Passenger Internet on aeroplanes used to be rare in the 2010s but has become almost commonplace today. On most flights, the basic idea is that the whole aeroplane is treated like a flying Wi-Fi router that connects to the rest of the Internet through a long backhaul link. This means your phone or laptop never talks directly to the ground. Instead it talks to a Wi-Fi access point inside the cabin, which then sends your data out of the aircraft via radio links.

How is the backhaul setup?

The backhaul link these days comes in two main forms: air-to-ground (ATG) and via satellite. In the ATG system, the aeroplane connects to special cellular-like towers on the ground. These towers have antennae pointing up at the sky. The plane's antennae -- often mounted on the underside -- maintains a radio link with whichever tower is currently in range ('radio link' just means they send and receive electromagnetic signals in the radio frequency range, 3-3,000,000,000 kHz). As the plane flies, the ATG system hands off the connection from one tower to the next, similar to how your phone switches between cell towers if you're on a call in a moving vehicle.

Obviously this arrangement works well over land, where there's a dense coverage of towers, but fails over oceans, deserts, and in polar areas -- all places with fewer or no towers. This is where satellites help.

Satellite systems are more flexible because they don't care whether an aeroplane is over land or water. In this system, the aeroplane uses a dish or phased-array antenna, usually inside a bespoke hump-shaped structure on top of the fuselage, that's pointed towards the sky. This antenna will transmit data to a satellite in orbit, which will relay the data to a ground station connected to the regular Internet.

Historically, these satellites have been in geostationary orbit, which is a circular orbit 35,786 km above sea level. This means, even at the speed of light, there's a noticeable data latency. A signal round-trip time of several hundred milliseconds is common. Newer systems use satellite constellations that are located in low-earth orbit, that is, 150-2,000 km above sea level. Since these satellites are much closer to ground stations, the latency is lower. Sometimes they also allow for higher bandwidth if the aeroplane's antennae can track fast-moving satellites and hand off between them.

How are connections provided to users?

Inside the aeroplane, the connectivity network resembles that in a small office. There's a central server or router, a satellite or ATG modem, and several Wi-Fi access points distributed through the cabin. When you connect, your device joins this local Wi-Fi network. A captive portal page appears in your browser where you either pay, log in with a frequent-flyer account or accept some terms of use. Once you're authenticated, the onboard network will start forwarding your traffic over the ATG or satellite link.

The airline or provider can shape or in many ways filter this traffic, and often does. For instance, providers may block high-bitrate video streaming or VoIP calls, force images to be compressed, and aggressively cache web pages to load faster over the constrained data link.

The in-flight bandwidth is almost always lower than home broadband connections and it's also shared among everyone on board. If a single satellite beam is serving multiple aircraft at once, they may all be competing for the same capacity. This is why performance varies so much between flights, routes, times of day, and providers. More sophisticated networks try to manage this challenge by dynamically allocating bandwidth and upgrading to higher-throughput satellites or satellite constellations. Even then the basic constraints of limited spectrum, distance, and shared use remain.

Don't signals interfere with operations?

When cabin crew ask you to switch your phone off for take-off and landing and to use 'airplane mode' the rest of the time, they're mainly trying to eliminate lots of uncontrolled radio transmitters inside the cabin. A phone whose mobile (that is, cellular) radio is on does several things that are awkward for both aviation and telecom regulators. It periodically 'shouts' loudly in radio terms, scans across multiple bands to find towers, and maintains a connection. In a cabin with a couple hundred phones, that creates a messy blast of radio noise at unpredictable times and frequencies -- much like a noisy playground. In the worst-case scenario, some of that energy could be very close to the frequencies aeroplane radios, sensors and navigation beacons use.

While contemporary aeroplanes are well-shielded and the actual risk is very low, regulators prefer a conservative approach that minimises known unknowns.

Further, a plane full of phones talking directly to towers on the ground still moves quickly and is visible to many towers at once. These quirks can confuse handover algorithms and overuse capacity in the cellular network. This is why telecommunications rules in many places explicitly forbid phones from connecting to terrestrial mobile networks at cruising altitude, and 'airplane mode' is a simple way to comply.

To its advantage, passenger Internet uses equipment that's part of the aircraft's certified systems. The Wi-Fi access points inside the cabin and the satellite or ATG radios that link the aeroplane to the outside world are all installed and tested together with the avionics. The engineers who design them also pick frequencies that are well separated from the bands that critical onboard systems use and design the antennae and cabling to minimise signal 'leaks'. They're also expected to prove (through testing) that none of these systems can fail in a way that also destabilises the aeroplane.

Taken together the in-flight Internet apparatus addresses the radio interference risks differently from how individual cellphones do, quelling regulators' anxieties while allowing you to stay online.

How are passengers able to access the Internet on aeroplanes?