Electronic Conspicuity Monitor

A live measurement of how aircraft make themselves visible electronically: ADS-L by radio and, as its mobile variant takes shape, by phone, alongside FLARM, FANET and the phone apps that already share positions. It starts from the questions the data can answer; each answer links to the evidence, and the method behind every figure is published, rules first and data after.

Live

1 · Should ADS-L carry the turn rate?

When a pilot circles in a thermal, anyone predicting where they will be a few seconds later needs to know that they are turning. Either the transmitter says so, or the receiver works it out from the positions it has already received.

FLARM and the OGN tracker send their turn rate; ADS-L has no field for it, so an ADS-L receiver has to derive it from the last two packets it heard. When every packet arrives the two are a second apart and the derived turn is good. When packets are lost, as they are when the pilot’s body shields the radio, the last two can be many seconds apart: in 8 seconds a paraglider in a thermal turns through a third of a circle, in 16 more than half, and the turn worked out from them goes wrong. The packet that does get through still carries the turn rate.

The chart measures this on real tracks that send every second, by pretending the packets in between were lost: the receiver keeps the start packet and the one 2, 4, 8 or 16 seconds before it. It then compares, at the same instants, an arc with the turn the receiver derives from that pair, an arc with the turn rate the start packet carried, and a straight line. The error is the distance between each prediction and the position that then arrived.

2 · Radio or phone: which keeps a paraglider visible?

A paraglider pilot sits in front of the wing with the instrument on the harness, so the pilot’s own body stands between the radio and half of the sky. A phone in a pocket has the same body around it, but it talks to whichever mobile cell is in view.

Being counted is the first half of the question. The second is how well an aircraft stays visible while it flies. For each source and channel, this measures the share of airborne time during which somebody watching would place the aircraft more than a given distance from where it really is. The distance is the point: thirty silent seconds leave a paraglider 300 m from its last position and a glider at 150 km/h more than a kilometre away.

Two estimates of “where it is” are compared. The solid bar uses what the last packet carried: the aircraft moves on along its course and speed, or stays put if the packet says it is circling. The outlined bar assumes the aircraft stays at its last point, as a plain map shows it. The difference between the two is the value of course, speed and turn rate in the packet. The rules were fixed before the data was collected and are in the method.

The pilot’s body can be measured directly. A paraglider circling in a thermal turns through a full circle every 20 to 30 seconds, so its transmitter points at a ground receiver from every side in turn, equally often. An antenna that radiated equally well all round would be heard equally often from every side; where the body shields it, packets go missing. The chart counts the FLARM packets received from circling aircraft by the direction of the receiving station relative to the aircraft’s heading. Gliders are the comparison, and an imperfect one: FLARM itself says the human body attenuates its signal, and in most gliders the antenna sits on the instrument panel with the pilot right behind it. The chart therefore compares two ways of carrying a transmitter, on a paraglider pilot’s harness and in front of a glider pilot.

3 · Does mobile coverage fade with height above the ground?

Mobile networks are built for people on the ground. The question is how high above the ground a phone app keeps an aircraft visible, compared with radio.

The chart shows the share of airborne time during which the position was more than 300 m off, by height above the ground at the moment the aircraft was last seen. Heights come from a terrain model of Europe at 15 arc-seconds. The bands follow the limits that mobile operators and a 2021 study for EASA have stated: coverage reliable up to 300 m, patchy up to about 1,000 m, and a link lost between 600 and 1,200 m in the EASA trials.

4 · Every so many seconds, or every so many metres?

A rule in seconds asks every aircraft to transmit equally often, whatever its speed. A rule in metres asks it to transmit once it has moved, which spares the small battery of a paraglider instrument or phone and keeps every aircraft’s position equally fresh on a map.

For each source and kind of aircraft, the chart sets two readings of the same flights side by side: the share of airborne time that a 6-second rule would count as stale, and the share during which the position was in fact more than 300 m away from the aircraft. Where the first bar is long and the second short, the rule in seconds condemns an aircraft that anyone watching could still see; where it is the other way round, the rule passes an aircraft that has already gone.

5 · How much free flight happens where phone apps work?

Paraglider and hang glider pilots carry their aircraft up a mountain in a rucksack, and nobody can check what is in it. They do carry a phone. What matters is how much of their flying takes place where a phone keeps them visible.

A square of the map counts as covered when phone apps logged at least two hours of airborne time there in the month and kept their position within 300 m for at least 95% of it. The flying time of every paraglider and hang glider, whatever it carries, radio included, is then split between covered squares and the rest. Squares with too little app traffic to judge count as not covered, so the share is a lower bound.

6 · Where do aircraft disappear?

Each square is about 25 by 20 km. Its colour shows how much of the flying time spent there was lost from view, more than 300 m off, on the channel chosen above the map.

Adoption of ADS-L and of the other systems

Each transmitter identifies itself with an address. Most use a fixed one, either the aircraft’s ICAO address or one assigned by FLARM or OGN, and each of those is counted once per month. Some transmit a random address that changes at every power-up or more often, so the same device can show up several times in a month: those are drawn as a lighter band on top, and the true number of devices lies somewhere inside it. A device flying out of range of every receiver goes uncounted.

The current month is still filling up, so it is drawn dashed and its low value is no sign of a drop. Light aviation is also seasonal, and winter months will dip whatever happens to adoption. The fair comparison is the same month one year apart, which appears as a grey line once there is a year to compare with.

Radio and phones

Besides ADS-L, the Open Glider Network relays FLARM, FANET and the positions that several phone apps send over the internet, among them SafeSky, SeeYou Navigator and VarioVoice. The pilots using those apps are the most likely users of the third accepted way to be conspicuous inside U-space, the mobile network, once the ADS-L variant for phones is finished. The map above can show them, and the chart below counts every source in the feed.

Radio and internet figures measure different things. A radio count stops where the receivers stop. An app count has no coverage limit, but it includes only the users who chose to share their position, and only for the apps that forward to OGN at all. Neither is the number of pilots. ADS-B, mostly airliners, is left out of the chart.

What ADS-L is

ADS-L is a radio standard that lets light aircraft broadcast their position to one another, to ground stations and to drones. EASA published it in December 2022, with a second issue in December 2025, as one of the three accepted ways for a crewed aircraft to be electronically conspicuous inside U-space, the airspace where drone traffic is managed by digital services. The other two are ADS-B and the mobile phone network.

Anyone may implement it without a licence. Its radio channels were derived in part from FLARM and PilotAware so that existing devices could add it, and it was developed by a working group of FLARM, OGN, Skytraxx and Avionix under EASA oversight.

Method and data

The full method, with every rule, the decisions taken and the date each was fixed, is on its own method page. In short: every figure comes from the public APRS feed of the Open Glider Network, which carries what its volunteer ground receivers hear by radio and what apps and platforms forward over the internet. A packet counts as radio when it carries the reception figures a receiver adds, signal-to-noise and frequency offset, and as internet otherwise. Each source is identified by the destination field of the packet, as listed in the OGN protocol repository.

Every count is a floor. A device out of range of every receiver, or an app that does not forward to OGN, is not seen. Random addresses can count one device several times, and they are shown apart for that reason. No track and no position is stored: the database keeps one row per device and month for the counts, and daily totals for the visibility measure. The live map refreshes every five seconds and the statistics every ten minutes.