IV.

Navigation Instruments

Many instruments are employed today to facilitate navigation; some are relatively simple to use and others require extensive instruction. In the latter category are some of the modern electronic and mechanical devices.

Navigation instruments are designed to fix position, measure direction and distance, determine speed, measure the depth of water, assist in plotting on charts, and observe the weather elements. Sometimes various instruments are used simultaneously to yield the required information.

The magnetic compass is one of the oldest instruments used aboard ships. Although it has been generally supplanted by the gyrocompass on large ships, the magnetic compass retains its original role as the basic navigational instrument because it is not subject to electromechanical defects, and hence, on most seagoing ships, it is a necessary standby instrument. The magnetic compass serves as a directional device by aligning itself in the direction of the Earth's magnetic poles.

Because of the location of the magnetic poles, the needle of a compass will point to the geographical North Pole only in a few localities. In other places, it will point east or west of north. The difference in degrees between the direction of the compass needle and that of true north is called variation, or declination. For the convenience of navigators, the declination in many parts of the world has been measured, and charts have been prepared that show by isogonic lines—curves connecting points of equal declination—the approximate east or west declination for any place. On such charts, the line of zero declination, along which the compass points true north, is called an agonic line.

The gyrocompass, which uses a gyroscope as its directive element, indicates true north. The gyroscope in this compass is a rapidly rotating body, free to move about one or two axes, perpendicular to the axis of rotation and to each other. Control elements are added to the gyroscope to convert it to a true direction indicator. The indications of the master gyrocompass may be repeated in various parts of the craft—for example, in bearing repeaters, steering repeaters, and radar repeaters.

The azimuth circle is an important auxiliary device used for indicating the azimuth, or bearing, of an object, its direction measured from the north point. It is a graduated ring with sight vanes that is designed to fit snugly over a compass or a compass repeater; it provides a means of taking bearings of both terrestrial objects and celestial bodies.

An instrument known as the log is used to determine either the speed of a ship or the distance travelled through the water, or both simultaneously. Various types of logs are used, some operating on a simple mechanical principle and others based on ingenious electromechanical techniques. The airspeed indicator fulfils the same function in an aircraft.

To determine water depth a navigator uses either the lead or the echo sounder. The lead, which consists essentially of a lead weight at the end of a suitably marked line, is used in coastal or shallow waters under conditions of low visibility. The echo sounder, which is found on almost all seagoing ships, indicates the depth of water by measuring the time interval between the emission of a sonic or ultrasonic signal and the return of its echo from the bottom. Aircraft height is determined by a barometric altimeter, a radio altimeter, or an inertial system.

The plotting equipment used by the navigator resembles to a certain extent the tools used in drafting. Dividers for measuring distances, compasses for drawing circles, plotters, protractors, and universal draughting machines are the rudimentary tools commonly found on the chart table of a ship.

For celestial navigation the navigator uses a sextant and a chronometer. The sextant is a double-reflecting instrument that measures the angle between two objects by bringing into coincidence rays of light received directly from one object and by reflection from the other. Its principal use is to determine the altitude (in degrees of arc) of celestial bodies above the horizon. The chronometer is a very accurate timepiece with a nearly constant rate of daily gain or loss. It is set to the time of a standard meridian, usually that of the former Greenwich Observatory, in London, and makes possible the determination of longitude at sea. Its daily rate of gain or loss is checked by radio time signals broadcast from various countries. (See Clocks and Watches). In addition to these instruments, most modern ships use several electronic navigation devices (see Electronic Navigation below).