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Functional Elements of the FBM System

The ability to strike a target with a ballistic missile warhead involves a precise determination of the launch point, a computation of the path to the target, and a system aboard the missile that will determine its course along the path to the target. In addition, the SLBM requires a system to launch the missile to the surface of the sea.

The ship’s navigation system provides the launch point, the ship’s fire control system computes the path equation, the missile guidance system follows this equation to release the reentry bodies that then fall on a predetermined ballistic path to the target(s), and the ship’s launcher system stows, protects, and launches the missile from the submarine.


1Successfully striking a target with a ballistic missile at long range requires accurate knowledge of both target and launcher positions. In the FBM weapon system, the constant change in launch position as the FBM submarine sails the world’s oceans complicates accurate determination of launch position. The navigation subsystem uses sophisticated navigation equipment and procedures to provide highly accurate ship’s position, attitude, and velocity while maintaining essential FBM submarine covertness, which translates directly into survivability. The heart of the navigation subsystem is the inertial navigator, a complex system of gyroscopes, accelerometers, and computers, which relates a ship’s movement over the Earth from an initial position to give a continuous report of ship’s location without frequent reference to external position fixes. POLARIS, POSEIDON, and TRIDENT I FBM navigation subsystems used the Ship’s Inertial Navigation Subsystem (SINS) as an inertial navigator from 1960 to 2005. Systems similar to the FBM SINS guided the USS NAUTILUS and USS SKATE on their historic voyages beneath the polar ice in 1958 and the USS TRITON on her 84-day underwater cruise around the world in 1960.

The TRIDENT I (C4) navigation subsystem is a version of the SINS-based design used on 616 Class POSEIDON SSBNs that was upgraded to improve SSBN security and navigation subsystem reliability. An Electrostatically Supported Gyro Monitor (ESGM) extended the interval between external fixes while maintaining position accuracy, thereby reducing submarine vulnerability to detection. (ESGM was subsequently backfitted on all POSEIDON SSBNs.) Adding closed-loop air cooling increased reliability. The sonar system and satellite receiver were re-engineered to improve their performance. The TRIDENT I navigation subsystem supported the FBM weapon system from 1982 to 2005.

For TRIDENT II (D5), the navigation subsystem was redesigned to support tightened weapon system accuracy goals and to maintain an extended fix interval. Major changes include adoption of the Electrostatically-Supported Gyro Navigator (ESGN) as the inertial navigator, addition of the Navigation Sonar System (NSS) with increased capability to measure velocity, the Global Positioning System (GPS) to replace the aging Navy Navigation Satellite System (NAVSAT), and installation of a digital interface with the FBM weapon system and ship. All new, state - of - the - art computers and a highly automated operating and maintenance/diagnostic system support these major elements of the subsystem.

All but five TRIDENT I navigation subsystems were upgraded to a modified TRIDENT II configuration in an effort called the TRIDENT Navigation Commonality Program (TNCP). TNCP modifications implemented current commercial displays, print, and mass storage technology. TNCP reduced long-term navigation support requirements, resulting in significant cost savings over the TRIDENT I and II life cycles.

The navigation subsystem is upgraded to a D5 Backfit configuration during the period 2000 to 2007. D5 Backfit replaced both TRIDENT I and TNCP configurations and reduced the navigation equipment complement from 22 to 8 electronics cabinets by insertion of Commercial-Offthe Shelf (COTS) technology. The D5 Backfit navigation subsystem maximizes reuse of existing software and maintains current fleet accuracy and availability performance levels with a significant life cycle cost savings.

FBM SWS navigation subsystems must be sent to sea on SSBNs with assurance that they will function as designed the first time and every time. To this end, a navigation test ship has been employed to test navigation systems in an at-sea environment before their deployment on a submarine. USS COMPASS ISLAND (AG-153) steamed well over 100,000 miles supporting development tests for the FBM navigation subsystems. USS COMPASS ISLAND was retired in 1980 and was replaced by USNS VANGUARD (T-AG 194). USNS VANGUARD steamed over 250,000 miles in support of POSEIDON and TRIDENT I navigation subsystems and in development of the TRIDENT II navigation subsystem before retiring in 1999. USNS VANGUARD and USNS RANGE SENTINAL were replaced by USNS WATERS in January 1999. USNS WATERS is a Consolidated Support Ship (CSS) that currently performs at-sea missions in support of navigation and flight test support operations.


Fire Control

The fire control system consists of a powerful distributed computing environment that is used to prepare and support the guidance system for missile flight and to coordinate SWS operations during a prelaunch sequence. The fire control system provides the guidance system with stabilized platform orientation data to support positioning of the platform gimbals, and prelaunch flight calculations that enable missile self-guidance capabilities after launch. The platform orientation data consists of initial guidance system azimuth and elevation position data and periodic navigation data used by the guidance system to determine and correct positioning errors. The guidance system uses the prelaunch flight calculations to determine the flight trajectory for the missile. The function of the fire control system is to collect, compute, and provide the proper data to C4 D5 the guidance system in each missile to ensure that each reentry body is released with the correct speed and direction required to reach a specific target. Delivery of reentry bodies to a target involves launching a missile from a mobile launch platform (the submarine) and guiding it through a flight path so it can release its reentry bodies to free-fall to their selected targets. Since portions of the flight calculations are constantly changing, the fire control system ensures that the most recent and correct flight data is entered into the guidance system before missile launch. At the same time, the fire control system continuously monitors the condition and operation of the guidance system.

The fire control system coordinates a prelaunch sequence by monitoring the SWS and the fire control subsystems required for a launch and by initiating the fire control system and SWS functions at the proper time. To accomplish these tasks, the fire control system receives data from the navigation, launcher, guidance, and missile systems. Using this data, the fire control system determines SWS readiness status, initiates the prelaunch sequence, verifies guidance system operability, provides prelaunch power to the missile, and computes and sends guidance, flight control, target data, earth rotation, guidance system correction coefficients, gravity, star location for inflight correction, time of day, polar motion (earth wobble), target area wind and air density, and fuze set data (reentry body detonation instructions) to the missile. Status reports and data from the navigation, launcher, guidance, and missile systems are monitored by the fire control system to determine if key events are completed prior to the missile launch. An operator control station in the FCS provides the primary fire control operator interface for controlling and monitoring SWS operations during a prelaunch sequence.

Missile Guidance

1The TRIDENT I (C4) Mk 5 and TRIDENT II (D5) Mk 6 guidance systems are both stellaraided inertial systems. They are composed of precision gyroscopes, accelerometers, a stellar tracker, and computer. After launch, the guidance system directs the missile on a corrected trajectory compensating for submarine position, in-flight effects such as high winds, and internal guidance calibratable parameters. The guidance system provides the reference for maintaining missile stability and triggering the reentry body separation for a ballistic trajectory to the target.

The POLARIS A1, A2, and A3 missiles all used what was known as Q-guidance where most of the final computation was done by the Fire Control System prior to launch with only simple tasks left for the guidance computer. The POSEIDON C3 missile used a much more accurate system of continuous computation of trajectory in flight based on position and velocity. Advances in integrated circuitry made it possible for C3 guidance to carry a computer with this capability. The stellar updates used in the C4 and D5 guidance systems correct even further for launch point, trajectory, and inertial measurement errors to produce even higher degrees of accuracy.



TRIDENT missiles are launched from the submarine by a steam generator system. A small, fixed solid-grain gas generator is ignited and its exhaust directed through cooling water into the base of the launch tube. The missile is ejected from the tube, through the water, and to the surface. At that point, the missile’s first-stage rocket motor ignites and sends the missile on its way. Each launch tube has its own launching system independent of the other tubes. Vital parts of each missile are accessible for inspection and maintenance even when loaded in the launch tubes and while the submarine is at sea.

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