Just how high do mobile signals go?

Caporegime
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I’m Currently in Cyprus on holiday and my son has just come to me asking why he’s had numerous texts from EE.

Looking at the date and times of the texts they correspond with him and his mum flying out here two weeks back (I joined them last Friday) , in a nutshell he’s got a text from EE saying for each country on the flight path “Hi Welcome to Austria, Germany, Bosnia, Croatia etc etc “

At the time the phone was in the hold - we’d clearly not switched it off - and for the duration of the flight the plane was at 37,000ft ~ 7miles up.

What puzzles me the most is I remembered mid flight to switch my iPhone to airplane mode, when I looked at it on the aircraft, it displayed “No Service” which at that altitude I’d expect.

Can anyone explain how his iPhone managed to get a signal from that height over each country he passed over?
 
Last edited:
Caporegime
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Thread title should have read do not to!

I’m Currently in Cyprus on holiday and my son has just come to me asking why he’s had numerous texts from EE.

Looking at the date and times of the texts they correspond with him and his mum flying out here two weeks back (I joined them last Friday) , in a nutshell he’s got a text from EE saying for each country on the flight path “Hi Welcome to Austria, Germany, Bosnia, Croatia etc etc “

At the time the phone was in the hold - we’d clearly not switched it off - and for the duration of the flight the plane was at 37,000ft ~ 7miles up.

What puzzles me the most is I remembered mid flight to switch my iPhone to airplane mode, when I looked at it on the aircraft, it displayed “No Service” which at that altitude I’d expect.

Can anyone explain how his iPhone managed to get a signal from that height over each country he passed over?

Nothing to block the signal. You had a few more layers.
 
Soldato
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Radio waves travel an amazing distance, which is why they are used in telecommunications... Is this surprising? The atmosphere is relatively shallow compared to how wide it is
Can also make a difference depending what frequency your phone can handle, lower frequency will have a higher range.
 
Associate
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Sounds like he didnt get most of those texts during the flight.

His phone was in the hold. Which puts it at on the underside of the plane. As it was flying out from the UK to your destination Cyprus, it was probably also sat there with "No Signal", because it probably wasn't getting one from a cell-tower, or if it did, intermittently. As such, it was probably actively polling "Any Cell Towers around here?" which will probably be with a higher signal transmission strength than normal.

Ones on the ground would have heard this at random points on the flight, and then begins the handshake and sending of "Welcome to PoboGrov Mobile" etc. By which time the plane has travelled out of receiving range (or a building gets in the way), and so the SMS would have been bouncing around the network until it eventually got delivered when the phone was on a stable network.

SMS's have a increasing rate of retry time.. in the first few mins they will retry often. Then as more undeliverables occur, they will drop lower down the retry queue and the time between attempts will go up. This can be hours, or even days.



PS, this is all made up from my head, I am in no way qualified on how mobile phones, cell towers or SMS's really work :)
 
Soldato
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First off i'll say i'm no telecommunications engineer of any sort.....

But as a guess looking at mobile towers aren't the signals sort of "aimed"? they point the dishes towards the horizon not up, so it makes sense you get less signal up there than you do horizontally. Just a guess though by looking through google images.

Source: Google images mobile phone tower
 
Permabanned
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Radio amateurs regularly bounce microwave signals off the moon, so at least that high..... :) Cloud, rain, meteor and aeroplane scatter do weird things to microwave signals, often allowing remarkable propagation of even extremely weak ones. Cell phone tower aerials are often slot type that aim the signals horizontally, but as above these get reflected and refracted to unpredictable places.
 
Soldato
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First off i'll say i'm no telecommunications engineer of any sort.....

But as a guess looking at mobile towers aren't the signals sort of "aimed"? they point the dishes towards the horizon not up, so it makes sense you get less signal up there than you do horizontally. Just a guess though by looking through google images.

Source: Google images mobile phone tower

They are, as there isn't many folks above them wanting signal :p Even so radio waves propagate in all directions to some extent, even spilling out I to space to join the background noise
 
Commissario
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The phone only has to get a signal very briefly from one cell in each network for the system to know that it's in a different country. That triggers the text and even if they weren't delivered straight away, they'd be queued on the network and would all be delivered when service was good.

Yes, it's quite likely for RF from cells to go that high. They generally use slot type aerials which concentrate the signal horizontally but RF does reflect, especially at the sort of frequencies used by mobile phones. I'm not surprised that there's been some bouncing high enough for a phone to receive.
 
Man of Honour
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Lot of reasons:

Though planes seem high - the altitude they fly at isn't actually that great compared to the distance radio signals are regularly received horizontally on the ground - if you take a small boat out you can still get mobile reception quite a distance off the shore - not sure off hand but I think like 6-12 miles is pretty normal.

Phones only need a tiny tiny amount of power to pickup messages - somewhere in the picoamp range or something IIRC

Planes not uncommonly have some variety of repeaters via other systems including satellite depending on settings, network and phone configuration, etc. you might be picking something up via that (more usually for data onboard sometimes with their own internal wifi for internet access so a less likely explanation).

Couple of different people I've watched videos of on YouTube who did circumnavigation of the globe in small planes were regularly picking up patchy mobile phone reception as far as 90 miles (might have been KM I'm not 100% now) off shore when flying in at a few 1000 feet on approach to countries over the ocean, etc.
 
Caporegime
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Sounds like he didnt get most of those texts during the flight.

His phone was in the hold. Which puts it at on the underside of the plane. As it was flying out from the UK to your destination Cyprus, it was probably also sat there with "No Signal", because it probably wasn't getting one from a cell-tower, or if it did, intermittently. As such, it was probably actively polling "Any Cell Towers around here?" which will probably be with a higher signal transmission strength than normal.

Ones on the ground would have heard this at random points on the flight, and then begins the handshake and sending of "Welcome to PoboGrov Mobile" etc. By which time the plane has travelled out of receiving range (or a building gets in the way), and so the SMS would have been bouncing around the network until it eventually got delivered when the phone was on a stable network.

SMS's have a increasing rate of retry time.. in the first few mins they will retry often. Then as more undeliverables occur, they will drop lower down the retry queue and the time between attempts will go up. This can be hours, or even days.



PS, this is all made up from my head, I am in no way qualified on how mobile phones, cell towers or SMS's really work :)
Interesting responses - thanks all.

Looking at the texts again, the times the texts arrived corresponds with the flight time and where they'd have been flying over when the texts arrived so I'm presuming they arrived live as such.
 
Soldato
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The phone only has to get a signal very briefly from one cell in each network for the system to know that it's in a different country. That triggers the text and even if they weren't delivered straight away, they'd be queued on the network and would all be delivered when service was good.

Yes, it's quite likely for RF from cells to go that high. They generally use slot type aerials which concentrate the signal horizontally but RF does reflect, especially at the sort of frequencies used by mobile phones. I'm not surprised that there's been some bouncing high enough for a phone to receive.

Yup. While the transmitter panels do focus their emissions slightly downwards to optimise coverage and signal quality for users, the physics involved means that you will get secondary and tertiary lobes of propagation pointing upwards. The signals from these lobes are far lower than that of the primary lobes, but they can carry for considerable distances in direct line of sight conditions where there is very low pathloss. Add in that said flight probably closed over the Alps at some point where the cell towers are at far higher altitudes than normal and you could well have occurrences of when the phone picked up the pilot channel signal from several towers and held a radio link long enough to send a location update request, triggering the "Welcome to Country X" text from the user's home network.

Because the texts couldn't be delivered, the SMSC will queue them waiting for the phone to attach on the home or valid roaming network at which point, the user will receive them all at once.
 
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