Improving mobile phone signal in GA aircraft

I have ljttle engineering knowledge about the way radio waves work in GSM but having worked ten years for Orange (as project manager), my understanding is that signal propagation is mostly horizontal due to the way ground stations are build – to cover as much ground as possible.

But again, I am no specialist :)

Agree with you that signal in the air is very useful. Not the same but sometimes I use my Garmin inreach for quick messages.

I saw a fascinating presentation from a Ericsson rep last fall on this very topic. The main problem you will face is that 3G and 4G ground antennas are not pointing upwards. Where I live sometimes I can’t get any signal above 3000ft, even holding the phone by the window. Being up and moving at airplane speeds is another part of the challenge: the unit sees too many masts, and just keeps hopping and doing nothing but handshakes.
With 5G there are some experiments to have antennas pointing upwards and improving the technology so there is useful bandwidth. If it is indeed implemented, it will be fantastic, possibly competing with ADS-B up to FL200 and bi-directional. But it’s still some years out.

The idea is sound, but can be elaborated a bit further. First of all, when flying, we know that all base stations are under us, so it makes sense to use a directional antenna that receives/transmits in the bottom hemisphere only, which gives a 3 dB gain compared to an omnidirectional one. Furthermore, an airborne terminal would be within line-of-sight coverage of too many base stations on the ground, which is generally a bad thing (may interfere with network registration – @Arne, this may be your case above 3000 ft). Thus, it makes sense to narrow the antenna beam even further so that it would only see the nearest base stations – something like 90-120° beam angle, or 6-11 dB directional gain. However, such an antenna protruding down from the aircraft belly will be subject to significant aerodynamic forces and will produce noticeable additional drag. I would suggest using one of the underwing inspection hatches (available on most aircraft) to mount the antenna as flush with the skin as possible.

Yes, it would be good to get coverage higher up. In my experience, the signal is usually lost between 8 and 9Kft.
Also IIRC , from what I learned too many moons ago, the direction an emitting antenna is one thing, but directly linked and most important is it’s polarisation, the plane the signals travel on, which is usually 90° off.

As mentioned above, I doubt that any provider is interested in improving the reception for a few small airplanes, unless a cheap and profitable solution appears. Not holding my breath on this one…

Living in the countryside, and with all “real” banks using SMS for online CC payment checking, I have “considerable” experience with mobile signal boosters. Most of them are imported from China by, basically, criminal gangs who import them as “hifi equipment”, change the company name every few months to stay ahead of consumer litigation, trustpilot reviews, and angry owners. They contain a simple RF amplifier and this is the #1 problem: you need a channel for every frequency being used by mobile phones, not just the 900MHz originally used by Vodafone. The more networks to support the more they cost, and you can’t just do a repeater for the whole band 900-2300MHz. The last one I bought has a config menu but no non-volatile storage so at any power interruption it stops working. And the receiver antenna needs to be well away from the output antenna. The boosters are mostly Ofcom-illegal. There are legal ones but they cost several times as much.

Also I don’t think a lack of signal is the main issue. It may be in some places e.g. right across Belgium you may see no signal at all above FL080 or so. In France you see a strong signal (so the stuff about the ground antennae pointing only down is clearly not true there) but there is no connectivity, suggesting that France blacklists any SIM card seen connecting to multiple towers, with some timing constraints like for more than 5 mins, and the ban is renewed every hour, so flying across France you can get data briefly, about 3 times I would expect every network to be doing this, otherwise arriving or departing airliners (anything up to 300 phones left on regardless) would create havoc with the tower interconnection bandwidth.

The Golze ADL is the best solution

Peter wrote:

They contain a simple RF amplifier and this is the #1 problem: you need a channel for every frequency being used by mobile phones, not just the 900MHz originally used by Vodafone.
…
There are legal ones but they cost several times as much.

An RF-only booster would indeed be of little use onboard. What does make sense, however, is a cellular modem physically located right next to the directional antenna and providing data connectivity inside the aircraft via Wi-Fi or even Ethernet.

That would be a mobile phone with an external antenna, and the wifi hotspot enabled

We have past threads on this – examples. I know many years ago someone was going to do a product which did a bit of this. You can improve the situation dramatically by hacking the GSM code to aggressively retry, but this is illegal IAW the GSM specification and possible only on a rooted device (feasible on android but that phone won’t run any banking apps, anything from Jepp, etc, and not really feasible on apple stuff) and AFAIK he gave up. The system would have allowed sending SMS messages and that is ok for tafs metars and, with suitable encoding, wx images.

Peter wrote:

That would be a mobile phone with an external antenna, and the wifi hotspot enabled

You mean like Golze 200? ;)
Shame it is being withdrawn (although, appears that for the good reasons).

Starlink is promising some connectivity direct to satellite from a phone and they’ve done some successful tests already.
https://api.starlink.com/public-files/DIRECT_TO_CELL_FIRST_TEXT_UPDATE.pdf
local copy

Maybe this is something that could be useful to us GA pilots in Europe someday.

Relevant part in the document:

The Direct to Cell network will expand Starlink’s vision by providing ubiquitous connectivity and seamless access to text, voice, and data for LTE phones and devices across the globe. Text service begins this year, followed by voice, data, and Internet of Things (IoT) services in 2025.