USS COD Articles - Navagational Systems

Navigational Systems

by John C. Fakan

The systems used to provide information for navigation include the gyrocompass and gyrocompass repeaters, the dead reckoning system, underwater log, LORAN, magnetic compass, sextant, and -- when in sight of fixed landmarks -- the two periscopes and the two Target Bearing Transmitters (TBT's) located on the bridge.


The gyrocompass receives its directional information from a rapidly spinning gyroscope driven by electric motors. Its directive action is based on the mechanical laws governing the dynamics of rotating bodies. When any object is spinning it tends to keep its axis pointed in the same direction, and if a force is applied to deflect its orientation it responds by moving at right angles to the applied force. The gyrocompass consists of a gimbal-mounted spinning gyroscope made north-seeking by placing a weight below the axis. As the Earth rotates gravitational pull on the weight attempts to change the gyroscope's axis of rotation. The resulting motion of the axis of the gyroscope at right-angles to the applied force causes it to move so as to align itself with the Earth's axis of rotation. A few hours of operation is usually sufficient to align the gyrocompass with the Earth's axis. Errors that would be introduced by changing latitudes as when the submarine is steaming towards the north or south are eliminated through periodic adjustments to compensating systems within the gyrocompass by the Navigator. An electrical servo mechanism and dial mechanically connected to the gyrocompass has the points of the mariner's compass marked on it and indicates the submarine's true course. Repeaters connected to the servo mechanism are located throughout the submarine to provide true course information where needed.

Dead Reckoning System:

The dead reckoning system consists of a dead reckoning analyzer (mechanical analog computer) and indicator in the control room and two dead reckoning tracers, one in the control room and one in the conning tower. (The name 'dead reckoning' is a contraction of 'deduced reckoning' which refers to the idea that the present position is deduced from a known prior position as modified by the known course and speed made good since the known position was established.) This system, when properly set, indicates at all times the latitude and longitude of the submarine on dials in the analyzer and traces the ship's movements on a chart placed on the tracer. The analyzer is an instrument for converting the ship's course and distance into direct readings of latitude, longitude and miles traveled. It receives course information from the gyrocompass and distance from the underwater log, a device that computes the apparent distance traveled from the continuously measured speed of the submarine. This speed is determined from the difference between the static pressure (the pressure due to the depth of water over the submarine) and the dynamic pressure (that due to the relative motion between the submarine and the water -- like the pressure felt when a hand is held out the window of a moving automobile). These two pressures are measured via the Pitot Sword which is deployed beneath the hull of the submarine as it moves through the water. (The pitot system is similar to those used by aircraft for determining airspeed.) Errors in the reported position are introduced primarily from ocean currents which are not detectable by the underwater log. Periodic position updates, or "fixes", are required to maintain sufficient accuracy in the dead reckoning position.


LORAN is a navigational system for determining position by means of radio. The word is derived from the first letters of the words "LOng RAnge Navigation." The principal of operation is based on the difference in travel time (to the point of observation -- eg., the submarine) of radio signals from a pair of transmitting stations located several hundred miles apart. For any specific time difference the receiver must lie somewhere along a special mathematical curve called a hyperbola. Determining the delay from a second set of transmitting stations results in a second hyperbola. The position of the observer must then lie at the intersection of these two curves. The DAS-3 LORAN unit aboard WW-II fleet submarines is a combined radio receiver and indicator and is capable of determining the differences in the arrival times of the signals within a microsecond (millionth of a second). This measurement determines a particular line of position (a hyperbola) as indicated on carefully prepared charts. Similar measurements made on a second pair of LORAN stations provides a second line of position. The intersection of two or more of these lines determines a "fix" in the conventional manner.

Magnetic Compass:

For more traditional means of navigation the submarine carries a magnetic compass in the conning tower, several chronometers, a sextant and appropriate almanacs and mathematical tables compiled especially for celestial navigation.

Target Bearing Transmitter:

The Target Bearing Transmitter, or TBT, is a permanently mounted binocular instrument that can be used to sight objects and determine their bearing relative to the heading of the submarine. Cod carries two of these devices on her bridge deck. The TBT contains electro-servo circuitry that electrically sends information about the target's bearing to equipment in the Conning Tower. The information is transferred at the push of a button mounted on the TBT. For navigation purposes the bearings of fixed land objects can be transferred to bearing lines on a chart to show the location of the submarine. Three appropriate bearings are required for a "fix", but even a single bearing can help the navigator determine the submarine's position.

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Last revised 09 Apr 2009
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