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Just how high do mobile signals go?

Discussion in 'General Discussion' started by Scania, Aug 7, 2018.

  1. Scania


    Joined: Nov 25, 2004

    Posts: 23,992

    Location: On the road....

    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: Aug 7, 2018
  2. Dis86


    Joined: Dec 23, 2011

    Posts: 22,543

    Location: Northern England

    Nothing to block the signal. You had a few more layers.
  3. d_brennen


    Joined: Jan 30, 2009

    Posts: 15,373

    Location: Aquilonem Londinensi

    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
  4. robj20


    Joined: Apr 9, 2007

    Posts: 8,377

    Location: Manchester

    Can also make a difference depending what frequency your phone can handle, lower frequency will have a higher range.
  5. skyripper

    Wise Guy

    Joined: Jul 19, 2011

    Posts: 2,058

    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 :)
  6. james_2k

    Wise Guy

    Joined: Aug 28, 2012

    Posts: 2,480

    normally lose phone signal at around 4-6000 feet amsl in my experience. certainly never got it back in the cruise, ever.
  7. CREATIVE!11


    Joined: Apr 21, 2007

    Posts: 6,385

    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
  8. Burnsy2023

    Man of Honour

    Joined: Nov 17, 2003

    Posts: 36,399

    Location: Southampton, UK

    The issue when you get up high isn't necessarily the range, but constantly handing off from tower to tower as you move between them.
  9. Chris Wilson


    Joined: Nov 28, 2003

    Posts: 4,479

    Location: Shropshire

    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.
  10. ScoTTyBEEE

    Wise Guy

    Joined: Jun 1, 2004

    Posts: 1,990

    Location: London

    I got charged data overnight from Morocco when I was over the sea in Spain (you could see Morocco from my window but it's hardly 'close').
  11. d_brennen


    Joined: Jan 30, 2009

    Posts: 15,373

    Location: Aquilonem Londinensi

    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
  12. james_2k

    Wise Guy

    Joined: Aug 28, 2012

    Posts: 2,480

    not saying it is, just my experience at work.
  13. sovietspybob


    Joined: May 25, 2008

    Posts: 2,582

    Location: North Wales

    I work on the north coast of Anglesey and sometimes get a welcome to Ireland text, i guess depending on weather conditions as it's not all the time but that's about 60 miles away!
  14. Regy53


    Joined: Jun 11, 2011

    Posts: 3,669

    Location: Northampton

    Its because the earth is flat
  15. Feek


    Joined: Oct 16, 2002

    Posts: 27,912

    Location: In the radio shack

    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.
  16. Rroff

    Man of Honour

    Joined: Oct 13, 2006

    Posts: 66,866

    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.
  17. Minusorange


    Joined: Nov 25, 2005

    Posts: 4,974

  18. Scania


    Joined: Nov 25, 2004

    Posts: 23,992

    Location: On the road....

    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.
  19. Terminal_Boy


    Joined: Apr 13, 2013

    Posts: 7,644

    Location: La France

    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.
  20. Combat squirrel


    Joined: Aug 7, 2004

    Posts: 10,139

    Could have put the phone in.......wait for it..........air plane mode, to save the texts! lol