Signal guide

APRS (position and messaging)

A ham radio system for sharing positions, weather readings and short messages in packet bursts.

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APRS sounds like a short, raspy data burst, about a second or less, like an old dial-up modem clearing its throat, repeating every few seconds on a busy channel.

What does it sound like?

Plate 05Packet data

A short 'brrrap' data burst every so often, like an old modem.

0 Hz1,0002,0003,000 Hz

12-second example. Recorded off the air on the developer's own receiver: an APRS packet on 144.390 MHz.

Field marksShort data burstsTones at 1,200 and 2,200 Hz144.390 MHz in North America

Hear it, step by step

This APRS clip, recorded off the air on 144.390 MHz, holds one packet about half a second long, plus a few faint sounds that are not part of it.

0 Hz1,0002,0003,000 Hz

Tap a step to hear that moment. The box shows where it is on the waterfall.

0.0 sA faint whistle throughoutA faint, steady tone near 2,390 Hz runs through the whole clip, wandering within about 20 Hz. It is not part of the packet, and the clip cannot tell us where it comes from.5.5 sStray sounds before the packetBetween about 5.5 and 6.5 seconds, faint sliding tones pass by, then a patch of slightly louder hiss with a low hum near 310 Hz. None of it carries the packet's data.6.6 sTwo tones, swapping every bitAt 6.56 seconds the packet's two tones, near 1,200 and 2,200 Hz, start trading places on every bit, about 290 times in a quarter second. On the waterfall that steady swapping shows up as even stripes 600 Hz apart.6.8 sFlags mark the startNext come two flags, the fixed bit pattern 01111110 that marks where a packet frame begins and ends. Each flag is 8 bits long, under 7 milliseconds.6.8 sThe packet, error-checkedFor 0.29 seconds the tones carry 41 bytes at 1,200 bits per second: addresses, two control bytes, then a short status report. A 2-byte error check follows and matches, and one last flag closes the frame at 7.11 seconds.

Who uses it?

Amateur radio operators, including cars, hikers, weather stations, balloons and the International Space Station.

Where and when can I hear it?

144.390 MHz in North America and 144.800 MHz in Europe; also via the space station when its packet radio is on (437.825 MHz as of September 2026; check ariss.org).

The science

APRS sends only unnumbered information (UI) frames of AX.25, broadcast to everyone rather than addressed as a connection, usually at 1200 bits per second using Bell 202 audio tones of 1200 and 2200 Hz through an ordinary VHF FM radio. Packets spread through digipeaters, stations that store and retransmit them; with a path setting such as WIDEn-N, each digipeater subtracts one from the remaining hop count and will not repeat the same packet again for about 28 seconds, which limits how far a packet echoes. Internet gateways called IGates feed what they hear into internet APRS servers, linking local radio networks around the world.

Why use it?

Because every station simply broadcasts, APRS is one-to-many: any station with information sends it, and every station in range receives it and can plot positions, weather and messages on a map, with no connection to set up. It runs on common FM equipment, and in North America everyone shares one frequency, 144.39 MHz.

Lore

Bob Bruninga, WB4APR, a retired senior research engineer at the U.S. Naval Academy, began in 1982 with an Apple II program that plotted Navy ship positions; after a port to the IBM PC in 1988, the system was renamed APRS in 1992. He later mentored midshipmen who built amateur satellites, starting with PCsat in 2001.

What do the decoded parts mean?

• Call sign: Who sent it, often with a number (SSID) like -9 for a car.
• Position: Latitude and longitude, sometimes with speed and direction.
• Symbol: What kind of station it is: car, house, weather station, balloon and so on.
• Path: Relay stations (digipeaters) that repeated it, such as WIDE1-1.
• Comment or weather: Free text, or readings like temperature, wind and rain for weather stations.

Easily confused with

• Packet radio (AX.25): APRS is AX.25 packet carrying positions and messages
• Touch-tone (DTMF): touch-tones are two pure tones, one key at a time

How to recognize it

Each APRS packet is sent using two audio tones, 1200 Hz and 2200 Hz, switched at 1200 bits per second (the old Bell 202 modem standard). To the ear that becomes a brief, gritty "brrrap." A packet starts with a short steady section that lets receivers lock on, then the data itself.

The channel is shared, so bursts come from many stations: a car reporting its position, a weather station sending readings, a relay station (digipeater) repeating another station's packet a moment later. In a busy area you may hear one burst after another; in quiet country, minutes can pass between them.

Easily confused with:

• Pager signals (POCSAG): harsher and buzzier, often longer, and heard on paging channels rather than 144.390 MHz.
• Other packet radio: the same sound, but on other frequencies and often at other speeds.
• DTMF tones: individual beeps, not a continuous burst of data.

Where and when to hear it

In North America, APRS uses 144.390 MHz; in Europe, 144.800 MHz; other regions have their own channels. Any scanner or FM receiver that covers the 2-meter ham band can hear it. Activity is highest near highways and cities and during public service events, when hams track race vehicles or support teams.

When the International Space Station's packet radio is switched on, it relays APRS (on 437.825 MHz as of September 2026, historically 145.825 MHz; check ariss.org for its current status), and during a pass you can hear packets from stations hundreds of miles away. High-altitude balloons flown by schools and hobbyists often send APRS too. Packets heard by internet-connected receivers (igates) are collected into public maps such as aprs.fi.

A little history

APRS was created by Bob Bruninga, WB4APR, a senior research engineer at the U.S. Naval Academy. It grew out of a map program he first wrote in 1982 and took its familiar form on the ham bands in the early 1990s. He built it on AX.25, the amateur packet radio protocol of the 1980s, but turned packet radio from point-to-point connections into a shared, one-to-many broadcast: every station says where it is and what it sees, and everyone listening builds the same picture. Bruninga continued developing it until his death in February 2022.

The system has since grown to carry weather data, short messages, bulletins, telemetry from balloons and satellites, and object reports such as the location of a storm or an event checkpoint.

Common questions

Sources

• APRS (Bob Bruninga's site, aprs.org)
• ARISS: current status of ISS stations
• Dire Wolf software TNC
• aprs.fi
• APRS Working Group: APRS Protocol Reference, Version 1.0.1 (2000), archived copy
• Dire Wolf software TNC User Guide (modem sections)
• ARRL News: APRS Developer Bob Bruninga, WB4APR, SK
• AX.25 Link Access Protocol for Amateur Packet Radio, Version 2.2 (TAPR, 1998)