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This question is a follow up to a previous question asked by @AvengingEarth

Would a smart phone work on an alien planet?

I'm interested in taking a step further.

All conditions are as in the previous question except that there are two people and each has their own smartphone. They have no network and no other equipment or computer to help them. Assume only the two phones. The owners have superuser skills but no way of changing anything except possibly software. However there is no network to download from, no extra computing power, and they would have to develop any software purely on the standard factory-provided software and firmware plus maybe a few games and things like Microsoft Office for mobiles.

My full question follows:

Would two smart phones be usable as an intercom on a human-life-supporting alien planet? If so, what range would be possible assuming there is no other radio communication going on anywhere on the planet.

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    $\begingroup$ About 50 meters. Then the wi-fi becomes spotty. This assumes that they know how to network the phones together using WiFi without an Access Point (most folks don't know) in the first place. Whichever phone takes on the role of owning the wireless network will have a significantly shorter battery life. $\endgroup$
    – user535733
    Sep 16, 2020 at 20:23
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    $\begingroup$ @ user535733 - You can assume their ability to use or discover any standard features of the phones to expert level. $\endgroup$
    – chasly - supports Monica
    Sep 16, 2020 at 20:27
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    $\begingroup$ Smartphones come with FM receiver/transmitter functionality, but accessing these features seems to require apps for that purpose (rather than adjusting settings). Even a superuser would need an app to modulate the signal, as far as I can tell). $\endgroup$
    – rek
    Sep 16, 2020 at 20:43
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    $\begingroup$ @rek My understanding is that they're FM-rx only, no transmit. Furthermore, the FM functionality is disabled in the radio's firmware, and completely inaccessible to the operating system. $\endgroup$
    – John O
    Sep 16, 2020 at 20:46
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    $\begingroup$ I imagine using built in flash to send morse code... I really need sleep. $\endgroup$
    – user6760
    Sep 17, 2020 at 3:35

3 Answers 3

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YES ... but ...
Generally phones need a network of some kind to operate, be it the cell carrier network or a wifi network. There are lots of intercom apps available. Make sure your explorers download and test functionality before departure!

There are other options, however. These "disaster communications" apps make use of the phones' wifi & bluetooth to form a kind of interwoven mesh, an impromptu network. Helpful indeed for when the cell network is down, or you find yourself with a friend on an exoplanet and all you have between you is a couple cell phones! According to this resource, and other similars, this kind of network is quite limited. Your team would have to be within about a hundred yards of each other to communicate this way.

If your intrepid explorers thought to pack a couple old Motorola phones, it appears they can indeed be used in this way, as walkie-talkies, as there is a native functionality.

Best bet: just equip your team with some actual walkie-talkies: maximum range can vary from three to maybe thirty miles, depending on signal obstructions.

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    $\begingroup$ As far as I can tell from reading the first link you provide, these apps still need a network. "Walkie talkie apps require Wi-Fi or data connections, ... they use very little data. As a result, they can even work on an ancient 2G connection" $\endgroup$
    – chasly - supports Monica
    Sep 16, 2020 at 21:15
  • $\begingroup$ @chasly-reinstateMonica -- That was exactly my point. Cell phones work best when they have access to a cell network! $\endgroup$
    – elemtilas
    Sep 16, 2020 at 21:17
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    $\begingroup$ I'm questioning your statement, "There are lots of intercom apps available. Make sure your explorers download and test functionality before departure!". If there is no network at the destination, there is no point in testing before departure. $\endgroup$
    – chasly - supports Monica
    Sep 16, 2020 at 21:19
  • $\begingroup$ @chasly-reinstateMonica -- That's why you need to read the whole answer! $\endgroup$
    – elemtilas
    Sep 16, 2020 at 21:21
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    $\begingroup$ I did. Your answer is very helpful. I was merely checking the relevance, if any, of the opening paragraph. Given the assumptions of no network, the first paragraph seems totally unnecessary. I was wondering if I had missed some relevance. Why would they "make sure" they did something that was of no use? $\endgroup$
    – chasly - supports Monica
    Sep 16, 2020 at 22:12
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Not very far for voice, but potentially very far for signaling. Let's break down typical I/O and signaling methods that an average smartphone has:

Radio:

  • GPS receiver: Very long range, but receive only. Useless
  • NFC antenna: Bidirectional but only functions at sub 5cm ranges. Useless
  • Bluetooth: Bidirectional and somewhat easy to use. Max range between 5 and 15 meters depending on phone vintage. With the right apps preloaded or sufficient programming skill, it would be comparatively easy to use as a walkie-talkie
  • WiFi: Bidirectional, easy to use, longer range and higher bandwidth than Bluetooth. Without obstructions, you might get 50 meters of range. Again, the right apps/programming skills can make this comparatively easy to use
  • Cell radio/gsm: Very long range (multiple km) but requires cell towers and service providers. It is likely impossible to reprogram the firmware of the transceiver without physically cracking open the phone and reflashing responsible chips. Developing the new software to enable p2p communication would take a company, a team of expert software engineers, and a bunch of time. Not feasible.
  • AM/FM radio: some phones can tune into radio when wired earbuds are connected. Unfortunately, this is receive only. If it's possible to reprogram, you'd again need a company, team of experts, and a lot of time. Not feasible

Other:

  • Speakers/vibration motor: quieter than someone can yell, so not very useful. Hypothetically, you could quietly pass messages by whispering a recoding into the phone and then tossing the phone to the other person
  • Flashlight: In dark environments, even small light sources can be seen from incredible, multi-km distances. You could use morse code or even an app which automatically converts text to flashlight morse; many such apps exist. Feasible with line of sight in the dark across potentially multiple kilometers.
  • Screen: Similar to using the flashlight, you can use the screen of a smartphone as a signalling mirror using the sun during the day. Has similar line-of-sight and morse-only limitations but has an incredible range
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  1. Learn Morse code.
  2. Write an app that translates text to morse and vice-versa. Add a mode where you can tap the screen for short or long periods of time to make a short or long beep.
  3. Get a couple of pint sized cups. You could make these out of clay. The cups should have a small role in the middle of the bottom, enough to pass a string.
  4. Using a very long thread which passes through both cups to tie the phones to each other as below:

Diagram of my superphone. Each phone is stuck to the bottom of a clay cup. The wire that connects them passes through the cup holes.

  1. ????
  2. Profit!

It works as long as the cord is taut. The range is the length of the wire. There is some signal attenuation over distance but should be good for a few hundred meters, which is more than what you'd get by wifi tethering. Also notice that this allows for airplane mode, which helps save battery while you are not communicating.

If you use the phone's accelerometer (supposing it has one) to detect vibration and read morse from it, you can even receive files!

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