APRS System
APRS (Automatic Packet Reporting System) is a tactical communication system that combines amateur radio with digital data to provide real-time information sharing. Originally developed by Bob Bruninga (WB4APR, SK), APRS is used for position tracking, two-way messaging, weather reporting, telemetry, and more.
What APRS Does
APRS is not just GPS tracking -- though that is its most visible application. The system supports:
- Position reporting -- Stations beacon their location, displayed on maps
- Messaging -- Two-way text messages between stations (like SMS over radio)
- Weather stations -- Automated weather data reporting
- Telemetry -- Remote sensor data (voltage, temperature, etc.)
- Object placement -- Marking locations of interest on the map (e.g., emergency shelters, road closures)
- Direction finding -- Tracking signal sources
- Announcements and bulletins -- Group messages and local information
How APRS Works
The Basics
- An APRS station generates a packet containing its data (position, message, weather data, etc.).
- The packet is transmitted on the APRS frequency using AX.25 packet radio at 1200 baud (Bell 202 AFSK modulation).
- Digipeaters (digital repeaters) hear the packet and retransmit it, extending its range.
- iGates (internet gateways) hear the packet and forward it to APRS-IS (APRS Internet System), a network of servers that distribute APRS data worldwide.
- Anyone can view APRS data on mapping websites (e.g.,
https://aprs.fi) or through APRS-capable software.
APRS Frequencies
| Region | Frequency |
|---|---|
| North America | 144.390 MHz |
| Europe | 144.800 MHz |
| Australia | 145.175 MHz |
| Japan | 144.640 MHz |
| China | 144.640 MHz |
| South America | 145.570 MHz |
These are shared, simplex frequencies. All APRS traffic in a region shares one channel.
Path and Digipeating
When an APRS packet is transmitted, it includes a path that determines how many times it should be relayed by digipeaters:
| Path | Meaning | Recommended Use |
|---|---|---|
WIDE1-1 | Relay through one fill-in digipeater | Urban areas with good digipeater coverage |
WIDE1-1,WIDE2-1 | One fill-in hop, then one wide-area hop | Most common; good for general use |
WIDE1-1,WIDE2-2 | One fill-in hop, then two wide-area hops | Rural areas with sparse coverage |
WARNING
Never use paths wider than necessary. Every digipeater retransmission uses shared channel time. Excessive path settings (e.g., WIDE2-5) flood the channel and degrade performance for everyone. In most areas, WIDE1-1,WIDE2-1 is the maximum recommended path.
APRS Equipment
Dedicated APRS Radios
Some radios have built-in APRS capability:
| Radio | Type | APRS Features |
|---|---|---|
| Yaesu FT5D | Handheld | Full APRS with GPS, messaging, SmartBeaconing |
| Yaesu FT3DR | Handheld | Full APRS with GPS |
| Yaesu FTM-400XDR | Mobile | Full APRS with GPS, large display |
| Yaesu FTM-300DR | Mobile | Full APRS with GPS |
| Kenwood TM-D710GA | Mobile | Full APRS with GPS, considered the gold standard |
| Kenwood TH-D75A | Handheld | Full APRS with GPS, Bluetooth, D-STAR |
TNC (Terminal Node Controller) + Radio
A TNC is a modem that sits between a radio and a computer (or GPS), handling the AX.25 packet encoding/decoding:
- Mobilinkd TNC4 -- Bluetooth TNC; pairs with a smartphone running APRSDroid or similar apps
- Coastal ChipWorks TNCPi -- Raspberry Pi-based TNC
- Argent Data Tracker3 -- Standalone APRS tracker (GPS + TNC + radio interface)
- Byonics TinyTrak4 -- Compact GPS tracker/TNC
Software TNC (Sound Card Packet)
Instead of dedicated hardware, a sound card interface and software can provide TNC functionality:
- Direwolf -- Open-source software TNC for Windows, Linux, and macOS. Connects to a radio via a sound card interface and provides full APRS encode/decode capability. Widely used for iGates and digipeaters.
- UZ7HO SoundModem -- Windows software TNC
Smartphone APRS
With a Bluetooth TNC (like the Mobilinkd) or a cable connection, smartphones can serve as the APRS display and controller:
- APRSDroid (Android) -- Full-featured APRS client
- APRS.fi (iOS/Android) -- View APRS data (receive only via internet)
APRS-IS (Internet System)
APRS-IS is a worldwide network of servers that collect and distribute APRS data received by iGates. This allows:
- Real-time viewing of all APRS stations worldwide on websites like
https://aprs.fi - Sending messages to RF stations from internet-connected clients
- Two-way messaging between RF and internet-connected stations
- Historical data queries and analysis
Connecting to APRS-IS
You can connect to APRS-IS with software on your computer:
- APRSISCE/32 (Windows) -- Full-featured APRS client with maps
- Xastir (Linux) -- Open-source APRS client with mapping
- PinPoint APRS (Windows) -- Lightweight APRS client
APRS-IS access requires a passcode derived from your callsign for transmitting (receiving is open to all). The passcode is a simple hash of your callsign and can be generated by most APRS software.
TIP
You can view APRS data without any radio equipment at all. Visit https://aprs.fi, enter a callsign, and see its position, path, and message history. This is a great way to learn how APRS works before investing in equipment.
Setting Up an iGate
An iGate receives APRS packets on RF and forwards them to APRS-IS (receive-only iGate), or does both directions (two-way iGate). Running an iGate is a valuable service to the APRS network.
Basic iGate Setup
- Radio: A 2-meter FM radio tuned to the APRS frequency, connected to a reasonable antenna (a simple ground-plane or J-pole at a decent height works well).
- Interface: A sound card interface connecting the radio's audio to a computer, or a hardware TNC.
- Software: Direwolf is the most popular choice for iGate operation.
- Computer: A Raspberry Pi is more than capable and is the most common choice for dedicated iGates.
- Internet connection: The iGate needs a reliable internet connection to communicate with APRS-IS servers.
Direwolf iGate Configuration
Direwolf is configured via a text file (direwolf.conf). Key settings include:
# Your callsign with SSID
MYCALL N0CALL-10
# Audio device
ADEVICE plughw:1,0
# iGate configuration
IGSERVER noam.aprs2.net
IGLOGIN N0CALL-10 12345
# Enable receive-only iGate
IGTXVIA 0Replace N0CALL-10 with your callsign and an appropriate SSID (see below), and 12345 with your APRS-IS passcode.
APRS SSIDs
The SSID (Secondary Station Identifier) is the number after the dash in a callsign. Convention assigns meaning to SSID numbers:
| SSID | Typical Use |
|---|---|
| -0 or none | Home station, fixed |
| -1 | Digipeater, home station alternate |
| -2 | Digipeater (alternate) |
| -5 | Smartphone |
| -7 | Handheld radio |
| -8 | Boat, maritime |
| -9 | Car, primary mobile |
| -10 | iGate, internet only |
| -11 | Balloon, aircraft |
| -12 | Laptop, portable |
| -14 | Trucking |
| -15 | Generic, software on HF |
APRS Best Practices
- Beacon rate: Don't beacon too frequently. For a fixed station, once every 30 minutes is sufficient. For mobile, use SmartBeaconing (which adjusts rate based on speed and direction changes) or beacon every 1--2 minutes.
- Keep paths short.
WIDE1-1,WIDE2-1is appropriate for most situations. - Use meaningful status text. The comment field in your beacon can carry useful information (your name, QTH, email, or club affiliation).
- Choose the right symbol. APRS has dozens of map symbols (house, car, truck, weather station, etc.). Use the one that matches your station type.
- Respond to messages. If someone sends you an APRS message, reply. Unanswered messages clutter the channel with automatic retries.
- Run an iGate if you can. The APRS network depends on iGates. Even a receive-only iGate running on a Raspberry Pi with a cheap radio contributes significantly.
Contributors
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