Originally Posted by x_navman
I've used Inmarsat in the marine environment.
The antenna needs to be kept "pointing" at the satellite during use.
to do this, the Inmarsat antenna contains a GPS receiver/antenna - if it knows where it is, and it knows where the satellite is, it can keep the antenna pointed at the satellite.
On a boat, the antenna is also stabilized, to account for the roll and yaw of the boat, but maybe this isn't needed on an aircraft.
The gps-based antenna pointer was an accessory but not a part of the core phone system itself, which works by itself if the antenna is correctly pointed manually. 8-10 years ago I used Inmarsat extensively in remote parts of North Africa, all I needed was a rough compass bearing to the satellite, elevation was a memory recall item of 35 degrees. Phone worked fine if antenna was pointed within 15-20 degrees of true satellite bearing, elevation had a greater tolerance.
Even 15 years ago it was possible to buy car-top antennas that did not need any pointing, worked as long as satellite was in line of sight anywhere above 15 degrees of the horizon. I'm sure modern Inmarsat antennas can connect without the need for satellite tracking.