APRS

Alinco DR-135T, Argent T3 digipeater iGate

If you read through any of my posts or pages, you probably noticed that I'm a Raspberry Pi fan. My appreciation for all things Pi continues into the arena of APRS, so the hardware for these projects includes the inexpensive and reliable Raspberry Pi 3B+.

The first project I made was an SDR-based, virtual TNC iGate using the Direwolf software package programmed to auto-start/restart itself. This great, pocket-sized iGate can run headless and does the job quite well. Add a monitor and keyboard, and Xastir mapping can give any local or internet-connected user a full picture of what's happening within RF range.

The second project was a transceiver-based digipeater iGate with an internal TNC. This setup can work independent of an internet connection to inform local RF stations of what's happening in the area, or with an internet connection, the information circle expands to provide local RF stations with a much larger view. Like the SDR setup, add a monitor and keyboard, and Xastir mapping gives the full picture. In either case, the digipeater iGate can provide anyone with a view of any part of the APRS-participating world. Pretty cool!

The ALOHA Circle

Before getting into the the projects, it's important to first understand the function of APRS and the principles of network saturation. While there are many good articles on these subjects, local APRS parameters should be set with the fabled Goldilocks in mind; not too big, not too small, but set just right. This is where the ALOHA circle comes into play.

The maximum reliable throughput on 144.390 MHz at 1200 baud is about 1,200 packets per 30 minutes. Then, for one digipeater, the maximum number of active stations within RF range, to achieve data capacity, or saturation, is about 50 stations. This is known as the ALOHA number. This number can go up or down some, depending on how frequently packets are transmitted by the given stations. 

The geographic space occupied by these stations at maximum reliable data throughput determines the size of the ALOHA circle. Digipeater stations should then take note of their individual ALOHA circle and adjust power and parameters to stay within their average anticipated ALOHA circle. This helps keep QRM to a minimum, and successful packet transmissions at a maximum.

Here, the yellow ALOHA circle is at 94 miles, with 54 of 263 APRS stations/objects reporting.

Bob Bruninga WB4APR, the inventor of APRS, has suggestions for various types of APRS units. 
  • WIDE1-1,WIDE2-1 for mobiles
  • WIDE2-2 for standard fixed digipeaters
  • WIDE1-1 for fill-in digipeaters
Properly set, a fixed system should digipeat no more data than what is needed for local stations to be informed of what's in their area, and a limited distance beyond their RF reach.

Alinco/Argent APRS Digipeater iGate

Components

  • Alinco DR-135T 144 MHz transceiver at 50 watts output
  • Argent Data Systems T3-135 internal TNC for the DR-135T, 1200 bps AFSK
  • Raspberry Pi 4B 2GB
  • 32 GB SD card
  • Diamond MX-72H duplexer (the APRS shares the 443.5 MHz repeater antenna)
  • Comet GP-6 antenna at 57 feet AGL

Software

  • Raspberry Pi OS
  • Xastir mapping

The Xastir software setup is pretty straight forward. Using the Interface Control, I set up one interface for the internet connection, and a second interface for communications with the radio TNC.


The "Device 1" has the digipeater settings. I have mine set for WIDE1-1, WIDE2-1 for RF-to-IG and IG-to-RF. The area my station is located actually has little APRS coverage to the south for about 15 miles, and the west for about 5 miles. So far, this seems to fill the gap quite well, without over-saturation, though I may drop it down to WIDE1-1 for an exclusively fill-in role.

The Argent Data Systems T3-135 TNC was easy to install in the Alinco DR-135T. When installed, the 9-pin connector on the back of the radio is used for data with a serial-to-USB converter connected to the Raspberry Pi. A USB cable from the TNC is used for programming the TNC and can be used for data in KISS mode. Using the configuration software from ADS, I set up the TNC as shown below.
 



APRS SDR RX iGate

Components

  • Raspberry Pi 3B+ computer
  • Geekworm heat sink armor case
  • Argon Artik programmable fan HAT (optional)
  • NooElec RTL-SDR receiver USB dongle
  • Heat sink scavenged from an old computer for the SDR
  • 32 GB SD card
  • Coax
  • Antenna

Software

There is a great software compilation at QSO365.co.uk for making an RX only APRS iGate. The complete image file, written in Raspbian Stretch OS with Direwolf installed, is a ready to use compilation with most settings already configured. Only a few station-specific edits are needed. 

The package is stable, either headless with access via SSH in a terminal window, or with the on-board GUI. Booting up starts Direwolf automatically and the iGate services start after a two minute imposed delay so everything can stabilize. The configuration instructions are easy to follow and appear to be accurate. 

Thanks, Keith M6NHU for your fine work making the QSO365 APRS RX iGate!

Some findings with this configuration.

  • The CPU load is averaging 5%
  • The CPU temperature is stable at 38C with no call for fan cycling (69F ambient)
  • Offline (no internet), Direwolf is receiving local traffic through the RTL-SDR
  • Off-antenna, Direwolf is receiving APRS-IS data
  • Runs continuous log journal in terminal with  sudo journalctl -o cat -af -u direwolf 

Mapping

Since reading the scrolling text of Direwolf isn't a realistic way of comprehending what the data is trying to convey, the next logical step is the addition of visual maps and overlays for the APRS iGate. Selecting the software was a pretty frustrating process because most of the software is old, some very old, with a DOS-like, or at best, a Windows 3.1 look. Maybe I'm just spoiled with the look and function of websites like APRS Direct and expect too much. So, no longer looking for a pretty face, I checked out several compilations and finally landed on Xastir.

Xastir was pretty straight forward to set up with the SDR and Direwolf using the "Networked AGWPE" configuration. Maps auto-download after making a selection. The choice of what information to show is diverse, although the representative icons are limited and out of date. Filters work well, and data can be logged in several ways.

Some findings with this configuration.

  • CPU load is averaging 9%
  • CPU temperature is stable at 40C with no call for fan cycling (69F ambient)
  • Direwolf auto-starts on boot or reboot
  • Xastir starts manually with  sudo xastir  from terminal
  • Map can be locked to a specified area
  • Map intensity can be adjusted
  • The Argon fan isn't a necessity, but it's a good option
Xastir (at 70% map intensity) mapping station and position overlays, and Direwolf journal

Findings

With over two weeks of almost continuous operation, the iGate seems to be working well. I did four 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 is feeding as it should, I closed the terminal window displaying the Direwolf journal and just view the Xastir map. In the picture below, the adjustable map intensity is set to 70%, so station activity stands out nicely, but you can play around with this to suit your liking. The raw packet data can also be viewed from Xastir, if desired.

Options

This setup can be run several ways with only minor adjustments.

  • With RTL-SDR, a receive-only APRS
  • With RTL-SDR and internet connection, a receive-only APRS iGate
  • With GUI hardware, the Xastir mapping gives a great picture of APRS activity
  • With VNC installed, access the station with an internet-connected device, from anywhere

Have fun with your APRS iGate!

No comments:

Post a Comment

Subscribe to: Posts (Atom)