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Why does inflight Wi-Fi cut out or slow down?

Nov. 01, 2020
12 min read
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Internet service in the air can be frustratingly spotty and there's a good reason: It's not easy to provide.

"Wi-Fi in flight is super-complicated," said Ekrem Dimbiloglu, Delta Air Lines' director of onboard brand experience, in an interview last year with. TPG. "You're going 500 miles per hour at 40,000 feet. And in the case of Delta, we're operating in 50 countries, a mixed fleet, and often offering gate-to-gate internet."

"At my home, there's no slowdowns generally, because we're not moving at 500 miles per hour," he laughed.

On past Delta flights between JFK, Seattle, San Francisco and Los Angeles I had excellent connectivity for email, web surfing and chat. But on other flights I was not so lucky. On a 2019 flight with KLM from Amsterdam to JFK, I spent about an hour trying unsuccessfully to connect to the internet. I helped other passengers connect by clearing their KLM-installed cookies, but no dice for me. I couldn't get to the payment portal.

The blue screen of death: No WiFi on a KLM flight.

A KLM spokesperson said they couldn't explain my persistent lack of connectivity.

For its part, Delta has installed "2Ku" antennae on most of its fleet; you can spot the antenna by looking for the dome on the top of any plane equipped with satellite Wi-Fi. By the end of 2020, the Atlanta-based carrier planned to have 700 aircraft with 2Ku technology.

Last year, Delta has just over 500 planes equipped with it in its fleet of more than 900 aircraft. New additions, such as the Airbus A350 and A220, are delivered with 2Ku installed.

A Primer on Inflight Internet

There are two ways a plane receives a signal to connect its passengers to the internet:

  1. Line-of sight-connection, also known as Air-to-Ground or ATG;
  2. Satellite.

Airlines do not want hybrid solutions; it's one or the other, because ATG and satellite equipment adds weight and drag which translates directly to cost. The actual services are provided by inflight Wi-Fi connectivity companies that you'll be familiar with, such as Gogo, ViaSat, Panasonic or SmartSky. Each airline has arrangements with one or more of these providers. Delta, for example, works exclusively with Gogo, JetBlue is exclusive to ViaSat, and United uses several different providers.

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The signal going up to the plane is the "forward channel." The signal returning from the passenger is the "received channel." There are technical and practical limits on the data that can be transmitted on the return trip. (For example, a large video upload to Facebook isn't likely to be supported by the internal policies of the airlines. They want you to have internet connectivity but not that much. Your connection will be throttled back to manage the load.)

The plane has a server onboard that transmits the forward signal to a local area network (LAN), just as you have at your office or home. Each passenger's device connects separately to the LAN, with routers placed throughout the cabin. According to Peter Lemme, a satellite-communication expert and former chairman of an aviation connectivity industry group, the LAN itself is not a choke point; your fellow passengers who are streaming Netflix aren't clogging the Wi-Fi onboard for those who are simply browsing the web. That's because the Netflix streamers are simply receiving the forward signal in the form of video; they aren't sending anything back.

How air-to-ground Internet works

ATG is an on-the-ground cellular network of towers around North America that transmit beams to aircraft passing through areas of coverage. (That coverage radius is a function of the altitude of the aircraft compared to the ground station.) The service delivers up to 9.8 Mbps of internet connectivity to the plane which is shared by the users. However, the ground-based system is potentially beaming the internet to every Wi-Fi-enabled plane that enters the cellular area. Nowadays, the ATG network is overwhelmed by forward-channel demand, which Lemme said is the biggest chokepoint.

A Gogo internet 4G tower (Image via Gogo).

In a nutshell, an aircraft passing through a tower's radius of coverage connects to the internet by receiving "forward signals" (internet going up to the plane) and "receiving signals" (passengers sending email, browsing, etc.). A signal travels from the tower to the plane then onward to the passenger via the plane's LAN. And then, with a click of a mouse, data makes the return trip.

Gogo is the leader in the ATG space with 200+ towers planted across the continental USA and Canada in a 4G system, as seen above. The benefits of an air-to-ground network are that only four small, lightweight antennae are added to the aircraft belly and body and can be installed overnight without taking it out of service.

ATG provides a lot of inflight Wi-Fi right now (depending on the carrier), but satellite service is nipping at ATG's heels. In 2019. Gogo announced the launch of a 5G network, which will be ready in 2021. This will be achieved by adding more antennae to their existing network of towers, effectively increasing the number of beams/bandwidth shot up into the sky.

Air-to-ground communications antennae (Image via Gogo).

How satellite Internet works

Satellite internet is a whole different beast. First, a satellite internet connection is much faster than an ATG connection but also more expensive. Internet connectivity is beamed from the ground to a network of satellites on either the Ka or Ku band, which are particular portions of the electromagnetic spectrum. The Ka band is used for Directv (i.e., all those little satellite dishes pointed out the window of houses and apartments).

A Gogo teleport sending the internet to satellites 22,000 miles up (Image via Gogo)

Gogo also has a satellite network, which it operates on the Ku band. The network consists of 28 satellites owned by companies such as SES and Intelsat and 17 ground stations (called teleports) owned by Gogo.

Internet delivered via the Ka and Ku bands provides up to 100 Mbps of connectivity. Gogo has installed two Ku-band antennae in the little dome on the top of the aircraft. Inside that dome, one antenna is dedicated to receiving the signal and the other is dedicated to transmitting the signal. "It's spectacular technology," Lemme said, calling it "the pinnacle" of airplane connectivity — and that is the reason Delta and many of its peers have gone all in for it. Gogo's 2Ku has quickly become the system of choice, installed on 1,100 aircraft globally by 20 of the world's largest airlines.

It's not without cost, however. Kenny Kirchoff, director of airborne solutions at Kymeta, a provider of satellite technology for planes, trains and automobiles, explained that the air inside the 6-to-12-inch-high domes is some 40 degrees warmer than the ambient temperature in flight, a direct effect of the friction of air running across the dome — and a direct indicator of drag. The dome's added drag, according to Lemme, makes fuel consumption rise by 0.3%. Over time, that is a big expense. That's definitely added to the cost per available seat-mile for the airline number crunchers.

Why Is It Sometimes Spotty?

"[R]egardless of the provider, setting realistic passenger expectations is very important and is strongly correlated with passenger satisfaction," John Wade, Gogo's president of commercial aviation, told TPG last year. "A lot of separate elements have to work together properly in order to provide internet to a passenger," Kymeta's Kirchoff explained. Here is why hiccups happen:

Competition between airliners
You're not the only passenger up in the sky wanting internet access, even if it looks like there are fewer than a dozen people on your flight using the internet. Lemme explained that a few planes could be sharing a signal on an ATG network, with more sharing the signal on a satellite network. "One airplane does not rule the roost," Lemme said. This is by far the biggest challenge for ATG networks, and the biggest reason for slower speeds or connectivity issues.

Encryption technology
There used to be tricks to load web pages faster, such as rendering images in lower resolution, Lemme explained. Nowadays, however, encryption technology means that it's not possible to tell what type of data is coming through. Accordingly, encryption technology alone has reduced the tricks available to load pages faster.

Coverage areas and satellite handoffs
Geosynchronous-orbit satellites powering satellite internet do not provide connectivity to aircraft flying above 70 degrees of latitude. This means that flights such as this one from Newark to Hong Kong will not have coverage after entering northern Canada. For domestic flights, any gaps in coverage are during the handoff points between ATG towers or for flights across the North Atlantic between two different satellites.

Rain and snow
Ka-band satellite technology (less so Ku-band) is susceptible to interference from precipitation, particularly in areas near the equator where there are rainy seasons with huge downpours. Precipitation that comes between the teleport and the satellite has a measurable impact on internet speed and connectivity.

It's a satellite: Not the best environment for computing
Lemme noted that satellites can be affected by cosmic rays and solar storms in a way that your home internet can not. Also, satellite communications require huge amounts of sophisticated computing power to manage geosynchronous orbit and multiple connections. "These are design challenges, driving cost and complexity," Lemme said.

Aircraft maneuvering
The satellite antennae currently installed in aircraft physically rotate to point at particular satellites to achieve connectivity. Turbulence aloft can physically jostle the antennae and cause temporary loss of connectivity, but Lemme noted turbulence is "hardly an issue." Aircraft banking in flight away from the satellite can cause a temporary signal blockage.

Distance from the satellite (latency)
"For providers like Gogo, bandwidth is easy to fix," according to Dr. Fabián Bustamante, a computer science professor at Northwestern University and a co-author of research on inflight Wi-Fi. (He was perhaps underestimating the investment needed to build more towers or antennae on the network.) "The real issues are packet loss and latency. And that's not going to be easy to fix, given the distance between the earth, the plane and the satellites."

Latency means the amount of time between transmission and receipt of a signal. In the case of a satellite internet connection, that signal must travel from the earth to a satellite some 22,000 miles in the sky, then back down to an aircraft. At its fastest this will take 1/4 of a second. Then the response goes from the aircraft to the satellite, and back down to earth. The round-trip takes half a second. "With gaming or voice communications, you're going to see latency issues," Lemme said. "But not so much with general business uses like surfing the internet." Passengers are used to the concept of buffering for a video, Bustamante said, which makes streaming video possible.

Air to ground...but your route is offshore
Flights from New York to Florida, for example, are likely to pass over the Atlantic Ocean, farther from the nearest ATG towers than would be ideal, Lemme said. If an airline uses air-to-ground for its Wi-Fi, there will be connection issues.

As noted above, the Gogo 2Ku satellite system provides up to 100 Mbps to the plane. But the airlines don't necessarily want passengers to use so much data. "As far as performance onboard, the speed a passenger gets today often depends on what speeds the airline wants to offer and pay for," said Gogo's Wade.

What about Free Internet at 37,000 Feet?

JetBlue has free onboard Wi-Fi on its fleet. Delta tested free Wi-Fi on 55 flights per day last year for two weeks to measure usage and adoption. "It generated rich data, including survey results and usage," Delta's Dimbiloglu said.

The economics of free Wi-Fi can be daunting, though. Pulling an aircraft out of service for four days of installation, plus the cost of the satellite antennae and labor, can approach $1 million per aircraft, Lemme said. And, on a good day, he said, airlines are paying $20-$50 per gigabyte of data transmitted to the aircraft. It's not inexpensive, but passengers are willing to pay for a solid connection.

"For Delta, this about passenger choice. Connecting to the internet via your device of choice and [receiving] world-class entertainment," Delta's Dimbiloglu said. "Our objective is for what you have at home, to have on the aircraft."

Mike Arnot is the founder of New York-based travel brand, Boarding Pass NYC and a marketing consultant to airlines, none of which appear in this story. He is also a private pilot.

Featured image by Alberto Riva