The system's charactersitics are highly classified in order to prevent others from discerning the operational parameters and therefore aid in interception. However, it is clear that the system must be in stable orbits (if not synchronous) and within the line of sight of ships and submarines at sea. One would assume this means from every major body of water where U.S. vessels may operate (even clandestinely...submarines again).
However, the system most likely also provides for routine communications at times where critical communications are not chewing up all bandwidth. Thus ground based control may allow changes of security modes and the like as necessary to communicate in differing traffic requirements.
The satellites are currently launched, maintained and operated by U.S. Air Force's Space Command which essentially handles all space borne assets for the U.S. Military.
According to Air Force Magazine 1, the satellites carry some 23 channels with 12 dedicated to Air Force usage, 10 for the Navy, and 1 for the national command authority (NCA).
The magazine also reports that the UHF Follow On program will replace the FltSatCom satellites over time.
The magazine also reports that the satellites are in geosynchronous orbit.