What Is APRS? Automatic Packet Reporting System via Satellite

📘 Definition

APRS (Automatic Packet Reporting System) is a tactical real-time digital communications protocol used in amateur radio. It transmits small data packets at 1200 baud using audio frequency-shift keying (AFSK) on the 2-metre VHF band. Each packet typically contains the station's position (from GPS), a brief status message, and optional telemetry. In the satellite context, the International Space Station runs an onboard APRS digipeater on 145.825 MHz worldwide — ground stations transmit packets that the ISS receives and retransmits, effectively giving each operator a "signal from space" footprint of roughly 2,500 km radius. The ISS APRS system has been operational since 2000 and is one of the easiest digital satellite modes to work.

145.825 MHz

ISS Frequency

1200 baud AFSK

Data Rate

Up to 256 bytes

Packet Size

RS0ISS / NA1SS

ISS Callsign

Understanding APRS

How the ISS Digipeater Works

The ISS APRS digipeater listens on 145.825 MHz for incoming packets from ground stations. When it receives a valid AX.25 packet addressed to ARISS (or with the appropriate digipeat path), it retransmits the packet on the same frequency. Ground stations within the ISS footprint (2,500 km radius) can then receive the retransmitted packet. This means your callsign and position report are literally broadcast from space.

Getting Started with ISS APRS

You need a VHF radio capable of 1200 baud packet (most modern handhelds work), a TNC (terminal node controller) or software modem like Direwolf, and a simple VHF antenna. Program 145.825 MHz, set your radio to 5W, and point a directional antenna at the ISS during a pass above 20° elevation. Use Orbital Radar's pass predictor to find the next ISS pass over your location.

Beyond the ISS

Several other satellites carry APRS or similar digipeater payloads, including some CubeSats. The SatNOGS network tracks and records many of these transmissions — listen to examples in the audio archive.

Frequently Asked Questions

Can I work ISS APRS without a licence? ▼

Transmitting requires an amateur radio licence (Foundation level or equivalent is sufficient). However, anyone can receive ISS APRS packets with an RTL-SDR dongle and a VHF antenna — no licence required for receive-only operation.

What does an APRS packet sound like? ▼

APRS packets produce a distinctive short buzzing screech — two alternating tones (1200 Hz and 2200 Hz) encoding binary data. Each burst lasts about 0.3–1 second. Listen to examples in the satellite audio archive.

How far can the ISS digipeat my signal? ▼

The ISS orbits at approximately 420 km altitude, giving a radio footprint of about 2,500 km radius. Any station within this circle when the ISS is overhead can potentially hear your retransmitted packet — meaning your 5W handheld signal can effectively reach stations over 5,000 km away.