FLARM and the Open Glider Network: receive and feed gliders yourself

A glider circles silently over the slope, two motor gliders head toward the airfield – and on your flight tracker app the sky is empty. That isn’t your app’s fault. Many general aviation aircraft don’t transmit ADS-B but FLARM. And it is exactly these signals that the Open Glider Network (OGN) picks up. This guide explains how FLARM works technically, what the OGN makes of it, how to receive gliders yourself and feed them into the network with your own station – and why privacy plays a bigger role here than with airliner tracking.

FLARM and OGN: the short answer

FLARM is a collision warning system for small aircraft. The onboard device knows its position by GPS, extrapolates its own flight path for the next few seconds from that and broadcasts it every second. Other FLARM devices in range compare this prediction with their own path and warn only when things really get tight. In Europe this runs in the license-free 868 MHz band.

The Open Glider Network takes advantage of the fact that these signals are in the air anyway. Ground receivers at airfields, at clubs, on mountains and at private homes decode the packets and send the positions to central APRS servers. From there, live maps, competition trackers, automatic logbooks and search and rescue services draw on them. If you set up your own station, you extend the coverage – much like feeding ADS-B.

Why gliders are missing from flight trackers

The big flight tracker services live mainly on ADS-B on 1090 MHz, supplemented by MLAT. How that works in detail is explained in the guide Flight tracking with ADS-B for spotters. Airliners practically always transmit their GPS position via ADS-B. In general aviation it looks different: many gliders have no transponder at all, others a Mode S transponder without position information. What the individual transponder modes transmit is covered in the article Transponder Mode A, C and S explained.

Such aircraft show up on classic maps at best via multilateration – and only if enough receivers are in range. Low over the slope or in an Alpine valley, that rarely works. More on this in the guide to MLAT and multilateration and in the overview Why an aircraft doesn’t appear on Flightradar.

FLARM closes exactly this gap. Because the system was invented for gliding, the equipage rate there is especially high. By now you also find FLARM in powered aircraft, helicopters, ultralights, paragliders and drones. The manufacturer lists air rescue and emergency services as a user group of their own – one reason why the guide to Helicopters on flight trackers is interesting for OGN users.

How FLARM works technically

Behind FLARM is a Swiss company based in Zug. It was founded in 2003, and the first devices came on the market in early 2004. Today numerous licensees also build FLARM-compatible devices.

The principle is simple and clever at the same time:

• Own position: A built-in GPS receiver provides position, altitude and course.
• Prediction instead of snapshot: The device transmits not only where it is right now but where it is flying in the next few seconds. That is crucial especially when circling in a thermal, because two gliders in the same thermal fly in very close quarters.
• Targeted warning: Other devices evaluate these predictions and raise the alarm only when the paths actually come dangerously close.
• Encrypted radio: The FLARM radio protocol is encrypted. The manufacturer justifies this with system integrity, data protection and safety.

Newer PowerFLARM devices additionally receive ADS-B and transponder signals so that glider pilots can see large aircraft early too. According to the manufacturer, the current models reach ranges of more than ten kilometers.

What is ADS-L?

For some years now, another abbreviation has appeared around FLARM: ADS-L. Behind it is a standard developed under the umbrella of EASA for electronic conspicuity in light aviation, which also works in the 868 MHz band. ADS-L matters mainly for U-space, that is, airspace in which drones and manned aviation are coordinated. FLARM stresses that ADS-L complements its own system rather than replacing it; many current devices can be retrofitted by license. The OGN already lists ADS-L among the supported sources. If you are interested in the drone side: the article Remote ID for drones shows how unmanned aircraft identify themselves.

The Open Glider Network at a glance

The OGN is a purely community project. Software, hardware and receivers come from individuals, clubs and the open-source scene. The project page says, roughly, that a station costs most glider pilots less than two aerotows – a comparison that says a lot about the target group.

The network consists of four building blocks:

Building block	Task	Who runs it
Ground receivers	decode radio packets in the 868 MHz band	clubs, airfields, private individuals
APRS servers	receive positions and pass them on	OGN infrastructure
Device database (DDB)	links device addresses with registration and type	entries by the owners
Display services	live maps, competition trackers, logbooks	various developers

Not only FLARM signals are received. The OGN also names its own OGN trackers, FANET (common with paragliders and hang gliders), PilotAware and ADS-L. The OGN protocol itself is openly documented – unlike FLARM.

The data is freely usable as long as providers follow the OGN’s terms of use. That is why there is a whole range of map views: the classic OGN live map, 3D viewers, services such as PureTrack or SafeSky and several automatic logbooks for clubs. Especially handy for spotters: these logbooks show which aircraft took off and landed at a field.

Privacy: who is seen and who is not

With airliner tracking, anonymity is hardly an issue. With glider and private pilots it is – private individuals sit in the cockpit here, and a flight track shows when someone was where. The OGN therefore has a tiered system:

• Not registered (default): The device is visible in the network but not identifiable on the OGN live map. It appears under a random name that starts with an underscore and changes daily.
• Registered in the DDB: The owner enters the device with its FLARM ID or ICAO address and decides which details are public. Only then do you see the registration or type.
• “Do not track” in the DDB: The position is hidden on the live map but still logged internally so that help is possible in an emergency.
• No-tracking flag on the device: From FLARM firmware 6 on, a flag can be set that makes OGN receivers discard the packets immediately. Then there is neither display nor log – and the OGN explicitly points out that in this mode it can’t help even in the search for a missing aircraft.

There is also FlarmNet, a database run by the manufacturer in which pilots can link their device address with name, registration, callsign or type. Cockpit displays load this file to name the traffic around them. FLARM points out that anything entered there should be considered public.

Building your own OGN station: hardware

You probably know the hardware of an OGN station from ADS-B reception already. The difference lies in frequency and antenna.

Component	ADS-B feeder	OGN station
Frequency	1090 MHz	868 MHz (Europe)
Computer	Raspberry Pi or similar	Raspberry Pi or similar, preferably the same one
Receiver	RTL-SDR stick	separate RTL-SDR stick
Antenna	tuned to 1090 MHz	tuned to 868 MHz
Software	readsb, dump1090 etc.	RTLSDR-OGN
Data destination	feeder networks	OGN APRS servers

A few points to keep in mind:

• A second stick is a must: An RTL-SDR covers only a narrow frequency window. 1090 and 868 MHz at the same time doesn’t work with one stick. Which sticks have proven themselves is shown in the SDR stick comparison for ADS-B – the criteria apply similarly to 868 MHz.
• Frequency stability matters: Sticks with a temperature-compensated crystal (TCXO) need hardly any correction. Older or cheap sticks can be off by 50 to 100 ppm according to the OGN wiki, which you have to calibrate out.
• The antenna makes the difference: A quarter-wave antenna for 868 MHz is about 8.6 centimeters long. Ready-made antennas for the band exist because 868 MHz is also used for many indoor radio applications. How to find a good location is in the guide Antenna location and mounting.
• Bias-T optional: If you want to power a preamplifier on the mast through the antenna cable, the OGN software supports bias-T power on suitable sticks.

If you are starting from zero, the basics article Building your own ADS-B receiver is worth reading first: operating system, network and enclosure work the same for both projects.

Building your own OGN station: software and registration

Setup follows a clear sequence that the OGN wiki documents in detail.

For the station name a fixed rule applies: three to nine characters, letters and digits only. The name of the town, mountain or airfield is customary.

If you already run your feeders in containers, it is worth checking whether OGN reception can be integrated there as well – the setup above follows the classic installation according to the OGN wiki. How to organize several networks in parallel on one computer is explained in the article Feeding multiple networks with Ultrafeeder. An overview of the classic ADS-B feeders is provided by the feeder software comparison.

Where an OGN station is worth it

Unlike with ADS-B reception, your targets often fly low, slowly and far from big airports. The range of an 868 MHz station therefore depends even more on a clear line of sight.

You can find out which small airfields and gliding sites are near you with the tips from Finding airfields on maps.

What spotters get out of it

For spotters, OGN opens up a world that is almost invisible on classic flight tracker maps:

• Plan gliding days: On the live map you see whether flying is going on at a site on the weekend and how many aircraft are in the air.
• Follow competitions: Live services such as Glide and Seek or Cloudracing display OGN data and are also suitable for following competition flights. For photographers at the finish point, that is a good timer for the final approach.
• Logbooks as a log: The automatic takeoff and landing lists tell you which aircraft use a field regularly – ideal for targeting rarities.
• Practice types and registrations: Registered devices show registrations and often the type. That helps with recognizing small types; the guide Identifying light aircraft provides the matching features.

How to approach general aviation photographically, which fields are suitable and which rules apply at a club site, you can read in the article Spotting general aviation. And if you want to understand what goes on behind the cockpit, the post Glider pilot license (SPL) shows the way to your own license. The history of gliding, hands-on, is at the German Gliding Museum on the Wasserkuppe.

What to watch for

• Keep your distance at the field: Gliding sites are active airfields with winch launches and cables on the ground. Stay outside the operating areas and ask the flight operations supervisor if in doubt.
• Allow for data gaps: OGN doesn’t cover everything either. Deep in valleys or in areas without a station, aircraft vanish from the map even though they are flying.
• Keep time in sync: Without a correct system time, the station works unreliably. If the service doesn’t start after a restart, it is often still waiting for time synchronization.
• Don’t overdrive: Too much gain brings more noise instead of more range. The automatic control is a good starting point; more on the signal chain is in the guide Boosting ADS-B range.
• Respect opt-outs: Data from the OGN is subject to its terms of use. Anyone who processes it further must respect the pilots’ decisions.

Conclusion

FLARM was built so that glider pilots don’t run into each other in thermals. The Open Glider Network turns these signals into an open tracking network that fills gaps left by the classic flight tracker services. With a second SDR stick, an 868 MHz antenna and the RTLSDR-OGN software, you build your own station and make the sky over your slope more visible to everyone. What matters is respecting the pilots’ privacy decisions – then spotters, clubs and rescue services all benefit alike.