Tutorial APRS AFSK

Tutorial: APRS and packet radio (AFSK 1200 baud)

What it is

APRS shares positions, weather reports, messages and object data over amateur packet radio. Most stations send short AX.25 packets at 1200 baud audio-frequency-shift keying on one shared channel; in Europe that is 144.800 MHz. You see a table of stations, a map and a log of every good packet. (LoRa APRS is a separate mode with its own entry.)

What you need

• Any SDR that tunes the band: an RTL-SDR covers roughly 24 MHz to 1.7 GHz at up to about 2.4 Msps (see Radio-RTL-SDR), and so do HackRF, Airspy, SDRplay and others (Supported-radios). The narrow signals here need only a small sample rate; the mode asks for 2 Msps (from 1 Msps).
• Antenna: a quarter-wave or 5/8 vertical for 2 m, outdoors if possible. A magnetic-mount whip on a car roof is a good start.
• Frequencies (from the database): Europe and Africa 144.800 MHz (the default), North America 144.390, Japan and Russia 144.640, New Zealand 144.575, Australia 145.175, Brazil 145.570 MHz, UHF 432.500 MHz in several countries, and the ISS packet digipeater at 145.825 MHz when it is active. The channel is 12.5 or 25 kHz wide; the panel has a Channel choice (12.5 kHz, 25 kHz).

Before you start

Set the gain so that the ADC level in the status line stays clear of full scale; use Auto-tune or Optimise gain (Your-first-reception). Too much gain makes strong neighbours (paging, FM broadcast, repeaters) leave spurs in the channel. Make sure the radio is on frequency: a cheap RTL-SDR can be several kHz off at 430 MHz, so enter its correction (ppm) in Radio settings or watch the carrier offset the panel shows. Narrow digital voice channels need an offset under about 1 kHz.

Step by step

1. Connect the radio and antenna and start OnAir. Open the group Amateur: voice & packet and choose AFSK (the entry reads AFSK, 144.8 MHz).
2. Enter the APRS frequency of your region (see the list) and set Channel to 25 kHz unless you know the channel is narrow.
3. Press Start. The panel says start the receiver to see stations before that; Signal shows no signal, signal, no good frame or the baud rate when a frame decodes.
4. Open Stations (a table), Map and Packets (the raw log).
5. On the map click a station to see its details; Listen is not needed because the data is decoded directly.

What you should see

Within a few minutes in a populated area you see stations such as digipeaters, weather stations, vehicles and handhelds. The status line gives the number of stations and good and failed the check packets; a good installation shows many more good ones than failed.

Reading the screen

Stations: Callsign, Symbol (the icon, for example a car or small aircraft), Frequency MHz, Last heard, Position and Comment. Map shows stations with a position; the number of stations with a position is shown. Packets lists the last good packets with Time, From > To,Path and Info, and copy all copies the log.

Options and settings

• Channel: 12.5 kHz or 25 kHz.
• The panel shows the rate being decoded: 1200 baud (AFSK) or 9600 baud.
• Zoom buttons and a time filter for old stations (minutes).

Troubleshooting

• Signal but no good frame. Probably an offset or the wrong channel width; try 25 kHz and a different gain.
• Failed the check, often. The packets collide with each other (it is a shared channel) or the signal is weak.
• Nothing at all. The region's frequency is different, you are outside coverage, or the antenna is inside. Move it outside.
• Stations without a position. Many APRS packets are messages or status only.
• ISS on 145.825. Only works during a pass and when the digipeater is on; plan with satellite tracking.

Tips

• Record the channel (Tutorial-Record-IQ-then-the-Signal-workbench) while it is busy and replay it to try settings.
• A scan (Main-window, tab Scan) helps to find an active channel.

Legal and privacy notes

APRS is public by design: positions and messages are sent in the clear to be seen. Receiving is legal in most countries. Do not stalk or publish private movements of individual operators. The map shows what is broadcast. Transmit works in OnAir (used on a real radio by the maintainer and in loop-back tests) but needs an amateur licence; see Transmit and Legal.

Under the hood

APRS packets are AX.25 UI frames (the link-layer protocol from the amateur packet network) sent as 1200 baud Bell 202 AFSK (1200 and 2200 Hz tones) in FM, with HDLC framing and a frame check sequence. The information field uses the APRS formats for position, weather, messages and objects. A 9600 baud (G3RUH-style) variant is also decoded.

Related

Mode-APRS-AFSK, Live-map, Tutorial-D-STAR, Radio-RTL-SDR, FAQ, Troubleshooting