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Very Low Frequency (VLF)

Very Low Frequency (VLF) communications transmitters use digital signals to communicate with submerged submarines on at frequencies of 3-30 kHz. The eighteen Trident submarines constitute about half the US strategic nuclear capability. The supporting infrastructure for these submarines includet connectivity links such as the Extremely Low Frequency (ELF), Very Low Frequency (VLF), and TACAMO Airborne VLF communications systems.

The Navy shore VLF/LF transmitter facilities transmit a 50 baud submarine command and control broadcast which is the backbone of the submarine broadcast system. The VLF/LF radio broadcast provides robustness (i.e., improved performance in atmospheric noise), availability, global coverage, and has seawater penetrating properties. The submarine VLF/LF broadcasts operates in a frequency range from 14 to 60 kHz and consists of five high powered, multi-channel MSK Fixed VLF (FVLF) sites and five multi-channel LF sites located worldwide. As part of the FY94 Base Realignment and Closure (BRAC), Navy Radio Transmitter Facility (NRTF) Annapolis closed in FY96 and NRTF Adak closed in FY97.

The submarine VLF/LF broadcasts are generated by the BCA or Alternate BCA from messages created locally by the C 2 processor, the SSIXS processor, or accepted for relay by the SUBOPAUTH. The BCAs and Alternate BCAs are connected to the transmitter sites by dedicated ISLs with the ability for JCS and USSTRATCOM to seize BCA, at any time, for EAM dissemination. At each of the transmitter sites, messages received over the ISLs are decrypted and input into the Integrated Submarine Automated Broadcast Processor System (ISABPS). Submarine VLF/LF broadcasts a continuous transmission sequence of prioritized messages which normally lasts two hours. It is generated by ISABPS and sent to the VERDIN transmit terminal. The VERDIN transmit terminal is used to multiplex, encrypt, encode, and modulate up to four 50 bps submarine broadcast channels into VLF/LF radio frequency signals which is amplified/radiated by the VLF/LF transmitter antenna. The Fixed Very Low Frequency Site Upgrades program maintains and upgrades antennas and transmitters at the FVLF sites. It consists of three individual programs: (1) The Solid State Power Amplifier-Receiver (SSPAR) program; (2) the Transmitter Keep-Alive Program (TKAP)/Service Life Extension Program (SLEP); and (3) Antenna Maintenance Program (AMP).

The SSPAR program could modernize the FVLF transmitter sites with solid state technology. This program could replace the current inefficient, unsupportable vacuum tube amplifiers and provide standardization for FVLF sites. The SSPAR program execution is being evaluated. SLEP is an interim program to extend the useful life of existing FVLF/LF equipment and systems until the delivery of SSPAR. The SLEP improvements includes switchgear, circuit breakers, solid state Intermediate Power Amplifiers (IPAs) and pre-IPAs, assorted electrical components, and updated technical manuals. SLEP has been completed at Jim Creek, Lualualei, Cutler FVLF, and Aguada, PR. H.E. Holt is scheduled to be accomplished in FY98. The AMP is administered by the Commander, Naval Computer and Telecommunications Command (NCTC) and provides for the ongoing maintenance and repair of FVLF/LF antennas and antenna components (e.g., insulators, top hats, guy wires, etc.).

As part of the overall DOD shore infrastructure reduction, NCTC and SPAWAR are examining modernization and cost savings alternatives within the submarine VLF/LF FSBS that could be accomplished which would maximize the return on initial investment. These study efforts, called the Smart Resource Management System (SRMS), are centered around consolidation of the shore VERDIN ISABPS equipment and more cost effective connectivity from the BCA/Alternate BCA to the BKSs and Broadcast Transmitting Stations (BTSs). Other initiatives being considered are Remote Transmitter Operation, Range Extension Mode, Power Management, Dynamic Channelization, and Split Array operation at the VLF sites.

SPAWAR is soliciting information from potential sources for the replacement, upgrade or modification of degraded, obsolete and maintenance intensive components, equipment and subsystems. The Very Low Frequency (VLF) transmitters are old vacuum tube technology with an average age of 37 years. The Very Low Frequency Ashore Lifetime Upkeep Effort (VALUE) is to extend the life of shore based submarine communications transmitter systems


Call Letters Location Frequency Power (kW)
NPM NRTF Lualualei, Hawaii 21.4 480
NAA NCTS Cutler, Maine 24.0 750/1000
NLK NRS (T) Jim Creek, Washington 24.8 192
NAU NRTF Aguada, Puerto Rico 40.75 100
NCS H.E. Holt, Australia

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