Also known as: APRS
APRS (Automatic Packet Reporting System) is an amateur-radio data network for real-time tactical information — position, weather, telemetry, and messaging. It is carried as AX.25 packet frames using AFSK, most commonly on 144.390 MHz in North America.12 Unlike traditional point-to-point packet radio, APRS is a connectionless broadcast system: stations transmit short beacons that any receiver can hear, and a mesh of relays spreads them, producing a live shared map of a region.
Overview
An APRS station beacons its own state — typically a GPS position, but also weather, telemetry, objects, and status text — in an AX.25 unnumbered information (UI) frame. Digipeaters hear these frames and rebroadcast them, following a path written into the frame (the well-known WIDE1-1, WIDE2-1 aliases) so a beacon propagates a controlled number of hops across a region without looping forever. Internet gateways (igates) forward received frames into APRS-IS, the global TCP/IP backbone, where they feed live maps and databases. The information itself lives in the packet’s payload as a compact human-readable/position-encoded text format defined by the APRS specification.
Technical characteristics
| Property | Value |
|---|---|
| Link layer | AX.25 UI (unnumbered information) frames |
| Modulation | 1200 bps AFSK, Bell 202 (1200/2200 Hz tones) |
| Band/frequency | 144.390 MHz (NA), 144.800 MHz (Europe), others |
| Integrity | 16-bit FCS (CRC) |
| Relaying | Digipeaters (WIDEn-N paths), igates → APRS-IS |
| Content | Position, weather, telemetry, objects, messages, status |
How it works
At the physical layer APRS is Bell 202 AFSK: a 1200 Hz tone for one bit value and a 2200 Hz tone for the other, transmitted as audio through an ordinary FM voice radio. The two audio tones ride the FM carrier, so any FM receiver — or an SDR FM demodulator — recovers the tones, and a decoder then distinguishes them by frequency to rebuild the bit stream. Bits are NRZI-encoded (a change of tone signals a 0), which removes the need to know absolute tone polarity. Above that sits the AX.25 frame: flag-delimited, bit-stuffed, and checked by a 16-bit FCS. A common software path is a soft TNC such as Direwolf, which does the AFSK demodulation and KISS framing in software rather than in a hardware modem; GopherTrunk performs the equivalent DSP internally.
History
APRS was created by Bob Bruninga (WB4APR) starting in the 1980s and refined through the 1990s, growing out of his interest in real-time tactical mapping for amateur events and emergencies.1 The design’s insistence on broadcast beacons rather than connected links — a departure from the packet-radio norm of the era — is what let it scale into a live situational picture. It remains one of amateur radio’s most active data modes decades later.
Deployment
APRS is used worldwide for position tracking (vehicles, hikers, balloons, and satellites), automated weather stations, event and emergency communications, and station-to-station messaging. The RF network of digipeaters and igates feeds APRS-IS, which in turn drives public tracking sites. It coexists with, and is built on top of, general-purpose packet radio infrastructure.
Decoding it with GopherTrunk
APRS is fully in scope for GopherTrunk and requires no proprietary components. The decoder FM-demodulates the channel, recovers the Bell 202 tones with timing recovery, reverses the NRZI and bit-stuffing, validates the AX.25 FCS, and parses the APRS payload into position, weather, telemetry, and message records. See the APRS / AX.25 decoder page.
Sources
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Automatic Packet Reporting System — Wikipedia, for the APRS data network, its AX.25/AFSK link layer, digipeater/igate relaying, payload types, and origins. ↩ ↩2
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APRS Protocol Reference (APRS101) — the APRS Working Group specification, for the packet payload formats, position encoding, and message types. ↩