Tutorial FLARM

Tutorial: FLARM (gliders and light aircraft)

What it is

FLARM is the collision-avoidance broadcast of gliders and many light aircraft. In Europe it transmits on 868.2 and 868.4 MHz. OnAir decodes the older protocol and shows aircraft in a table and on the live map, as the Open Glider Network does. Receive only.

What you need

• A radio that tunes 868 MHz and samples at least 1 Msps; both channels come from one capture centred on 868.3 MHz, so an RTL-SDR works. See Supported radios.
• An antenna for 868 MHz (a quarter-wave whip, about 8.6 cm, is enough to start); outdoors and high gives much more range.
• Frequencies from the project's database: 868.2 and 868.4 MHz (868.0 to 868.6 MHz SRD band), and 917 MHz for North America ("FLARM hops over the 902-928 MHz band"). The mode starts at 868.3 MHz; check the panel for what is decoded outside Europe.
• A correct computer clock: the packet key changes every 64 seconds, so the clock must be right to within a few seconds.

Before you start

• Set your receiver position, the same setting as the ADS-B screen (latitude and longitude, or a locator such as KO24). Without it the packets give no positions: the receiver completes them around your position, within about 300 km.
• Keep the gain moderate; 868 MHz is shared with other short-range devices.

Step by step

1. Choose the group Aviation, entry Gliders, 868.2 / 868.4 MHz (mode FLARM).
2. Enter the receiver position. If you type something invalid OnAir suggests the format (for example 54.687, 25.280 or a locator).
3. Press Start. The status first reads "listening on 868.2 and 868.4 MHz (tune 868.3 MHz, at least 1 Msps)".
4. Open Map or Table.

What you should see

Where gliding takes place and the weather is flying weather, aircraft appear within a minute or two. If you see "energy on 868.2 / 868.4 MHz, no decodable packet yet", there is a signal but no packet has been decoded: check the clock and the position.

Reading the screen

• Table: Aircraft (device ID), Type, Altitude, Speed, Track, Climb, Distance, Level, Heard and Packets.
• Map: the aircraft on the live map with their tracks.
• Counters: packets, "other packets", "FLARM (new protocol)" (only counted, without positions) and "privacy-flagged aircraft".

Options and settings

Clear forgets everything heard. The position field accepts decimal degrees or a Maidenhead locator.

Troubleshooting

• No aircraft: it may simply be quiet; check the status line and that the frequency is 868.3 MHz.
• Energy but no decode: the computer clock is wrong, or no position is set.
• No position for an aircraft: the new protocol is counted only.
• Aircraft missing that you can see overhead: FLARM units with the no-track or stealth flag are never shown, logged, saved or exported.
• Outside Europe: other regions use other frequencies; check the panel.

Tips

Short antenna cables matter at 868 MHz. Combine with ADS-B to see powered traffic alongside gliders.

Legal and privacy notes

FLARM positions are broadcast openly, but some pilots do not want to be tracked, so OnAir respects the no-track and stealth flags and never shows those aircraft or anonymous addresses. Do not try to identify privacy-flagged aircraft. Sharing to the public map is opt-in; privacy-flagged aircraft are never sent. See Privacy and Legal.

Under the hood

2-FSK at 100 kchip/s Manchester coded, packets with a time-dependent key (changing every 64 seconds) and a checksum; the old protocol is decoded, the new one is only counted.

Related

FLARM mode page, Tutorial: ADS-B, Live map, Privacy.