Satellite terminals and modems are the equipment on a ship, an aircraft or a remote site that sends signals to a satellite and receives signals from it. A terminal has two main parts. The antenna is the dish or flat panel that points at the satellite. The modem turns data into radio signals, and radio signals back into data.
On land, a satellite dish is bolted in one place and pointed once. A ship is harder. It rolls from side to side, pitches up and down, and turns. A dish pointed even a degree or two away can lose the signal from a satellite 36,000 km away. So a ship's dish sits inside a white dome, on a motorised mount with motion sensors. The sensors measure the ship's movement many times a second, and the motors turn the dish to cancel that movement. Newer terminals use flat antennas that steer their beam electronically, with no moving parts. Aircraft use flat, low antennas, some steered electronically and some turned by motors, because a large dish would stick out too far from the plane's body.
The modem does the other half of the work. It packs data into radio signals in the satellite's frequency band. When the signal weakens, for example in heavy rain, it changes to a slower but sturdier coding, so the link stays up. Many modems are software-defined radios: much of how they work is set by software. So a software update can let an older terminal use a new satellite.
For instance, take a bulk carrier crossing the Bay of Bengal towards Visakhapatnam in rough weather. The ship rolls up to 15 degrees each way, about once every 10 seconds. The terminal's sensors read the motion 100 times a second, and the motors keep the dish within a fraction of a degree of the satellite. Later, the ship sails out of the area covered by one satellite beam. The modem receives the details of the next beam from the network, switches to it and reconnects in about a minute. The crew's internet pauses only briefly.
Terminals that can use both geostationary and low-orbit satellites, described in LEO and GEO satellites, have a harder job. A low-orbit satellite crosses the sky in a few minutes, so the terminal must follow it and then switch to the next one.
On aircraft, the terminal connects to the cabin's in-flight Wi-Fi. Uncrewed boats and other unmanned systems carry small terminals, so that their operators can control them from far away.