Synchronousorbitcommunicationsatellitesarecommunicationsatellitesthatarelaunchedintoacircularorbitsynchronizedwiththerotationperiodoftheearth(23hours,56minutes,4seconds).Theorbitalheightofthiskindofsatelliteis35786kilometers,andtheintersectionanglebetweentheorbitalplaneandtheearth'sequatorialplaneiszero,thatis,thesatelliteislaunchedovertheequatorandmovesinthesamedirectionastheearth'srotation.Fromanyobservationpointontheground,thesatelliteisstationary..
Ifthecommunicationsatelliteissenttoacircularorbit35,800kmabovetheequator,anditmovesinthesamedirectionastheearth'srotation,thetimeforonerevolutionoftheearthisexactlythesameasthetimefortheearthtorotateonce(thatis,24h).Fromtheground,thesatelliteisstationary.Thiskindofcommunicationsatelliteiscalledasynchronouscommunicationsatellite,alsocalledageostationarysatellite.
Globalsatellitecommunicationscanbeachievedwiththreesynchronoussatellites.Leadtwotangentsfromthegeostationarycommunicationsatellitetotheearth,andtheanglebetweenthetwotangentsis17.34°.Suchageostationarycommunicationsatellitecanprovidecommunicationfornearlyone-thirdoftheearth.Ifthreegeostationarycommunicationsatellitesarelaunched,separatedby120°fromeachother,theycancovermostpartsoftheworldandrealizeglobalcommunication.