In-flight Wi-Fi is getting faster and cheaper, and is an increasingly common offering on budget and flagship airlines alike. "Sorry I missed your email – I was on a plane" is an excuse that simply doesn't cut it anymore. But how does in-flight Wi-Fi actually work?

To simplify, there are two ways for an internet signal to reach your device at 35,000 feet. The first is via ground-based mobile broadband towers, which send signals up to an aircraft's antennas (usually on the base of the fuselage).

As you travel into different sections of airspace, the plane automatically connects to signals from the nearest tower, so there is (in theory at least) no interruption to your browsing. But if you're passing over large bodies of water or particularly remote terrain, connectivity can be an issue.

The second method uses satellite technology. Planes connect to satellites in geostationary orbit (35,786km above the planet), which send and receive signals to earth via receivers and transmitters. These are the same satellites that are used in television signals, weather forecasting, and covert military operations.

Information is transmitted to and from your smartphone via an antenna on the top of the aircraft, which connects to the closest satellite signal. Information is passed between the ground and the plane via the satellite. Wi-Fi signal is distributed to plane passengers via an on board router.

In both cases, the US has a much more developed infrastructure than anywhere else in the world – so US carriers have a better (and cheaper) Wi-Fi offering than those in Europe.

Why is in-flight Wi-Fi so slow?

Technology is developing fast, but it has struggled to keep up with the sophistication and sheer number of Wi-Fi-guzzling devices.

Back in 2008, when in-flight broadband company Gogo (then known as Aircell) launched its first onboard Wi-Fi service on a Virgin America plane, the 3 Mbps connection was adequate for a few laptops (and streaming video was prohibited). But now, with every passenger toting at least one device to connect to countless apps, websites and services, there's a much greater strain on resources.

These days, a satellite connection offers around 12 Mbps, but satellites are expensive to maintain and upgrade – so that technology is lagging behind too.

According to UK communications regulator Ofcom, the average UK household internet speed reached 28.9 Mbps in 2016 – so in-flight Wi-Fi has a long way to go to catch up.

Airlines that offer free inflight Wi-Fi

1. Emirates
2. JetBlue
3. Norwegian
4. Turkish Airlines
5. Air China
6. Philippine Airlines
7. Hong Kong Airlines
8. Nok Air

Why is in-flight Wi-Fi so expensive?

All of that technology doesn't come cheap – and nor do the in-aircraft systems. Antennas also increase drag, adding fuel costs to the airline's bill.

Those fees – plus engineering and maintenance costs – are usually passed on to customers. The price of in-flight connectivity varies between airlines, although some offer free trials – for example, the first 10MB on an Emirates flight is free.

Will it get faster in the future?

Yes. Communications firm Inmarsat is working with Deutsche Telekom to develop the European Aviation Network [EAN], a high-capacity satellite Wi-Fi network backed up by ground towers, which promises "a reliable high bandwidth broadband service in the air" throughout Europe. The EAN is slated to enter commercial service during 2017 – and British Airways has reportedly already signed up.

"Over half of the world's aircraft will be equipped for in-flight Wi-Fi within the next six years," says Inmarsat. "It is set to become a billion-dollar revenue sector by 2020."

Gogo, meanwhile, currently has a monopoly on US in-flight Wi-Fi, with a network that covers the whole country. It has been criticised for its painfully-slow upload and download speeds, but its new 2Ku service promises upgraded antennas and satellite services, delivering up to 70 Mbps – much faster than your average connection on land.

The Telegraph, London