APRS on 144.800 MHz in Europe: cars, weather stations and digipeaters on the map

What you hear

In FM, short bursts of a harsh buzzing tone, each a fraction of a second to about a second long, the sound of 1200-baud packet radio: two audio tones, 1200 and 2200 Hz, switching at every bit. On a busy urban frequency there is a burst every few seconds, many of them twice or three times in a row, because the digipeaters repeat what they hear. By ear every packet sounds the same; the interest is in the data.

Decoded, each burst is one packet from one station: a callsign with a suffix that hints at what it is, a symbol, and the payload. Cars and handhelds send their position from a GPS, with heading, speed and altitude, usually in the compact Mic-E format of mobile radios. Weather stations send temperature, wind, pressure, humidity and rain, every few minutes, around the clock. Digipeaters and internet gateways send a beacon with their name and position. There are also objects someone placed on the map, a repeater, an event, a hazard, status texts, messages between stations and telemetry from remote sites. The path at the start of the packet shows which digipeaters have relayed it.

Who transmits and why

Radio amateurs, through the Automatic Packet Reporting System, a network that grew out of American packet radio of the 1980s and spread worldwide in the 1990s. In Europe the agreed frequency is 144.800 MHz; North America uses 144.390. Digipeaters on hills repeat the packets so that a single handheld covers a whole region, and internet gateways pass everything into a worldwide feed from which the familiar APRS map sites are drawn. The uses are as varied as the packets: knowing where the club's cars are during an event, following a balloon, running a weather net, messaging when the phone network is down, or simply saying that the station is on the air and where.

When and where to listen

Around the clock. Weather stations and digipeaters beacon all day and all night; cars and walkers appear in the daytime and in the rush hours; weekends with outdoor events are the busiest. This is VHF, so the receiver hears what is within line of sight, some tens of kilometres, plus whatever the local digipeaters relay from further away: a receiver on a hill or in a large city delivers a packet every few seconds, one in a valley may wait a minute. The mode is narrow FM, and the frequency has to be exact; APRS does not tolerate a receiver tuned off by a few kilohertz.

The receiver must have the 2 metre band. Most public receivers cover shortwave only, and the ones with VHF are fewer and sometimes serve one band at a time, so the choice is narrower than for an HF station.

In ShackDeck

Station catalogue → Fax and digital signals → APRS 144.800 (Europe). The program picks a public receiver that has 2 metres, tunes it to 144.800 MHz in narrow FM and sets the APRS mode; no receiver, antenna or account of your own is needed. The APRS panel under the waterfall lists the packets as they arrive, with the callsign, what the station is and the details it sent; every station gets a symbol on the map with its callsign as a label, weather stations with the temperature next to it, and a click on a symbol opens a popup with the heading, speed, altitude or weather. A click on a row in the list centres the map on that station. Entries stay on the map for an hour after their last packet. If the receiver picked is busy, the station's card offers the others with the frequency.