NAVIGATION

science of directing a craft by determining its postition, course, and distance traveled. Navigation is concerned with finding the way, avoiding collision, conserving fuel, and meeting schedules. Navigation is derived from the Latin navis, ship, and agere, to drive. It originally denoted the art of ship driving, including steering and setting the sails. The skill itself is even more ancient than the word, and it has evolved over the course of many centuries into a science and a technology that encompasses the planning and execution of safe, timely, and economical operation of watercraft, aircraft, and spacecraft. Many animals can find their way home from considerable distances by remembering what they saw or smelled along the way. For example, a honeybee returning to its hive with food performs a dance to communicate to its fellows the direction (relative to the Sun) and distance (as far as six miles [10 kilometres]) they should fly to reach the source of the food. Many species of birds migrate thousands of miles with great precision, orienting themselves by the Sun and stars. Such pathfinding feats have undoubtedly been performed since long before human sailors first ventured out of sight of land, but detailed investigation of these activities has made no direct contribution to the analogous skills that have been developed and refined by human navigators. Early mariners who embarked on voyages of exploration gradually developed systematic methods of observing and recording their position, the distances and directions they traveled, the currents of wind and water, and the hazards and havens they encountered. The facts accumulated in their journals made it possible for them to find their way home and for them or their successors to repeat and extend their exploits. Each new landfall became a signpost along a route that could be retraced and integrated into a growing body of reliable information. For these pathfinders, the danger of running into another vessel was of negligible concern, but as heavy traffic developed along established routes, collision avoidance became a serious matter. In all fields of navigation, emphasis shifted from finding the way to maintaining an appropriate distance between craft moving in various directions at different speeds. The larger a ship is, the easier it is to see, but, by the same token, the larger a ship is, the more time it requires to change its speed or direction. When many ships are in a small area, an action taken by one to avoid colliding with another may endanger a third. This problem has been alleviated near busy seaports by confining incoming and outgoing ships to separate lanes, which are clearly marked and divided by the greatest practical distance. Airplanes travel so fast that even though two pilots may see one another in time to initiate evasive action, their maneuvers may be nullified if either one incorrectly predicts the other's move. Ground-based air traffic controllers are charged with the responsibility for assigning aircraft to selected paths that minimize the likelihood of collision. Civil air navigation is profoundly influenced by the requirements of following the instructions of these controllers. The advent of steam-powered ships during the first half of the 19th century added the problem of fuel consumption to the navigator's duties. In the air as well as at sea, the need to carry large amounts of fuel for long trips reduces the cargo capacity; even if plenty of fuel is on board, economy requires that its consumption be kept to a minimum. In spaceflight, fuel is a major navigational consideration. The route to be followed and even the moment for lift-off must be chosen to keep the fuel requirements as small as possible. Adherence to a predetermined schedule, a matter of vital importance in space navigation in connection with fuel consumption, has become important in sea and air navigation for a different reason. Today each voyage or flight is a single link in a coordinated network of transport that carries people and goods from any starting place to any chosen destination. The efficient operation of the whole system depends upon assurance that each journey will begin and end at the specified times. Modern navigation, in short, has to do with the whole of a preconceived passage, from start to finish, and is concerned with four basic objectives: selecting the course (and staying on it); avoiding collision with other moving ships and crashing into fixed obstacles; minimizing fuel consumption; and conforming to an established timetable. Edward W. Anderson The Editors of the Encyclopdia Britannica This article deals with the methods and devices integral to the science of navigation. It also provides a brief historical survey of their development. science of directing a craft by determining its position, course, and distance traveled. Navigation is concerned with finding the way, avoiding collision, conserving fuel, and meeting schedules. Early mariners relied on natural phenomena to mark a course and guide the ship. The earliest sailors stayed within sight of shore and followed landmarks described in written accounts. Detailed knowledge of the prevailing winds allowed later mariners to set longer courses. The Phoenicians and Polynesians could sail out of sight of land because they learned how to use the positions of the stars and the planets to guide them. The earliest instruments are still the principal navigational tools: compasses, logs, and charts. The magnetic compass was the first navigational aid that gave a constant reference point, helping navigators confirm wind directions on cloudy days and allowing for more accurate navigation in open water. When ships began to travel longer distances, it was discovered that true north was almost nowhere the same as magnetic north and that the discrepancy varied considerably from place to place. Magnetic compasses also were inaccurate whenever vessels pitched in heavy seas; the introduction of iron for shipbuilding caused further difficulty. Modern compasses used in both sea and air travel are stabilized by gyroscopes and housed in binnacles that compensate for the effects of the craft's motion and magnetism. Ship speed was first calculated by dropping overboard a log attached to a reel of line knotted at regular intervals. As the log drifted astern, the line was played out; the number of knots exposed while a sandglass emptied gave the speed of the vessel in knotsthat is, nautical miles per hour. The patent log came into use in the 19th century. This instrument is towed by the ship below the water surface; it measures the ship's speed by the rate of rotation of its vaned rotor. Pitot tubes are commonly used to calculate speed through the air as well as through the water. The L-shaped calibrated tube faces forward, and the force of the air or water outside the tube on a liquid inside the tube measures the speed of the vessel. Extensive charts were first drawn up for the Mediterranean. By the Middle Ages detailed accounts of harbours, landmarks, and dangerous waters accompanied harbour-finding charts. These charts did not account for the curvature of the Earth, so that when sailors ventured out of the Mediterranean latitudes they found that such charts did not accurately represent distances or directions. Charts based on the Mercator projection corrected the directional inconsistencies while continuing to display rhumb lines (which were the same as compass bearings) as straight lines. Upon the advent of steamships and airplanes that traveled through the higher latitudes, some charts were based on gnomonic projections, which depicted great circle routes as straight lines. In modern navigation, two methods are used for determining a position: fixing a position and dead reckoning. Information deduced from both methods is usually compared to obtain an accurate position. Fixing a position requires charts detailing known locations, together with instruments that calculate a vessel's bearing relative to those locations. In offshore calculations, the navigator can measure the direction or bearing of a landmark with a sight fixed to a compass. This bearing can be indicated on a chart by a line. A second line drawn from a second bearing will intersect the first line and thus fix the position of the ship. Known as Hadley's Quadrant, this is actually an octant with mirrors which allow it to also be used Fixing a vessel's position on the open sea became possible when latitude and longitude could be accurately determined. Latitude was determined by measuring the altitudethe angular distance above the horizonof the polestar or the noonday Sun with a quadrant. Modern versions of the quadrant or sextant can provide latitudinal position within a few hundred feet. Longitude could not be as accurately reckoned until the 1700s. Because a vessel's position relative to celestial bodies constantly changes owing to the Earth's rotation, precise chronometers and tables showing the calculated positions of those bodies throughout the year had to be developed before longitude could be accurately fixed. Observation of the altitudes of a few identifiable stars at a known time made it possible to calculate the longitude of the position from which the measurements had been made. In the 20th century, radio beacons have become standard reference points. Radio-beacon systems and satellite-transmission networks allow aircraft and ships to determine their position by timing the arrival of radio pulses from synchronized transmitters. Long-rangesignal systems that cover the globe have been developed, permitting any sea or air vessel to fix its position if it has the appropriate receiving equipment on board. Dead reckoning determines a ship's present position from an accurate history of its headings and speeds. Steering a craft with the necessary precision is a complex process. Modern steering mechanisms use gyroscopes, which maintain constant orientations regardless of the craft's motion, to measure headings. Systems have been devised for the continuous measurement of a craft's acceleration in each of three directions that are maintained by gyroscopes. These measurements can be converted to velocities and distances along each of the three directions; the result is a running record of the location of the craft relative to its initial position. This technique of using accelerometers and gyroscopes is known as inertial navigation. Computerized inertial-guidance systems are extremely accurate. Submarines fitted with inertial-navigation systems have traveled under the polar ice cap with errors of less than a mile a week. Inertial navigators are also used to guide missiles and spacecraft. Dead reckoning has become more important as air traffic has swelled. Air-traffic controllers and space expeditions rely heavily on the ability to project a course and control the craft with precision throughout its flight.