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London, UK
reply to ISurfTooMuch

Re: Overwhelm the registration system?

They dont connect to the cell sites on the ground anyway. There is too much distance between the handset and the cell site.

Not only that, but cell sites broadcast their signal horizontally, not vertically, with antennas that are specifically designed to achieve this kind of signal distribution. Typically these antennas have a vertical beam width of only a couple of degrees (and at that with some added down tilt), and a horizontal beam width of some other dimension depending on what the operator is trying to achieve. Typical horizontal beam width is 120 degrees.

True, there are things called "side lobes" which basically means there will be some signal leakage from the antenna in directions that are not planned as part of the coverage area, but these lobes are typically very weak in comparison to the main lobe, so are not likely to be of much use unless you were perched on top of the cell tower.

Any kind of in-flight mobile communications would likely have to be done with some kind of on-board "base station", with probably something like satellite backhaul. Though, I have heard of terrestrial based backhaul using antennas that point up in to the sky and down from the belly of the aircraft, but I have never actually seen anything like this before - only antennas under domes on top of the aircraft.

Happy to be proven wrong, this is just based on my understanding of it all.