Navigation

V

Navigation in Pilotage Waters

Piloting is the most exacting form of navigation because it entails the movement of ships under many potentially dangerous conditions. The greatest care and exactness are necessary for success in piloting, especially in poorly charted coastal waters or under unfavourable weather and visibility conditions. One of the chief concerns of the navigator in pilotage waters, where traffic is heavier than at sea, is to avoid collision with other ships.

A

Line of Position

A basic concept in piloting is known as the line of position, a line indicating a series of possible positions of a craft and determined usually by observation. The line may be straight, curved, or irregular, as when the line produced by plotting a series of soundings taken over a period. One line of position is not sufficient to determine the exact position of a ship. The point of intersection of two or more lines of position, taken simultaneously or adjusted for time lapse, is a positive position known as a fix. The navigator in pilotage waters strives constantly to plot such intersections of lines. Fixes then serve as reliable guideposts for future movements or decisions.

Visual piloting is accomplished generally by employing an azimuth circle on a gyrocompass repeater to take bearings of identifiable and chartered objects. These bearings are then plotted on a chart of the area to indicate graphically the position of the ship. A single navigational object may define a fix if both a bearing and a range can be taken simultaneously by the use of a rangefinder in addition to the azimuth circle, or by radar. In cases where only one line of position is available without an accompanying range, the navigator must resort to the use of the so-called estimated position, which is not as reliable as a fix but is more reliable than a dead-reckoning position. An estimated position requires continued extra caution in the navigation process until a fix is determined.

A line of position may be obtained by any one of the following methods: a range within which two known fixed objects appear in line, and the ship is placed somewhere on this line; a compass bearing of an object observed visually or by radar; a range obtained by rangefinder or by radar; a single sounding or a series of soundings of the bottom (usually referred to as a chain of soundings); a horizontal angle, measured by a sextant, between two known objects; a vertical angle, measured by a sextant, of an object of known height; an echo of the ship's whistle or siren; a radio direction-finder bearing; lines of position derived from one of several electronic systems; and astronomical lines of position.

B

Fixing the Position

Any combination of these methods of determining a line of position permits a fix. Fixes may be arrived at by cross bearings, by finding the bearing and distance of the same object, by a bearing and a sounding taken simultaneously, by horizontal sextant angles, and by two bearings of a single object taken at different times but adjusted for time lapse when plotted. The last-mentioned technique is known as a running fix.

In addition to these graphic methods, a ship's position can be deduced by the use of horizontal angles in conjunction with a three-arm protractor. Such a protractor consists of a circle, graduated in degrees, to which is attached one fixed arm and two arms pivoted at the centre. If horizontal angles taken on three identifiable fixed objects shown on a chart are set on the protractor and the latter is positioned on the chart with the objects lined up on the three arms, the position of the ship is fixed at the centre. Aids to navigation may consist of various types of beacons, buoys, lighthouses, and light vessels; their characteristic shapes and colours provide at least partial daytime identification, and characteristic phases and colours of lights provide identification at night. Where these aids are absent, the navigator must resort to taking bearings of mountain peaks and of chartered structures such as water tanks or church spires, and taking tangent bearings of islands or points of land.

C

Tides, Tidal Streams, and Ocean Currents

The practice of navigation is complicated by the presence of tidal effects and ocean currents. These effects, which may be favourable or unfavourable, tend to deflect the ship from its charted course and to reduce or increase its speed. A comparison of dead-reckoning positions and fixes reveals the extent of such effects and often helps the navigator to predict and adjust for future influences. See Ocean and Oceanography; Tide; Wind.

VI

Celestial Navigation

In this classic method, used most commonly in the open sea, the navigator measures the positions of celestial bodies. Stars have been identified and grouped into constellations, partly for this purpose, since ancient times (see Astronomy). Celestial navigation makes possible voyages across thousands of kilometres of unmarked water, but its one great limitation is that poor visibility, caused by clouds, fog, rain, snow, mist, or haze, may prevent the essential sightings of celestial bodies.

A coordinate system similar to the Earth's coordinates of latitude and longitude has been adopted to describe the positions of heavenly bodies. This system consists of declination, which corresponds to terrestrial latitude, and hour angle, which corresponds to terrestrial longitude. For practical purposes of navigation, the positions of the stars relative to one another are regarded as fixed on the celestial sphere; the motions of the Sun, the Moon, and the planets are indicated in this system as a mean rate of progression across the sphere.

The principal maritime nations publish yearly nautical almanacs that tabulate the coordinates at any particular time of the celestial bodies used in navigation. The tables also provide other pertinent astronomical information.

To use the nautical almanac, the navigator must establish the time of an observation accurately by means of the chronometer. The measurement of time is based on the rotation of the Earth and the consequent apparent rotation of celestial bodies around the Earth. In navigation, the primary system of time is based on the apparent movement of the Sun westward at 15° of longitude per hour. Thus, a time difference is established between two places on the surface of the Earth based on their difference of longitude. The longitude of New York, for example, is roughly 75° west and that of Greenwich is 0°. New York is therefore 5 hours to the west of Greenwich.

The navigational triangle, or astronomical triangle, which constitutes the most important part of celestial navigation, is a spherical triangle, the three points of which represent the position of the observer, the geographical position of the celestial body, and the geographical pole that is nearer to the observer. The solution of such a triangle provides the basis for the derivation of an astronomical line of position. Spherical trigonometry was formerly required to solve such a problem, but this triangle can today be solved simply by using the nautical almanac in conjunction with one of several short tabular methods. The tabular methods include precomputed solutions of the astronomical triangle to accommodate any position of the observer and any celestial body observed.

In the most modern approach to celestial navigation, the circle of equal altitude and the astronomical position line are used in conjunction with the solution of the navigational triangle. The circle of equal altitude is a circle on the surface of the Earth, and at every point on this circle the altitude of a given celestial body is the same at a given instant.