I have been working on this setup for a while now.
The heart to it is the custom code I wrote for the ESP8266 to serve as a UART - TCP client that connects to a server in the internet. The GCS connects to the same server. The server is running a TCP relay software.
I had to do all this work due to the fact that it's not possible to connect directly to the Wifi module from the Internet since it is getting Carrier Grade NAT. This means it's not possible to do NAT and connect to it. So I had to have the ESP8266 act as a UART - TCP client instead of a server.
I get a reliable connection for hours. Latency is well under a second.
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Many of 3G modems support voice calling, SMS, 3G video.
So you can just call your drone and enable 3G video / 3G data to work.
Call centers implemented 3G video 10 years ago.
Latency in case of 3G network is subject to change, so you need to
buy CIR from your carrier (Committed Information Rate)
The goal was not to transfer video, but have full control of your quad at all times. So a wifi module+3G router costs about $30 and gives you a reliable unlimited range.
"Calling" your drone is not that simple. The main issue is that the APM is acting as a server. In order to connect to it, it must have a valid IP or you need to have ask your carrier to do Network Address Translation specifically for you, which they wont.
So the APM needs to be a client and connect to some TCP relay, and, the GCS is also a client connecting to the same relay.
Thank you my friend.
So it looks like my "Call Your Drone"
technology solution is still worth $0.99
This is the setup
apm wifi 3g setup.png