1. A method to maintain trajectory synchronization comprising: Comparing, using a processor, an active trajectory with a synchronized trajectory, wherein the comparison is triggered by at least one disturbance, periodic trigger, communication event between different trajectory systems, or downlink/uplink event; if the comparing indicates a difference between the active trajectory and the synchronized trajectory, then performing the actions of: updating the synchronized trajectory with the active trajectory; and transferring the synchronized trajectory or sending a command to modify a flight plan to an external system; Wherein one ground computer performs the action of when a downlink is received, initiating a comparison process where the downlink is compared to a downlinked synchronized trajectory and a ground predicted trajectory for the aircraft, and if the comparison indicates a difference between the synchronized trajectory and the ground predicted trajectory, then initiating a synchronization process, else a local synchronized trajectory is updated with the downlinked synchronized trajectory; wherein the disturbance is at least one of a change detected by an aircraft, change data from an external source, or a periodic trigger; wherein the change detected by an aircraft is at least one of atmospheric condition change, pilot input change, airline input change, or other state measurement; wherein the change data from an external source is at least one of flow management, a separation from other aircraft command, change in convective weather, a special use airspace command, a scheduling requirement, a trajectory downlink, or a command uplink; wherein comparing the downlinked synchronized trajectory with a ground predicted trajectory is matching a plurality parameters comprising ground route, lateral route, altitude restrictions, and speed restrictions; wherein the synchronization process performs ground initiated amendment to a flight plan and uplinking of a command to the aircraft to modify its plan only after a substantial or all discrepancies between the flight plan and the downlinked synchronized trajectory are resolved.