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Author Topic: Raspberry Pi 3B+, Ham radio and APRS setup  (Read 280 times)

K5XOM

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Raspberry Pi 3B+, Ham radio and APRS setup
« on: January 28, 2020, 12:39:42 PM »

Has anyone used a Raspberry Pi and ham radio for APRS operation?
I have a Pi and a extra HT. Can someone could give me more details/instruction on what else is needed besides a GPS unit. And what might be the easiest GPS to use. And how does the Pi operate the radio? How are the connections made? I have done some searching but have not got all the details down yet.
thanks Ken
K5XOM
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W9YW

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Re: Raspberry Pi 3B+, Ham radio and APRS setup
« Reply #1 on: February 04, 2020, 05:55:14 PM »

In any useful APRS, you need a message, a way to modulate it to AX.25 via a TNC (modem), then as this is still at the audio level, a way to send this modulated audio over an HT to someone that can discriminate/read your message.

A GPS source is often one datum inside the message. The rest is your callsign, and information related to your situation and how far the message should reasonably travel over the air, being repeated, etc.

The Raspberry Pi itself has no GPS, but it can form messages from a GPS source. I use an Android smartphone, which communicates the data over Bluetooth to my Pi. The MobilinkD is a Bluetooth TNC that runs for most of a day on a single charge, and plugs into the speaker/mic jack on your HT or other rig.

I can bypass the Pi, but I'll tell you where it comes in handy in my configuration in a second.

A basic circuit has APRSdroid (the Android app) stealing the GPS info from the phone's; the APRSdroid app then periodically coughs the entire set of packets with all of the info to the MobilinD TNC, which "presses" the PTT, sends the data on 144.390MHz, and then releases the PTT. At the same time, the radio is picking up signals and through the MobilinkD TNC, passes them via Bluetooth to the APRSdroid app, which in turn, displays messages from other operators. When the phone is also connected to the Internet, it can map where people are, and if in motion, where they're going, and a history of where they've been picked up (if they provide GPS info, and most all do-- fixed or mobile).

This circuit summary: HT-TNC-Phone with app on receive; phone-TNC-HT on transmit. Bluetooth in my case, between HT/TNC and my Android phone.

If you want more fun, you can put a Pi in the circuit, via Bluetooth or another connection method, to do the logging, and information transmission for fixed point operation (you put in the static GPS/lat-lon data of the fixed location). Used in this way, it's great for a home/QTH-based stationary APRS beacon. Rules say: you have to have a way of monitoring, lest it go key down or something and control operator intervention becomes necessary. In practice, once going, few ever need to touch a Pi running a fixed-position APRS beacon/relay. It is nonetheless an operator's responsibility to prevent problems for others.

Direwolf on Pi is the best place to start, and they have a good group on groups.io to offer support. Lots of good brains there. READ FIRST. Use your search engine to understand basic APRS, how it works, and typical setups. Then look on the Direwolf forum for APRS tips and tricks. The Pi, using a simple USB receiver, can also do plentiful public safety scanner work, AND APRS at the same time. Direworlf covers this. Xastir is another Pi-based app that works well for APRS on Android. Good luck! 73 de W9YW

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