I’ve written previously that the RV-8 has an APRS tracker embedded in one wingtip. APRS is a realtime automated reporting system which supports reporting location (GPS coordinates and altitude) with or without messages attached. The system consists of trackers, digipeaters, and iGates. Trackers initiate packets (and can be the destination for delivered packets). Digipeaters repeat and forward packets. iGates are internet gateways to get packets between the network of radios and the internet.
For most trackers, they are configured to leverage the digipeaters to relay packets. However, when a tracker takes flight – in an airplane or balloon – the range of the little transmitter gets big fast! So, the rule of thumb is for trackers in aircraft to only transmit direct to iGates. Otherwise, a single aircraft could easily hit several digipeaters which in turn would repeat the packets causing a lot of unnecessary APRS traffic. The problem is an aircraft starts and finishes on the ground so if its tracker is configured to only use iGates, then there will be a period it will not be received. This could easily be up to 1000′ or more above the ground.
It turns out there has frequently been a 100 mile void in the vicinity of 53VG. The “good citizen” solution was to setup an iGate. Easier said than done when some conditions were put into effect such as keep the cost as low as possible. Here is the list of parts that are needed:
- a radio to receive the APRS amateur radio frequency 144.390mhz
- a computer
- a TNC (packet decoder)
- outdoor antenna
- audio cable between the radio and the TNC
- antenna coax between the antenna and the radio
I had an old 2 meter amateur radio hand held so that was $0.00. A number of internet articles described using software and a computer’s sound card to emulate a TNC. I also decided to put an old computer into service, but I wanted it to be very low power since it would be running 24×7 so the computer I chose was my old OLPC XO-1 that has been idle for 3 years so again that was $0.00. The audio cable was $5.99 plus $3.99 for a 2.5mm-3.5mm adapter to fit the radio. The antenna is a J-Pole I bought off of eBay from KB9VBR for about $40.00 (including the shipping). The coax was a big ticket item since I wanted really low loss and needed to run 50 feet so that $60.00 and another $5 for a PL-259 – BNC adapter. I also needed some paint and a mast (I had the sheet metal) so lets add another $15.00 as a guess. The total was $130.00 but that was without a radio or computer to buy. I did some pricing on eBay and an old 2 meter radio can be won for about $50.00 and a low end computer like the old Asus eee’s are going for $125.00. So, if you started with nothing, you would spend about right around $300.00 for everything.
The antenna was a PITA. I am not fond of extension ladders and 32′ extension ladders at full extension are definitely not my cup of tea. I get half way up and the things always start to flex and I get rubber legs. Thus, it took short stints over three days to get the antenna bracing in place, the mast and antenna installed, and the coax run.
The iGate software was also less than simple. I did a test run using my Windows machine and had everything running in about an hour. However, setting up Debian linux on the OLPC and getting the soundmodem software working was slow. Getting Debian working correctly on the OLPC XO-1 is well documented but takes a few hours. I started with DebXO and then had to run through about 8 steps of updates. To keep memory usage down to a minimum I chose the LXDE GUI. The slowest part was tuning the soundmodem software. I had to balance the volume levels of the radio with the microphone levels of the computer. It was compounded by how few good APRS packets I could receive. IT took nearly three days of running the setup to narrow in on the optimal settings.
Most iGate software have a map display. This is nice if you are planning to look at it often but since an iGate is really a server you want to “set it and forget it” and the OLPC has limited solid state storage as well as horsepower, I wanted a very sparse solution. After trying Xastir, aprsd, aprx, and aprsg, I settled on aprx. Configuration was easy and it connected to the soundmodem driver. The only things I added were SSH support and a small HTTP server. I created a symlink from the “page directory” of the HTTP server to the log directory of aprx. Now I can remote manage the server using SSH and I can check up on the server with any browser (including my iPad and iPhone).
You can see the stats for the iGate over on aprs.fi. Given how little APRS packet traffic there is in the area, most of the iGate’d packets come from a prety big digipeater up the road. None the less, the iGate will definitely help with any aircraft transiting the area and visiting 53VG !
Update: for anyone interested, here is a gzip of the bootable SD card from 18-Jan-2012 (410MB).