While out hiking recently I got to thinking about the lack of phone signal often in the mountains here. A phone would be really useful if hiking solo and I fell and couldn’t walk, but often there is no signal.
Then I wondered if a drone could solve the problem. Who better to ask than those here who probably know will about drones.
Is there such a thing as a light weight drone (say less than 0.5 kg) that could fly up to 400agl or whatever height is max allowed, where it could hopefully pick up a GSM/3g/4g signal and relay that to my phone down below so I could make a call or use the internet? Often only a little bit of extra height is all that is need to get a signal.
Such a thing might also be useful after landing on a remote, deserted airfield and needing to order a taxi.
Does it exist?
That type of drone certainly does exist per se, but I am not aware of any commercially available one that would incorporate a GSM signal relay system. Here’s a hack for that:
In essence, you set one phone to hotspot mode, attach it to a drone and send it up. The second phone on the ground then gets the signal. This being WiFi, you’d need to make calls via WiFi calling if your provider allows that or via Skype, etc.
No idea if one of the small drones (Mavic, etc) can lift a phone. The guys in the video use DJI Phantoms which can carry a lot more.
Edited for typos
Strip out some of the user interface and the ‘essential’ components of a phone are very light.
A drone would lift it without issue.
However in real terms, by the time you work out that you need one often enough to make sure you then get one, you’re probably better getting a sat phone.
It’s still a cool thought though.
The lightest phones on the market weigh around 85 g. Strip out the unnecessary things, and you can probably get down to 50. For this kind of weight, you don’t even need a drone – just take a large balloon, inflate it with hydrogen from a trivial chemical or electrolytic generator, and tether it with a thin fishing line. The lift of a hydrogen balloon is approximately 1 g per litre of volume, minus the weight of the balloon itself and the tether. Take a still bigger one, and you can even attach some solar cells and let it hang up there for days non-stop.
Interesting idea.
A relay for GSM / GPRS / 3G / 4G is tricky. Basically you have to build a packet repeater for each of the frequency bands, and nowadays there are many bands. These products are – like a lot of good things – illegal, but you can buy them on Ebay, from, of course, China. The packaged is labelled “HIFI equipment”. Once upon a time I had a 900MHz one (the common Vodafone band around here) in the loft, and it worked, sort of… it needed much better (bigger) antennae than what came with it.
A WIFI link is much easier, and as stated above you would need to use VOIP on the phone, which is easy enough for outgoing calls but a multiple challenge for incoming calls. Also WIFI doesn’t go all that far unless you have a big directional antenna (which is illegal of course, like many good things) which you could have since it could be very lightweight.
However I think the main issue would be straight distance from the nearest tower. There is a system limit beyond which it won’t work. It varies according to the location.
Finally, I am convinced that the networks in some countries blacklist a phone if they see it visible to too many towers. I think France did this for years and possibly still does. It prevents any connectivity when flying at say FL100, even though you likely have a strong signal most of the time.
You can get a Thuraya 7100 satphone on Ebay for not much. I had some which I sold for 300 quid. Then you get a PAYG SIM card, which has an “interesting” tariff whereby they take off an annual subsidy to the UAE Royal Family but, last time I checked, you don’t get cut off after say 90 days which is the custom on terrestrial GSM networks these days 
You can get data on it at 9.6k. It costs about €1/minute, I think, but calling the Thuraya number from say a normal GSM phone can be 5x to 10x as much. Thuraya-Thuraya calls are really cheap. More data here.
Another approach, for emergency use, might be to program a drone to fly within distance of a GSM tower, carrying a phone which is trying to send an SMS, and return.
Peter wrote:
These products are – like a lot of good things – illegal
Again, this may depend on the country. They are not illegal in Sweden, but they are subject to a license and the approval of the cellular network operator.
Put an old cheap iPhone (or a HUAWEI Hotspot, maybe with external antenna) below is a drone, activate personal hotspot, use VoIP for your call.
Hello everyone !!!
Something to use for SMS that includes your location is on the link below.
https://github.com/uavpal/beboptwo4g
https://github.com/uavpal/beboptwo4g/wiki/Installation
You need to buy https://www.parrot.com/us/drones/parrot-bebop-2 of course
Peter wrote:
Also WIFI doesn’t go all that far unless you have a big directional antenna
WiFi goes quite a lot further in the wilderness.
WiFi range is quite short in most places, because it’s competing with dozens of other WiFi access points on the same channel (all naturally transmitting at full power), as well as things like microwave ovens, while trying to go through walls and other building structure. It’s why for a while at least 5GHz WiFi tended to go much further than 2.4GHz, despite being more attenuated by walls – there were just far fewer access points in that band. (The wifi strength meter on most devices is actually a signal-to-noise ratio meter).
But out in the wilderness, a hotspot stuck on the bottom of a drone, with nothing on the same channel and perfect line of sight to a single device on the other end? You’ll get much more distance out of wifi.
Just to give you an idea of how much more you’ll get on an uncluttered frequency, a while back I was doing some poking around with an SDR transmitter, which has a maximum output power of 10mW (a tenth of the power of a typical WiFi access point). I set up the software to transmit on the 70cm amateur band, just repeating my callsign. Using an antenna made from cut off bits of mains wire, perched on my desk, I could take my handheld radio and walk about quarter of a mile before I lost the signal, even though the signal had to go through three feet of Manx stone just to get out my house. Of course, WiFi is wider band (so that little power won’t go as far) but out in the middle of nowhere, it’ll probably do well enough you could establish a VOIP connection via a drone hovering a couple of hundred m above you.
The otters point of Peter still holds true: For all practical reasons a Satphone is cheaper, more practical, less complex and lighter in these situations.
