Inmarsat 40 degree line
Those concentric circles with degrees on them on the map denote 'angle of elevation' for a ground observer at sea level to assist with positioning a flat antenna on some terminals.As you get closer to the satellite the angle increases.The zero circle denotes the point at which the satellite will be on,or very slightly under the horizon.Given that the satellites are in geostationary orbit ( approx 36,000 kms above the equator) the difference in the elevation angle at 35,000 feet instead of sea level will be negligble.
Which means that you could probably use a hand held satphone out of a western facing window on the aircraft.
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Last edited by TWT; 15th Mar 2014 at 11:33.