Satellite ADS-B explained: how flights over oceans and the poles become visible

You look at the flight tracker map in the evening, and over the North Atlantic a dense string of pearls made of aircraft symbols is heading toward Europe. Only a few years ago, exactly this part of the map was a big white hole – or at best full of estimated positions. The fact that you can now see where an aircraft is flying even in the middle of the ocean is thanks to a technology that was actually built for air traffic control: space-based ADS-B. This article explains how satellites receive ADS-B signals, who runs the systems, what has changed over the Atlantic as a result and where the limits are.

Satellite ADS-B: the short answer

With satellite ADS-B, receivers on board satellites listen in on aircraft’s normal ADS-B transmissions and relay them to ground stations. The aircraft itself does nothing different than over Frankfurt or Munich: its transponder regularly sends position, altitude, speed and identification on 1090 MHz. The only new thing is the receiver, which no longer stands on a roof or mast but orbits a few hundred kilometers above the Earth.

How ADS-B works in general and how to receive the signals yourself is explained in the guide Flight tracking with ADS-B for spotters. This one is about the part of the world that no ground receiver reaches.

Why ground receivers end at the coast

ADS-B is a radio signal with quasi-optical propagation. A receiver on the ground hears an aircraft only as long as it is above its horizon. Depending on flight altitude and antenna location, that is enough for a few hundred kilometers – no more. How strongly location and antenna affect range becomes obvious immediately when you build your own ADS-B receiver.

On land, this problem can be solved with many receivers. Over the open ocean, in the Arctic, in Antarctica or over large deserts, though, there are no places where you could set up radar installations or receivers. According to common estimates, ground-based surveillance covers only about a third of the Earth’s surface. The rest was long an area where air traffic control did not see its aircraft but had to rely on reports from the crew.

How ocean flights were monitored before

Over the North Atlantic, crews reported their position at fixed waypoints – earlier by shortwave radio (HF), later increasingly by datalink. Ten minutes or more could pass between two reports. Because controllers did not know exactly where an aircraft was between those points, they had to plan large safety margins: with this so-called procedural separation, aircraft were roughly 40 nautical miles or more apart longitudinally.

The datalink over which crews and controllers communicate over the ocean today is explained in the article CPDLC explained simply. Position reports via ADS-C over satellite communication are a separate technology – they are actively sent by the aircraft to air traffic control and are not to be confused with satellite ADS-B.

How reception from space works

The principle sounds simple but is technically demanding. A satellite in low Earth orbit sees a huge area at once – and with it very many aircraft at the same time. The individual ADS-B messages are short and are sent without coordination among themselves. Over heavily flown regions they overlap, and the receiver still has to tell them apart.

On top of that come distance and motion: the signal has to travel several hundred kilometers to orbit, and the satellite moves fast relative to the aircraft. The receivers are therefore built specially for this task and work with antennas that cover several narrow reception zones.

Why the top antenna counts

Many airliners have transponder antennas on the top and bottom of the fuselage and transmit through both. For a ground receiver the bottom antenna is the important one, for a satellite the top one. Aircraft that transmit only through an antenna on the underside of the fuselage – this mainly affects smaller general aviation aircraft – are therefore harder to receive from space. For long-haul airliners, this is rarely a problem.

The transponder itself stays the same. What Mode S, Extended Squitter and the various transponder modes mean is in the article Transponder modes A, C and S explained.

Aireon: satellite ADS-B for air traffic control

The most important provider is Aireon. The company was founded by Iridium in 2011. Later several air navigation service providers joined as shareholders, including Canada’s NAV CANADA, the UK’s NATS, Italy’s ENAV, Denmark’s Naviair and the Irish air navigation service. In July 2026, Iridium took over the remaining shares; since then Aireon has been a wholly owned Iridium subsidiary.

Technically, Aireon does not sit on satellites of its own. The ADS-B receivers fly along as an additional payload on the Iridium NEXT communications constellation:

• 66 active satellites in six orbital planes with eleven satellites each, plus spare satellites in orbit.
• About 780 kilometers altitude on near-polar orbits with an inclination of 86.4 degrees. That is why the system also covers the polar regions, which satellites on near-equatorial orbits never pass over.
• Crosslinks between the satellites: A received message does not have to wait until the satellite passes over a ground station but is passed on via neighboring satellites.

The satellites were launched into space in eight launches between January 2017 and January 2019. In spring 2019 the system went into regular operation, and Aireon received approval from the European Union Aviation Safety Agency (EASA) as a provider of surveillance services for air traffic control. Positions reach controllers after a few seconds instead of only after many minutes.

Spire: ADS-B from a small-satellite fleet

Spire Global takes a different approach. The company, founded in San Francisco in 2012, builds its own small satellites in CubeSat format, the so-called LEMUR satellites, manufactured in Glasgow. From 2018, Spire put ADS-B receivers into space on its satellites. The company sells the data under the product name AirSafe, mainly to airlines, data analysts and tracking providers.

The differences from Aireon are clear:

Feature	Aireon	Spire
Platform	Payload on Iridium NEXT	own small satellites (CubeSats)
Orbits	66 active satellites on fixed, near-polar orbits	Fleet with varying orbits
Data relay	via crosslinks between satellites	via ground stations
Coverage	continuous worldwide, including polar regions	not continuously gap-free
Main customers	air traffic control, plus commercial users	Airlines, data customers, tracking services
Certification	certified for surveillance in air traffic control	not certified as a primary surveillance source for air traffic control

Besides these two, there are test projects and new constellations. As early as 2013, the ESA satellite Proba-V carried an experimental ADS-B receiver from the DLR (German Aerospace Center), and small test satellites from the Danish manufacturer GomSpace also received ADS-B from space. Further European projects have been announced but are not yet in regular operation.

What has changed over the North Atlantic

The North Atlantic is one of the busiest oceanic airspaces in the world. On the eastern side it is controlled from the Shanwick control center in Prestwick (Scotland), on the western side from Gander in Newfoundland. Many flights between Europe and North America use routes set anew every day, the North Atlantic Tracks.

At the end of March 2019, NATS and NAV CANADA activated Aireon’s satellite data for this airspace. For the first time, controllers saw their aircraft in near real time, even hundreds of kilometers off the coast. The consequences:

• Shorter longitudinal spacing: Instead of about 40 nautical miles, the new procedures allow much smaller spacing between aircraft flying one behind the other, around 14 to 17 nautical miles depending on the procedure.
• Tighter lateral separation: For the lateral minimum, several steps from 23 via 19 down to 15 nautical miles were planned.
• More preferred altitudes and speeds: Because controllers can separate more flexibly, crews more often get their optimal altitude and speed. That saves fuel and CO₂ on every Atlantic crossing.

The role air traffic control plays elsewhere in Europe is explained in the articles on Eurocontrol and on DFS, the German air navigation service. A look at the long development from position reports to satellite surveillance is offered by the history of air traffic control.

After MH370: tracking becomes mandatory

The disappearance of Malaysia Airlines flight MH370 in 2014 showed how little could be known about an aircraft’s position over the open sea. The International Civil Aviation Organization (ICAO) subsequently introduced rules for flight tracking by airlines: larger airliners must be located regularly over oceanic areas, normally at least every 15 minutes. Satellite ADS-B is one of several ways to meet this requirement.

What you see of this on the flight tracker map

For spotters, satellite ADS-B is mainly an explanation of why the map over the ocean looks different today than it used to. Where a displayed position comes from is usually revealed by a flight’s detail window. Typical sources are:

Source	Where it typically appears	Accuracy for spotters
ADS-B from the ground	over land and near the coast	very high, updates within seconds
MLAT	over land, for aircraft without an ADS-B position	high, if enough receivers are in range
Satellite ADS-B	over oceans, polar regions, remote regions	high, availability depends on service and subscription
Estimated position	where no data is available	only rough orientation

How MLAT works is in the article MLAT/multilateration explained. Which symbols and colors the services use for this is shown in the article Flight tracker symbols and colors explained.

Satellite data is expensive. Many flight tracker services therefore show it only in certain subscription tiers, with a delay or in commercial products. If a flight over the Atlantic suddenly disappears or draws an estimated line, that is often not an outage but a question of data access. Which providers offer what is compared in the flight tracker apps comparison, and what the individual tiers at Flightradar24 include is explained in the article Flightradar24 subscriptions explained; all the typical causes of missing aircraft are collected in the article Why an aircraft doesn’t show up on Flightradar.

Spotting in practice: keeping an eye on long-haul flights

Satellite ADS-B gets exciting when you are specifically waiting for a long-haul flight – such as an aircraft from Air Canada or WestJet from Canada or an Icelandair flight coming across the North Atlantic. With an ocean position you know early whether the aircraft is on schedule and can plan your trip to the spot at your leisure.

Along the route lie airports that have been closely tied to Atlantic flying for decades: Shannon Airport on Ireland’s west coast was once an important fuel stop for transatlantic flights, and Keflavik Airport lies midway between Europe and North America. Many cargo flights on the polar routes between Asia and North America make stopovers at Anchorage Airport – at the spot Point Woronzof you can experience them up close, more on that in the guide Plane spotting in Anchorage. There in particular, satellite data is often the only source that shows a flight over the Arctic.

For the exact landing time and runway choice, though, you should rely on ground data. How accurate the ETA figures on flight tracker sites are is explained in the article How accurate are ETA forecasts?. And if you want to set alerts for particular registrations, the guide Setting up notifications for aircraft gives an overview.

What to keep in mind

• A satellite is not just a satellite. ADS-B via satellite, ADS-C via satellite communication and estimated positions are three different things. Look at the source information before you blindly trust a position over the ocean.
• No signal doesn’t mean no aircraft. Military aircraft, aircraft with ADS-B switched off or older transponders do not show up in satellite data either.
• Legal rules still apply. Whether and how you may receive and share ADS-B data is regulated by the respective country – details in the article Receiving ADS-B data: the legal situation.
• Your own receiver stays valuable. For aircraft in your area it delivers the fastest and most accurate data – and networks like the community trackers or ADS-B Exchange live on receivers on the ground.

Conclusion

Space-based ADS-B has closed the last big white spots on the flight tracking map. The principle is simple: the same signals your rooftop receiver hears are picked up from orbit. With it, Aireon delivers certified surveillance data for air traffic control, and Spire supplies commercial data customers. Over the North Atlantic, aircraft consequently fly with tighter separation and more efficiently. For you as a spotter that means: long-haul flights can be tracked earlier and more reliably – the rest is handled by ground receivers and a good spot at the fence.