UHF APRS iGate - If someone else won't, just do it yourself.

  

    Many digipeaters and iGates on the APRS network will, of course, repeat your packet to extend your coverage, and will send your location to the APRS-IS network for viewing through internet platforms. However, there are many objects on the APRS network that do not travel via RF paths. This can include MMDVM hotspots, CWOP weather stations, users of Android/iOS APRS applications, and even severe weather events.

    A great use case for this could be during a SKYWARN severe weather net, where spotters can quickly see temperature shifts and rain rates between weather stations located on opposite sides of town. This is also useful during SKYWARN nets as a way for the Net Control Station to add reports to APRS-IS, such as road blockages, flooding, and locations of emergency response centers; objects that cannot easily be added to the APRS network via traditional RF sources. Spotters will have immediate access to this information via their radios they are already using, helping them stay informed of rapidly changing conditions.

    In my area, the digipeater/iGate owners choose not to forward these objects, whether it be to avoid flooding the APRS frequency, personal reasons, or to avoid loops of packets. Whatever their reasons may be, I was disappointed, as I knew my radio could decode these objects, and I felt like it'd be fun to see some more APRS traffic, aside from the standard beacons of these existing iGates, or maybe 1-2 hams a week driving two miles down the road. 

    Building the iGate would in theory be pretty simple, but without a deep understanding of APRS, can be a bit complicated. Especially when trying to work with other iGates/Digipeaters on the same frequency. The decision was made to place the iGate on its' own frequency. I had a Motorola DMR mobile sitting around, and I knew this would be robust enough to run a 24/7 iGate. I also had a sound card with the correct Motorola connectors, used from our previous SSTV project. Since the Motorola is UHF only, we chose a frequency in the lower part of the band, as UHF is a severely underutilized band, especially in my area. I also had a 17' Diamond base station antenna I had picked up at Hamvention, and a Raspberry Pi 3b that would work as the server.

    Due to a lack of coax that'd be long enough without being lossy, I mounted the antenna on a buried section of top rail behind my garage. My garage has a pass through on the rear wall, right above a shelf near some power. WiFi performance was a bit of a concern, so I chose to use a headless Raspberry Pi running a lightweight version of Raspbian with no GUI. Since Direwolf is configured via a text file, a GUI would've just been a waste of system resources.

    After installing Raspbian and connecting to the Pi via SSH, I used the version of Direwolf available in the Debian repositories. This version was a bit outdated, so I just grabbed the latest release from the Direwolf Github. The entire Direwolf configuration exists in a single text file, and there are excellent guides available from the author of Direwolf, breaking down every available feature the software has.

    After fighting for a little bit with the correct filtering and configuration for IS -> RF traffic, the iGate was live. There was some tinkering with the TXDELAY setting that was required, as the packet would tend to be cutoff, due to the delay of triggering the PTT circuit via the sound card, and waiting for the Motorola to begin its actual transmission. 

    Although I have to remember to switch back to the standard APRS frequency once out of my coverage area, this iGate has been exactly what I wanted. I encourage those who are frustrated with a lack of APRS coverage to build their own iGate, and begin sending non-RF objects over the air. It makes the whole system a lot more fun. This entire project can be done with pretty much any FM radio, sound card, and PC that you have laying around. I have even added K2IE's noaacap software to the system, which continuously checks National Weather Service offices for my zone, and will send out an APRS packet if a severe alert is issued, completely independent of APRS-IS.

    I reached out to the author of noaacap and let him know my use case, as he had asked for feedback on his Github page. I also let him know my choice of using a non-standard frequency. For anyone hesitant on setting up an iGate, for fear of interfering with another iGate owner, I posted an excerpt below from his email that really got me thinking. The iGate owner in question is someone who quoted "not wanting to fill the airwaves with too much traffic" as their reason for blocking IS -> RF traffic, but has their own digipeater setup beaconing every 5 minutes or so. 

"With regard to running another iGate on 144.39, why not go ahead and do it?  Let the other op complain, but as you say, he is the one jamming the airwaves with continuous becaons.  No amateur owns a frequency and he'll find his becaon is transmitting a lot less if it has to wait for your traffic.  A fixed station should not beacon more than every 30 minutes.  It is not like a location needs to be updated.

If you need some targeted traffic from IS to RF, then that is a valid reason."