Geo-stationary satellite

A geostationary satellite orbits the equator at about 35,786 km altitude, where the orbital period equals one sidereal day, so the satellite appears fixed over one longitude. A fixed antenna can therefore point at it without tracking, which is why these orbits carry Inmarsat maritime communications and the SBAS overlay broadcasts of WAAS and EGNOS. The trade-off is coverage: a geostationary satellite cannot serve high latitudes much above 70 to 75 degrees, where it sits too low on the horizon, so GMDSS for the polar regions relies instead on the Iridium low-earth-orbit constellation.