Tutorial LoRa APRS

Tutorial: LoRa APRS

What it is. LoRa APRS carries APRS packets (positions, messages, weather) over LoRa on the 70 cm amateur band. Small trackers and iGates use it. OnAir lists the stations it hears and shows them on a map.

Read this first. Receive only; OnAir does not transmit. Amateur radio signals are meant to be received, and receiving needs no licence in most countries. Check the rules where you live. See Legal.

What you need

• A radio that reaches 433 MHz at 1 to 2 Msps. An RTL-SDR is enough. The mode table covers 400 to 2500 MHz.
• A 70 cm antenna.
• Frequencies: 433.775 MHz (the start; Europe, SF12, 125 kHz). The mode tooltip lists the known channels: 433.775 MHz SF12 4/5 (EU), 434.855 MHz SF9 4/7 (PL), 439.9125 MHz SF12 (UK) and 915 MHz SF12. LoRa APRS uses sync word 0x12.

Before you start

SF12 signals are long (up to a second or more) and weak. Moderate gain is fine; patience matters more.

Step by step

1. In the mode list open Amateur: voice & packet and choose LoRa, 433.775 MHz.
2. Pick the radio and start the receiver.
3. Open Settings. Under Networks, make sure LoRa APRS is ticked; the other networks (Meshtastic, MeshCore, MeshCom, LoRaWAN) can stay off to save CPU. Under Band plan pick your region, or use Custom to set the LoRa setting by hand (bandwidth, spreading factor, coding rate, sync word, preamble).
4. Watch Stations, Map and Packets.

What you should see

Stations appear with their callsign-SSID, position and last packet. Beacons come every few minutes. The map plots stations that sent a position.

Reading the screen

• Stations: the station table with callsign, symbol, position, comment and last heard.
• Map: stations on the map.
• Packets: the raw APRS packets with the radio data (SNR, frequency offset, spreading factor).
• Settings: band plan, LoRa setting and networks.

Options and settings

The Settings tab also searches every known LoRa APRS channel inside the captured band. Custom keeps one LoRa setting of your own for networks not using the usual presets.

Troubleshooting

• Nothing. Local activity may be low. Check the frequency for your country and the sample rate.
• Packets but no map positions. The packets carry no position or only a message.
• High CPU. Untick other networks.
• A weak station shows errors. Improve the antenna.

Tips

Leave it running; trackers beacon slowly.

Legal and privacy notes

APRS is public by design, but positions of individuals are personal data. Do not publish a person's track without consent.

Under the hood

LoRa physical layer; the payload is a plain-text APRS packet, with a short LoRa APRS header. See also Tutorial-LoRa-raw.

Related

Mode-LoRa-APRS, Tutorial-MeshCom, Tutorial-LoRa-raw, Tutorial-Meshtastic, Live-map, Modes, Main-window, Supported-radios, Troubleshooting, FAQ, Legal.