Has anybody built something like this yet?
FPV pilots (like me) use analog A/V links of course, and one of the existing audio channels can be used for telemetry. Telemetry can be then decoded on the ground and the GPS coordinates can be used to aim an antenna tracker.
Using an audio modem chip to encode (parts of) Mavlink data and output it as analog audio for the A/V transmitter, will mean saving on a redundant, interfering, and range limiting external wireless modem set.
The audio modem will of course have to be more or less immune to noise, and transferred data will require error detection and correction code.
I wouldn't mind trying if I had the time, but it would be overwhelming as my 1st Arduino project ever, and I'm not even sure which components I would need. I don't even have an electronics background.
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I did something like this many years ago, although my approach was somewhat crude.
My A/V transmitter was in the 2.4GHz band and included stereo audio. Over one of those audio channels, I sent telemetry data at 38.4kbps. Over the other channel, I fed audio derived from a prop sensor so I could directly hear changes in engine RPM. I didn't fly FPV, but I did fly at a substantial altitude and distance within visual range.
The data was collected from multiple devices on board, including a GPS, and formatted into a semi-synchronous serial stream (framed, regularly formatted data, but with gaps allowing a normal async serial port to recognize the data) using multiple PIC microcontrollers. I included a CRC (real CRC, not checksum) for error detection, but no error correction.
The resulting serial stream was then level converted to audio levels and fed directly into the audio input on the transmitter.
At the ground station, I fed the audio output from the A/V receiver into what basically amounted to a bit "slicer", that reconstituted the square shape of the bits and converted them RS-232 levels. I then fed this directly into a serial port on my laptop. The laptop was 486, if that gives you any idea how long ago this was.
The entire system actually worked better than you might expect, but obviously it was far from being a well-engineered solution. IIRC, on a typical flight I usually had less than a 5% overall frame error rate.
I would hope there are better implementations out there these days, but I haven't really looked. If you do want to roll your own, Arduinos now are more friendly than the PICs I was using back then. And oscilloscopes can be had rather inexpensively as well, which I highly recommend.