Fun With JS8

It’s been a while, but I am returning to the Drift Buoy, and using the JS8 APRS interface for the telemetry.

Things to work out:

  • Physical packaging
  • Solar / battery / power control
  • QRP transmitter power-stage design
  • ultra-short antenna matching
  • uController and transmitter synthesis/driver
  • GPS, environmental sensors
  • Program that builds JS8 frames

I’ve made progress here and there on this list, but found myself stumped by the JS8 messaging.  Fortunately the JS8Call program is open-source and the author (Jordan Scherer) is happy to discuss it.  I’ve been able to use the JS8Call program to send APRS EMAIL messages, and using the JS8Call API have captured the “symbol list” that shows what FSK tones are being generated for each frame (messages may take several frames).  I have written a simple “JS8 Explorer” program (borrowing and re-coding much from the JS8Call program) that converts this symbol list to bits.  The Explorer then:

  • Strips off the framing fields
  • Unscrambles the Forward Error Correction
  • Strips off the CRC bits

We now have a 75-bit frame that can be decoded into its various bits and pieces.  There are several types of frame.  I am still figuring out many of the details, but am continuing to hack away at it.

Here’s a screenshot of JS8 Explorer decoding a “Dense Code” frame:



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