The UHF FLEETSATCOM system provides a substantial resource to support important communication needs of the U.S. Navy and other Department of Defense users. Because of the significant investment being made in this system, there is strong motivation to examine techniques that make efficient usage of its components. This paper reports on one such effort, the Terminal Access Control System (TACS) that was developed at the M.I.T. Lincoln Laboratory. TACS utilizes time division multiple access (TDMA) to derive up to nine 2400 bit/s circuits from each of the several 25-kHz-wide frequency channels available on the FLEETSAT satellites. Using TACS, access to these circuits would be controlled in real time by a central control station in response to user demand. This application of demand assignment further multiplies the number of subscribers that can use the system by a significant factor. A description of the access control techniques and efficiency of performance is provided. A master control station and subscriber unit were constructed and used in an extensive laboratory test program.

The advantages of utilizing the extremely low frequency (ELF) band for communications to submarines are principally the low attenuation rate of ELF signals in seawater and the low attenuation rate in the earth-ionospheric waveguide. ELF transmitters, however, require high input power. Efficient signaling schemes and sophisticated receiver designs help minimize the required transmitter power and hence system cost. One such signaling schemeconvolutional encoding and binary antipodal modulation-and an experimental receiver were successfully field tested and are described. Tests included real-time reception of messages aboard a submerged submarine in the Atlantic Ocean. It is concluded that the receiver, which included adaptive nonlinear noise processing, notch filtering, ocean channel compensation, and sequential decoding makes extremely efficient usage of available signal strength and demonstrates the technical feasibility of communication in the ELF band.