Tone Encoding/Squelch vs. APRS Packet Reception Reliability

We’ve been using ham radios on our bikes for years, but last year I put together an interface that connects a TinyTrak3+ GPS encoder to the helmet mic amp. This year I’m building two more, about which I’ll write later.

The problem is that listening to APRS data bursts isn’t all that pleasant, although it’s bearable, but it’ll get much worse when we use 144.39 MHz as our intercom frequency so we can both talk and be tracked: we’d hear all the APRS traffic within digipeater range.

Now, admittedly, talking on 144.39 isn’t standard. The local APRS wizards have given tentative approval, as we can’t figure out a better way to talk, give position reports, and not carry two radio / battery / antenna / electronics packages on each bike. As long as we don’t do a lot of yakking, we shouldn’t interfere with the digital traffic very much… and we don’t do a lot of talking while riding.

So I figured I’d send a 100 Hz tone under the audio and enable tone squelch, so we wouldn’t hear packets from anybody else. We’d still hear each other blatting away, but if I set the TT3+ encoders to send a position report every 10 minutes, it ought to be bearable.

The catch with this is that some receivers / APRS decoders can’t handle subaudible tones. I considered Digital Coded Squelch, but one of our radios doesn’t include that feature, alas.

To get some idea of how tone would work with the APRS setup around here (which is where we do most of our riding), I set up an HT on the bench with the TT3+ and my interface. The antenna is an HF/VHF discone, indoors, on the basement floor, beside a window. The GPS receiver can see a slice of sky from its perch just outside the basement window under an awning. That’s about as terrible a setup as we have on our bikes: low power, bad antenna, obscured line-of-sight.

Each test ran 10-14 hours, the TT3+ sent a packet every 5 minutes, and I checked the raw packet results on aprs.fi.

With tone off and the TT3+ waiting for 3 seconds of audio silence before transmitting, 39% of the packets got through to the APRS-IS backbone.

With tone on and, thus, the TT3+ unable to hear / avoid other traffic, 47% of the packets got through on one test and 42% on another. The higher rate was overnight, when (I think) there’s less traffic on 144.39.

Putting the gadgetry back on the bike, parking it beside the garage, and letting it run for 5 hours on a Saturday afternoon showed that 81% of the packets made it to the backbone. Some of the packets were received by stations over 30 miles away, which probably coincided with the the closer receivers hearing transmitters hidden from the more distant ones.

The only conclusion I can come to is that tone squelch isn’t going to hurt anything around here, where the APRS wizards have done a great job of getting the decoders to cope with subaudible tones. How it’ll work elsewhere is up for grabs, but we’ll burn that bridge when we come to it.

And it turns out that the radios take about half a second to wake up and activate the audio output with tone squelch enabled, so we don’t actually hear the data bursts: they’re almost always finished and we may hear dead air for a fraction of a second. Because the TT3+ can’t do collision avoidance, we sometimes hear other packets from other transmitters before the squelch closes again, but it’s not objectionable. Whew!

Update: with the TT3+ set to transmit every 3 minutes, it works fine!