ADS-B has changed traffic awareness in general aviation. Pilots who fly with a receiver see targets on their EFBs that would have been invisible ten years ago. But ADS-B was never designed to show you everything. Supplemental traffic sources fill the gaps that ADS-B leaves behind, and understanding those gaps is the difference between a partial picture and a genuinely useful one.
This post is part of our complete guide to electronic conspicuity and portable ADS-B in general aviation.
What ADS-B Alone Misses
ADS-B Out works by broadcasting GPS-derived position data on 1090 MHz (or 978 MHz UAT in the US). If the other aircraft is transmitting, you see it. If it is not, you don't. The system's biggest limitation is also its simplest: it only shows aircraft that participate.
Large categories of airspace users fly without ADS-B Out. In Europe, there is no mandate equivalent to the FAA's 2020 rule for most of the GA fleet. That means a significant portion of the aircraft sharing your airspace are electronically silent to a standard ADS-B receiver.
Gliders and sailplanes are the most obvious gap. Across Europe, thousands of gliders fly cross-country routes during the soaring season. Most carry FLARM, not ADS-B. A Discus-2 thermalling at 4,500 ft ahead of your PA-28 will not appear on an ADS-B-only display.
Ultralights and microlights operating in Class G have no transponder obligation in most European states. A Eurostar or C42 flying a local sortie from a farm strip is legal, airborne, and invisible to ADS-B.
Drones represent a growing and rapidly expanding traffic category. Commercial operators flying agricultural surveys, infrastructure inspections, or mapping missions at low level increasingly use remote identification and network-based tracking, but they do not broadcast on 1090 MHz.
Paragliders and paramotors fly at the altitudes GA traffic uses, particularly near hills and ridgelines. They carry no ADS-B equipment.
Military traffic may fly with ADS-B switched off under national security exemptions.
The result is that a receiver tuned only to 1090 and 978 MHz gives you a filtered view of the airspace. In busy GA environments, that filter can hide the traffic you most need to see.
The Supplemental Traffic Landscape
Several technologies exist to track the aircraft that ADS-B misses. Each serves a specific community, and each operates on its own protocol.
FLARM
FLARM is the standard collision-avoidance system for gliders in Europe. Developed in Switzerland and widely adopted since the mid-2000s, it broadcasts position, altitude, and predicted flight path on 868 MHz. FLARM is mandatory at many European gliding clubs and competitions. PowerFLARM extends the concept with ADS-B reception built in, but the core FLARM signal remains separate from ADS-B and unreadable by a standard 1090 MHz receiver.
A network of ground-based FLARM receivers relays this data to centralised servers, making it available over the internet to aggregation platforms.
Open Glider Network (OGN)
OGN is a community-run network of ground receivers that collects FLARM, FANET, and other signals from unpowered and lightly powered aircraft. Volunteers operate receiver stations across Europe, feeding position data into a shared database. OGN coverage is particularly strong in gliding-dense regions like the Alps, southern France, and central Germany. The data is open and freely available to platforms that integrate it.
FANET
FANET (Flying Ad-hoc Network) is a peer-to-peer protocol used by paragliders and paramotors. Devices share position data locally among nearby FANET-equipped aircraft and also feed into aggregation networks via ground stations. For GA pilots flying near ridge soaring sites or hillside launch points, FANET data reveals traffic that operates at similar altitudes without any radio transponder.
Mode-S and MLAT
Mode-S transponders broadcast a unique aircraft address but do not always include GPS position. Multilateration (MLAT) uses multiple ground receivers to triangulate position from Mode-S signals, generating position data for aircraft that have transponders but lack ADS-B Out. This captures a segment of older IFR-equipped aircraft and military traffic that responds to SSR interrogation but does not broadcast ADS-B.
Community and App-Based Data
Pilots using mobile apps can share their position via cellular data. This voluntary reporting adds another layer of visibility, particularly for aircraft that carry no electronic conspicuity equipment at all but whose pilots run a tracking app on their phone or tablet.
How Aggregation Works: The SafeSky Model
Individual data sources are useful. Combined, they become powerful. This is where aggregation platforms change the picture.
SafeSky is the most established aggregation network in European GA. It collects traffic data from more than 30 sources, including ADS-B ground stations, FLARM receivers, OGN, FANET, Mode-S MLAT, PilotAware, and live position reports from its own network of over 75,000 active pilots. The platform merges these feeds into a single, deduplicated traffic stream, correlating data from multiple sources to confirm positions and reduce false targets.
When a receiver has access to SafeSky's aggregated feed, every data source contributes to one unified traffic picture. A glider transmitting FLARM appears alongside a Cessna broadcasting ADS-B and a paramotor reporting via FANET. The pilot sees all three on the same display or EFB overlay.
For a deeper look at how this multi-source approach improves flight safety, see how multiple data sources create a safer flight.
Real-World Scenarios
Ridge Soaring Near Gliders
You are flying a Robin DR400 along a ridge in the French Alps at 5,000 ft. Three gliders are working a thermal 2 NM ahead, circling between 4,500 and 6,000 ft. All three carry FLARM. None broadcast ADS-B. On a standard ADS-B receiver, the ridge ahead looks clear. With supplemental traffic sources integrated, all three gliders appear on your display with altitude and track information, giving you time to alter course or adjust altitude.
Busy Uncontrolled ATZ
Saturday morning at a regional airfield with no tower and an active glider club, a flight school, and visiting traffic. A K-21 glider is on a winch launch, two Rotax-powered microlights are in the circuit, and a drone operator is conducting an agricultural survey 1 NM south at 300 ft AGL. ADS-B shows the two flight school PA-28s and a visiting Cirrus. Everything else is invisible unless your receiver pulls in FLARM, OGN, and drone registration feeds through an aggregation layer.
Drone Zone Awareness
Commercial drone operations are increasing across Europe, particularly for photovoltaic farm inspections and powerline surveys. These operations are often conducted under specific operational authorisations with defined volumes of airspace. Network-based remote identification data, when fed into an aggregation platform, can alert a GA pilot to active drone operations along their route before they reach visual range.
The Data Quality Question
Supplemental traffic data is not identical to direct ADS-B reception. Pilots should understand the differences.
Latency. Data routed through the internet carries more latency than a direct radio broadcast. ADS-B signals arrive at your receiver in milliseconds. Internet-aggregated positions may be 1 to 5 seconds old by the time they reach your display. For strategic awareness and planning, this is highly effective. For last-second conflict avoidance, the latency matters, and visual scanning remains essential.
Accuracy. Position accuracy varies by source. ADS-B with GPS provides the highest precision. FLARM positions from ground stations are accurate but dependent on receiver geometry. MLAT accuracy depends on the number and spacing of ground stations in the area. App-based reporting depends on the phone's GPS and cellular coverage.
Coverage gaps. Aggregation networks depend on ground infrastructure. In remote or mountainous areas with limited cellular coverage and few ground receivers, supplemental data may be sparse or unavailable. SkyRecon addresses this by continuing to display locally received ADS-B traffic on its built-in 3.4" screen even when internet connectivity drops, ensuring you always have a baseline traffic picture.
For more on why your ADS-B receiver may not be showing you everything, we break down the specific categories of invisible traffic in detail.
What to Look for in a Receiver
Not every portable ADS-B receiver supports supplemental traffic. Many are radio-only devices that display ADS-B and TIS-B, full stop. If supplemental traffic matters to you, look for:
- Internet connectivity (Wi-Fi or built-in cellular) to access cloud-based aggregation.
- Integration with an established aggregation platform that covers the types of traffic in your flying region.
- Dual-band reception (1090 + 978 MHz) so you get maximum ADS-B coverage as your baseline.
- A display or EFB output that merges all sources into one picture without requiring you to cross-reference multiple apps.
SkyRecon includes SafeSky Inside with access to more than 30 supplemental traffic sources beyond ADS-B. Its dual-band receiver handles 1090 and 978 MHz. Its built-in 3.4" display and GDL90 output to ForeFlight, SkyDemon, and EasyVFR present all traffic, regardless of source, in a single merged view with configurable proximity advisories.
For a broader look at how supplemental traffic data fits into your overall airspace awareness strategy, read how supplemental traffic data enhances airspace awareness.
Build the Complete Picture
ADS-B is a foundation. Supplemental sources are the rest of the building. Flying with only ADS-B data in European airspace means accepting blind spots that technologies like FLARM, OGN, and SafeSky have already solved. The aggregation model works because each data source covers a different segment of the traffic population, and when combined, the gaps shrink dramatically.
Explore the full landscape of electronic conspicuity and how these technologies fit together in our complete guide to electronic conspicuity and portable ADS-B. If you are ready to fly with the complete picture, SkyRecon is available for preorder with a €100 deposit.