sending reports with bearing information. I need to look further at that.