K0TFU

How to Build a Portable APRS Command Center

What is a Portable APRS Command Center

Here’s my design objectives for a portable APRS command center:

Capabilities

  • Receive APRS messages, beacons, telemetry, weather reports
  • Send APRS messages
  • The command center should know where it is and beacon it’s location via APRS
  • Be an APRS digipeater to extend APRS coverage
  • Send to and recieve from the APRS-IS network service if internet connectivity is available
  • Provide a map with live display of relevant APRS traffic
  • Allow multiple users concurrent access to all these capabilities
  • Use GNSS to always know where the command center is
  • Provide TNC, GPS, time, and internet connectivitity for ad-hoc users and devices in the command center

Flexibility

  • Powered from any common source: utility, solar, generator, or any 13.8V DC supply
  • Work with several types of radios, from a handheld to a 50W mobile with a portable mast and high-gain antenna
  • Be functional with or without internet connectivity
  • Work with a variety of internet connectivity options: phone hotspot, WiFi, or ethernet
  • Small and light enough to put in a backpack and run from a park bench
  • Powerful enough to run an event like a marathon, parade, or bike race
  • Portable enough to run from inside a moving vehicle

Components

I selected a RAspberry Pi to function as the core of the command center. They are inexpensive, very capable, small, light, and don’t consume much power. I’m using a Pi 4 with 8GB of memory, this use case doesn’t require the higher performance or power consumptions of a Pi 5. With a portable USB keyboard and mouse, and a small monitor, you can use the PI directly. Or you can just bring a laptop and treat the Pi like a server.

Digirig Mobile - works with many radios, allows us to use high performance TNC software.

Software - graywolf - all in one APRS toolkit with offline maps - tnc-server so we can use any hardware TNC - gpsd to export gps location data to graywolf and on the network for any consumer - chrony for time keeping from the gps device and sharing with any network clients - a custom built homepage to manage/monitor

Radios and Antennas

To meet our flexibility requirements, the command center should work with a variety of radios and antennas. The Digirig is part of this solution. I’ve run this command center with

  • a $20 Baofeng GT-5R with a SignalStick antenna
  • a Wouxun KG-UV9PX with double the transmit power of the GT-5R and a superheterodyne receiver, connected to an Ed Fong roll-up J-Pole hoisted up a tree
  • A Kenwood TH-D75 with either the built-in TNC or using the Digirig as an audio interface so we can take advantage of the improved performance of the software TNC in graywolf, connected to a mag mount Comet CA-2x4SR antenna on the roof of my truck
  • A Yaesu FTM-500DR that’s mounted in my truck and wired to a Comet SBB5 that’s mounted on the left front fender
  • A 50W Kenwood TM-V71 with a Diamond X300 antenna with 6.5 db of gain on 2m mounted on a 25 foot portable mast

Inexpensive USB GNSS receiver. I selected with with a small magnetic mount and a built-in USB cable. You can stick it on the car roof, or set it on a folding table at a bike race and it will work

Power

A multi-purpose DC power box - 13.8V DC - solar MPPT charger - solar panels - AC charger for utiliy or generator power input - fused distribution blocks with powerpole connectors

For 13.8V DC power, the radios are easy, the computers and network are more difficult

Get a 12V car charger with 2 USBC ports and plug it into your DC power box. Or build a 4 port USBC charger that takes 12V input (separate project, but highly useful)

For utility power, the computers and network are easy, the radios are more difficult.

Get a short extension cord with a few plugs on the end. Use a 4 port USBC charger to run the PI, travel router, laptop, and whatever else.

battery eliminator and 12V USBC power cable and distrobution block for handhelds

mobile radios need a switching power supply

Calculate and measure max power consumption when operating from DC

Suggest running everything from your DC power box, and then recharging it from utility power if it’s available. You get battery backup of your entire setup this way.

Network

  • travel router
  • optional KVM
  • optional ethernet switch powered from 12V

Connects to any internet source available * starlink * phone hotspot * a wifi network * an ethernet port

The Quick Reference Card

make a quick reference card, print it off, and laminate it. This card has two parts, how to get it all set up and working, what to connect to what, etc. Second part shows how to use all the services, ie if you wanna use aprs.fi in the command center, how do you set that up, how do you connect to the wifi of the command center, etc. where is the url of the command center home page, QR code for the command center WiFi.

put a link to this on the command center homepage too

Moving it around

  • i put each of the main components in pouches, and store them in a packout. the packout is durable, water proof, and stacks up with my other gear like the 12V DC power box.

Use Cases and Utility

  • APRSDroid using command center WIFI and TNC
  • APRS.fi for ios using command center WiFi and TNC
  • tablet as additional live map display
  • laptop operations
  • live tracking of sag wagons at a bike race