UKC

Flying back from Sweden to the UK today I glanced at my phone halfway across the North Sea and notice it was showing my position as a pulsing blue spot moving across the sea. It was in flight-mode, the wifi/roaming was off and (unsurprisingly) there was no phone signal.

I can only assume it was picking up something from the aircraft - anyone know what and how?

Cheers


Chris

In reply to Chris Craggs:

GPS? Some phones disable it in flight mode, some leave it on. It's a receiver, not a transmitter, so there's no worries about it interfering with the aircraft systems.

In reply to Chris Craggs:

GPS. On a decent phone (i.e. not some iPhone models), flight mode does not disable the GPS receiver.

In reply to both:

Doh!

Though in my defence I try to keep the GPS turned off.


Chris

In reply to Chris Craggs:

I've used my Ipad in Canada without wifi/or any other connection and it showed both map and position.

In reply to captain paranoia:


Debatable that it's a sign of a "decent" phone. Any RF receiver tuned to receive a specific frequency will also broadcast on related frequencies (a side effect of downmixing in the radio front end) which is the sort of thing flight mode is meant to prevent...

In reply to wintertree:

A 'decent' phone will use a dedicated GPS RF stage, rather than bodge it into its mobile data RF stage, which, requiring broadcast power levels, must be turned off. Since GPS is receiving signal levels below thermal noise, it doesn't need much power in the GPS RF stage.

In reply to wintertree:

'Broadcast' slightly overstates it, 'leak' might be a more appropriate and descriptive word.

I'm a little surprised handheld GPS works inside an airliner.
jk

In reply to jkarran:

The GPS (or at least the little 'where am I?') arrow was deffo turned off - so I'm still a bit confused - is it still on, even when its off?

Chris

In reply to Chris Craggs:
Well, if all your data channels were turned off, and GPS was turned off, where was it getting position data from? That simply isn't possible, so you must have left something on.

Now, whilst ground-based WiFi networks can be geolocated by Google sniffer vans and cloud-sourced data, I doubt if the plane's internal WiFi server is geolocating itself (linking avionics stuff to cabin services is a Bad Idea), so the most likely service is GPS.

We also have to ask what map the blue dot was displayed on, and where the mapping data was coming from (resolution, cached?)

Maybe someone nearby had GPS and WiFi turned on, and location reporting turned on, and Google cleverness was able to know that your WiFi could see the other WiFi, and tell your device where you both were. But you'd need WiFi enabled...

In reply to captain paranoia:

Latest version of iOS doesn't disable gps when airplane mode active.

In reply to Chris Craggs:

If the GPS was switched off and with GPS which is receive only it's the data processing that's power hungry so off is off or at the very least, it's a pretty deep sleep state. Most likely one of the other RF services you switched off with flight mode, Bluetooth, WIFi, or the core phone radio service was simply switched to listening but not transmitting and was picking up location data from other devices nearby that have GPS enabled.

The remaining possibility is that it's using the built in inertial sensors to infer position from the last known WiFi/GPS position and acceleration data integrated over time. Older aircraft navigation systems have used this technique to good effect as have submarines, both use much more stable electronics than are found in a phone as errors significant accumulate quickly from very small sensor offsets. Over that sort of time period with the low quality sensors in a phone I'd be surprised if it could be done but your dot could be 30miles out on the map over the North Sea and you'd hardly notice so it's just about plausible.

jk

In reply to all:

Cheers some great and really interesting answers here.

I must admit the position showed (on Flightradar24) was further south and east than I expected as outward we flew over Oslo but this was down towards the Danish coast.

Food for thought,


Chris

In reply to Chris Craggs:

What phone is it? On my Samsung S5 the GPS works in flight mode and it shows my position fine on the plane, as long as I'm in a window seat. I quite like keeping track of where I am!

In reply to Chris Craggs:

Perhaps flightradar plays the "last known" flight plan out in real time when it doesn't have any data connection to go on. Could explain why it wasn't accurate later in the flight, pilot might have deviated for weather or somesuch?

In reply to Chris Craggs:

I wonder how accurate any compass and inertial sensor in the phone might possibly be? My guesstimate would be that if these were used (as they are on some devices to interpolate between gps sampling) then they would probably be at least hundreds of miles out on even such a short flight they would probably get general direction about right though.

In reply to ultrabumbly:
My phone compass is surprisingly accurate but it can be seriously out if placed next to a big chunk of ferrous metal on the aircraft.

The blue dot on my map gps was spot on the other day over the southern uk. Surprised it wasn't shielded by the fuselage or the heating elements in the windshield.

In reply to Tim Davies:

would being inside the plane and the plane being symmetrical not cancel out any effects from the structure? I want to get on a plane and try this now.

I'd have thought it would be the inertial part of any system that might be less accurate as it would be designed with running and driving type accelerations in mind.

In reply to ultrabumbly:
Think the GPS receiver needs a fairly clear view of the satellites to work.

Not used the GPS to resolve the ground speed.

In reply to ultrabumbly:


As jkarran points out, the IMU (inertial measurement unit) in smart devices is not good for offset; this drifts in the order of a few seconds or tens of seconds. And when you then do the double integration from acceleration to position, with a drifting offset, you get a hopeless position.

The 3D accelerometer is really intended for changes (playing games by tilting, etc, detecting steps), rather than absolute position. At least, not without some significant and cunning processing to determine and remove the drifting offsets...

The 3D magnetometer and compass in the IMU aren't bad, but obviously susceptible to ferromagnetic disturbances.

A modern, sensitive GNSS receiver will acquire and track signals coming in via the windows in a plane or train, provided you're close enough to give a reasonable view. The obscured sky view means the position fix won't be brilliant (tending to see low elevation satellites), but it will be good enough to show your position on a map of the world...

In reply to Chris Craggs:

The "gps" icon on mine indicates that its using "location services" which is a google experience enhancement by location thing not a gps thing. Could it be you just switched that off?

In reply to Chris Craggs:



How were you using Flightradar in flight mode?

In reply to krikoman:


Flightradar was turned on when we took off - I put the phone into Flight Mode, then later checked the App which was when I spotted it appeared to still be 'working',


Chris

In reply to Tim Davies:



I've just come back from Slovenia and tracked my flight using a cheap £40 GPS device and my bike computer both had excellent reception, though I did have a window seat and shoved them both between the window blind and the window.

I was picking up 12 satellites at some point on the journey.

I've done this many times with a number of devices including my phone, if you move them away from the window they quickly lose the fix.

In reply to Chris Craggs:



Then you must of had a signal to the internet, somehow. Did the plane have wifi?

In reply to krikoman:


On Ryanair - you gottabe kidding!


Chris

PS Wifi was turned off too!