I recently migrated my receiver from dump1090-mutability to dump1090-fa. Looking at the sbs1 output, ground track isn’t being calculated while on the ground (MSG,2), example:
dump1090-fa will emit the ground track there if it’s available.
The difference you’re seeing is probably that dump1090-fa understands the distinction between ground track (direction of movement) and ground heading (direction the nose is pointing) present in newer ADS-B versions, and considers them different data values. In this case I expect the transponder will be sending a heading rather than a ground track.
The port 30003 output is a very legacy output and has no way to represent this, and no way to extend the format (basically every time I’ve tried to enhance it, it’s broken something that uses it, so I gave up). You may want to look at reading data/aircraft.json if what you want is snapshots of aircraft state rather than individual messages. If you really want individual messages, then I’d suggest handing the Beast format on 30005 directly and decoding them yourself.
Thanks for your response. I had seen the option “–write-json” but worried about shortening the life of the SD card by constantly writing to it. However, I now realize that the installer configures it to write to /run in the Raspberry Pi, which is a tmpfs filesystem (i.e. RAM).
Back to the original question, dump1090-mutability’s SBS output would (somehow) consistently give a ground track for aircraft on the ground as long as there was position data. And dump1090-fa, on the same hardware and location, consistently doesn’t, which led me to ask the question.
That said, the JSON output is far better and now I realize there is no downside in using it.
