I have installed the EchoUAT and Skyfyx. The interface was originally for Navworx. (yes, I can pick winners..). Transponder monitor was used for pressure altitude, the GTX327 set squawk and Ident serially. FAA reported Pressure Altitude as unreliable, about 9% error rate. I got the GTX327 tested, it passed 24 month tests. I contacted uavionix, then got a 'Navworx Multiplexer' cable. This cable has a small PCB that takes 9600 baud Pressure Alt (From Dynon D100) and 9600 baud Transponder info (from GTX327) and combines them into a 115,200 stream. Input is a 9 pin serial connector, output a flat molex.
Thus the EchoUAT is no longer using the 1090 transponder monitor. I (not a lawyer) think my device is not infringing. It *could*, but now does not. Does this mean I can keep using it?
I do find this transponder monitor an interesting problem, as the query on 1030 for Mode A or Mode C is different, but the response on 1090 looks the same. Seems the designers didn't allow for multiple queries in a radar environment. Somebody else (Freeflignt?) proposed having a small 1030 transmitter on the plane to make queries. I'm not sure how the FCC could approve that. The uAvionix patent proposes to pick up 1090 from power wires. I don't see how much that differs from using an antenna per the Garmin '301 patent. A wire acts as an antenna... a good or bad one depends upon geometry.
Mode S is a different design, and has much more info in the response.