Terrestrial System
The BTS system onboard the aircraft sends data in the form of internet protocol (just like VoIP) to the onboard air-to-ground system. The air-to-ground system encodes the voice data and modulates it using PSK. The signal is then broadcast using an omni-directional antenna. In some configurations the signal is sent directly from the onboard system to the ground network. In other configurations, the signal is received first by a satellite and is then sent to the ground network. If a satellite is used to connect the onboard system to the ground, there will be a delay of 200-300ms each way.
When the ground network receives the signal from either the satellite or the aircraft, the voice data is recovered through demodulation and decoding. The signal is then routed through the ground network to the destination; just as mobile phones calls on the ground are routed.
