Sallyanne, keeping streams separated and communicating through satellites is a done deal. Back in the 80s it was a done deal on the DSCS satellites and ground stations. As you say, it requires a proper protocol. It's been invented already, several times. Inmarsat M is another protocol which manages this.
Privacy is a minor problem. If you data that is converted to a stream of bits then encrypting it to send and decrypting it on receipt is no big deal. Every time your browser shows the little lock that's being done for you on the Internet.
Volume is a problem. And price is a problem. And spectrum is a problem.
How much data do you want to send? Maybe spectrum that is being abandoned at HF as HF broadcasting becomes a lost art could be recycled if the data streams are small enough. Perhaps you can send deviation from flight plan rather than actual position.
Volume of data and limited spectrum availability drive price. Do we put airlines out of business simply to send data that statistically is not needed? Sending a megabyte of data for every byte that is actually needed and used seems to me to be pointless.
So how much is that one byte worth? Can it be reasonably amortized over the entire data volume being proposed?
{^_^}