APRS Digipeater iGate

Alinco DR-135T, Argent Data Systems T3-135 TNC

The APRS-SDR receive-only iGate project was fun to build, and great for seeing what's happening, real time, in the state. But one thing it doesn't do well is help non-iGate APRS users. This is where a simplex APRS iGate repeater can really help, by sending data as well as receive data. 

The APRS iGate repeater requires two things. First, a transceiver to receive and repeat data to and from local stations via RF. The second is an APRS-IS internet gateway connection. With the iGate, local stations can also receive wide-area data originating from the APRS-IS gateway, data outside the reach of their RF radios.

For example, take this poor guy who's arm fell off. They are out in the middle of nowhere, beyond the reach of getting help. But because his friend has an HT with APRS, and there is a nearby APRS iGate repeater, the ambulance guys can see right where they are, even though the stations are too far apart to receive each other's RF signals directly. Thanks to the simplex APRS iGate repeater, help is on the way. What a lucky break for this guy!

Components

• Alinco DR-135T 144 MHz transceiver
• Argent Data Systems T3-135 internal TNC for the DR-135, 1200 bps AFSK
• Raspberry Pi 4B 4GB
• Armor case heat sink for the Pi
• 32 GB SD card
• LMR-400 flex coax
• Diamond X-50 antenna

Software

• Raspberry Pi OS
• Direwolf
• Xastir mapping

Busy downloading, configuring, doing other stuff... I'll get back soon.

APRS SDR iGate Update

The distance from stations heard in Michigan seems to indicate some assisting propagation over Lake Michigan.

After four days of almost continuous operation, the APRS SDR receive-only iGate seems to be working well. I did two reboots to test the auto-connect between Direwolf and APRS-IS and it worked perfectly. The Xastir software was restarted manually and the configuration settings loaded and returned to my settings as expected. 

Seeing that the data was feeding as it should, I closed the terminal window displaying the Direwolf journal, so now I'm just viewing the Xastir map. I have the map intensity is set to 70% so station activity stands out better. I may play around with this a bit, and I may look for a different map.

On the iGate Data page, I have some performance statistics listed, but a picture is still worth a thousand words, and maybe three bar graphs. Here is a station heard radius provided by APRS Direct. The radius they are showing is about 18 miles. I agree with their assessment, as this is the area from which a majority of stations are heard by my station.

 

Here's a few stations that fall outside of the majority curve. The KD9JSX-14 mobile station was heard from a distance of 32 miles. The AB9HH-10 station in Milwaukee, at a distance of 42.7 miles, accounts for over 65% of all packets received. The KB9OIV-1 station comes in from 46.7 miles, and three Michigan stations are heard from 89.4 miles over Lake Michigan.

Raspberry Pi APRS with SDR

For complete information on this project, see the APRS page.

While experimenting with the Raspberry Pi 3B+ APRS receiver, the Pi is temporarily running Gqrx SDR software with an AFSK1200 decoder. 

How does the RPi 3B+ handle the demand of processing with the SDR? The CPU is averaging a pretty busy 55% load. The Argon programmable fan is doing a good job keeping everything cool as it cycles on at 42C with 10% fan speed, and off at 38C. The fan cycle time is about 1:3 minutes on/off in a 69F degree environment. 

In comparison, monitoring the CPU load on the K9KMS MMDVM repeater while it's in use (not idle), I found it to be less than a 2% load. This MMDVM repeater runs on a Raspberry Pi 4B, so it's not exactly an apples-to-apples comparison, but running the Gqrx software to get the APRS data is obviously much more demanding. This will likely result in a shorter life expectancy for the RPi using this approach, so scratch Gqrx off the list and find something to process data, not radio frequency.

On Sep 21, 2020 at 17:39 UTC, the decoder ran until 00:30 (6+ hours) and logged 1,029 hits, an average of 171.5 hits per hour. Not too bad considering the surrounding forest and lowland lakefront area.

Read about this project's exciting continuation on the APRS page!