Antennas on other people’s roofs

A box on the balcony

Every airliner tells the world twice a second where it is, how high and how fast, plus its callsign and a code unique to the airframe. The signal is called ADS-B, it goes out on 1090 MHz, and since 2020 airliners in Europe and the United States have to carry it. It is not encrypted and nobody needs permission to listen. A receiver the size of a USB stick with an antenna on the balcony picks it up.

What it hears is decided by the curve of the Earth. Radio travels in straight lines, so an aircraft cruising at ten kilometres is visible from roughly four hundred kilometres away, while one circling just above an airfield disappears behind the horizon after a few dozen. A hill, a block of flats or a chimney cuts a slice out of the view, which is why every station’s reach is a lopsided shape rather than a circle.

So map.flights stands on people who run such a box at home and send the data to us too. Someone building a first station has no monitor or keyboard to hand: they flash the card, plug the box in and scan a QR code with their phone. The station opens its own Wi-Fi, the owner taps the antenna position on a map and sees what it receives within seconds, without an account.

When something is wrong, the station says so itself: a weak power supply, a receiver another program has already grabbed. The profile is public without the address. Only a coarse map cell and the country leave the building.

Overnight

Until early September the network grew by a few stations a day. Then, between 6 and 7 September, the count jumped from 66 to 111.

On 28 September there were 170 stations in 24 countries on six continents. A typical one hears aircraft out to about 370 kilometres, which is close to what the curve of the Earth allows.

In the United States some stations listen on a second frequency too. Small aircraft flying low there may send their position on 978 MHz instead of 1090, and an owner who adds a second receiver sees on the profile how many of them it caught. We count those separately, because a zero on the profile has to mean that we looked and found none, not that we never looked.

More receivers than the leaderboard shows

Anyone who ticks map.flights in adsb.im starts sending data straight away, and many never get round to registering. Those receivers have no name and no profile, and they stay out of the leaderboard. On the evening of 28 September, 239 of them were connected at one moment, while 147 registered stations were connected the same way at that moment.

Plenty of stations feed several networks at once, and that is fine by us. The data belong to whoever receives them, and one antenna can help as many maps as its owner likes.

An ordinary station contributes a few hundred thousand positions a day, the busiest one almost nine million. In bytes it is modest: typically tens of megabytes a day, just over a gigabyte for the busiest stations in crowded airspace.

Even an anonymous receiver gets a place on the map of the network if it takes part in computing the positions of silent aircraft. It is drawn in a paler colour, only to the nearest 25-kilometre cell, and we never credit it with a range or a score, because we have never measured it.

A record that wasn’t

In September one station near Prague suddenly reached 822 kilometres. It had not grown a better antenna. It had heard an aircraft whose satellite navigation was being jammed, and the aircraft was broadcasting a position far away over Ukraine.

The aircraft gave itself away, though: it signalled that it didn’t trust that position itself. Since then a position the aircraft itself doesn’t trust no longer counts towards a station’s range. Every position still counts towards how many aircraft a station heard. Hearing it was real; only the distance was not.

More than half of the positions from our own network come from people we don’t even know by name.