Can you measure speed on a plane?
Yes. Your phone's satellite receiver still works at 11,000 metres. Hold it against the window, press Start, and you get the aircraft's ground speed in knots, its altitude, its track and the distance flown so far.
Ground speed isn't airspeed
This is the aviation version of what sailors deal with, and it's why the same route takes different times each way.
An aircraft flies through air that is itself moving. Airspeed is its speed relative to that air, and it's what the wings care about. Ground speed is its speed relative to the earth, and it's what your arrival time cares about. GPS can only see the second one.
The difference is wind, and at altitude the wind is huge. A jet stream can run at 200 knots or more. That's why a transatlantic crossing eastbound lands early and the same route westbound takes an hour longer and burns far more fuel.
Typical cruise numbers
With a strong tailwind an airliner's ground speed can pass 600 knots, well over 1,100 km/h, without flying any faster through the air than usual. That isn't the aircraft breaking records. It's the air carrying it.
| Aircraft | Cruise airspeed | Cruise altitude |
|---|---|---|
| Airbus A320 / Boeing 737 | 450 kn (830 km/h) | 11,000 to 12,000 m |
| Boeing 787 / Airbus A350 | 490 kn (900 km/h) | 12,000 to 13,000 m |
| Boeing 777 | 490 kn (905 km/h) | 11,000 to 12,500 m |
| ATR 72 turboprop | 275 kn (510 km/h) | 5,000 to 7,000 m |
| Cessna 172 | 120 kn (220 km/h) | up to 4,000 m |
| Concorde (retired) | 1,176 kn (2,180 km/h) | 18,000 m |
Why is the altitude different from the cabin screen?
What you see here is height above the WGS-84 ellipsoid, a mathematical model of the earth's shape. The cabin display shows pressure altitude against a standard atmosphere, which is what the whole air traffic system uses so every aircraft measures against the same thing.
They aren't the same, and a few hundred metres of difference is normal. Neither is wrong. They answer different questions. GPS altitude is also less accurate than your position, usually two or three times worse.
Getting a signal in the cabin
- The window is essential. An aircraft fuselage is an excellent Faraday cage. Away from the glass you get nothing.
- Press it against the pane and give it a minute. A cold fix takes longer up there.
- A wing seat is the worst. The wing blocks a big chunk of sky below the horizon, which is where satellites show up during a turn.
- Expect nothing at the gate and a good fix once you're level in the cruise and the aircraft stops manoeuvring.
Track and heading
The heading reading shows track over ground, the direction the aircraft is really moving. With a crosswind that differs from where the nose points, sometimes by ten degrees or more. If you've ever watched a plane approach a runway visibly sideways, that's the same thing at low speed.
Frequently asked questions
Can I use GPS on a plane?
GPS is receive-only and transmits nothing, so it interferes with nothing. But aeroplane mode is required on almost all carriers and usually turns location off, and some airlines ban GPS devices entirely. Check your flight's rules and follow the crew.
Why is my ground speed higher than the aircraft's cruise speed?
Tailwind. Ground speed is airspeed plus whatever the air is doing, and at altitude that can be over 200 knots. Eastbound transatlantic flights routinely show 600 knots over the ground.
Why does the altitude disagree with the cabin screen?
The cabin shows pressure altitude against a standard atmosphere, which is what air traffic control uses. This shows GPS height above a model of the earth. A few hundred metres apart is normal and neither is wrong.
It shows nothing at all. What's wrong?
You're almost certainly away from a window, or location is off with aeroplane mode. Hold the phone against the glass and give it a minute.
Does this work in a small aircraft?
Better than in an airliner. A light aircraft has far more glass and much less metal above you, so reception is usually excellent. Switch to knots, which is what the instruments use.