Jonathan Sawyer / Eowyn Sorenson

There are no highways at sea. when a boat sails out from the comforting familiarity of the harbor it moves into a wide-open, ever-changing world. Perspectives shift, distances become hard to gauge, and landmarks along the receding shoreline take on a new and often. perplexing aspect. Ahead, all kinds of unseen dangers may lurk - sand bars, rocks, sunken wrecks and powerful tidal currents. To keep track of his exact location in this alien environment and to lay a safe course to his next destination, a boatman applies a basic and venerable skill -. he navigates.

Basic navigation aids include a chart and a compass. A chart shows the physical characteristics of an area and is used to plot a vehicle's course and position. An aeronautical chart provides such information as the location of airports, the height of mountains, and the location of radio transmitters that aid navigation. A nautical chart provides the depth of the water, the location of buoys and lighthouses, and other sailing information. A navigator uses either a magnetic compass or a more complex electronic instrument called a gyrocompass. Gyrocompasses are more precise than magnetic compasses.

Vehicles also have such basic navigation aids as an extremely accurate timepiece called a chronometer and a device to determine speed. The speed gauge of a plane is known as an air-speed indicator. The one on a ship is a log and on a land vehicle is a speedometer.

There are five primary methods of navigation: (I) dead reckoning, (2) piloting, celestial navigation, (4) electronic navigation, and (5) inertial guidance. A navigator uses one or more of these methods, depending on such factors as the type of vehicle and the weather.

Dead reckoning (DR) involves estimating a vehicle's position by considering how far and in what direction it has traveled. This method is not extremely accurate. However, navigators generally use dead reckoning along with any of the other methods. In dead reckoning, the navigator determines the vehicle's position in relation to its last fix. A Fix is a vehicle's known position. Starting at the fix, the navigator draws a line on a chart that represents the direction and distance traveled by the vehicle. The vehicle's DR position is at the end of this line. The navigator determines the direction with a compass. The distance is calculated by multiplying the vehicle's speed by the time traveled. Dead reckoning does not consider such factors as current, steering errors, or the wind. Any of these factors may cause a vehicle to be at a location other than the calculated DR position. The navigator must then use a more accurate method to establish a new fix. Dead reckoning helps the navigator keep track of a vehicle's position between fixes. It is also used to plot future positions and to estimate when the vehicle will arrive at its destination. Some larger ships use a device called a dead reckoning tracer to plot their position electronically. The tracer monitors the ship’s gyrocompass and log to ascertain the direction and distance traveled. This information is automatically and continually drawn on a chart or displayed on dials.

In piloting, a navigator finds the position of a vehicle in relation to one or more landmarks A-n up-to-date chart is a necessary aid. The chart shows the positions of natural landmarks, including mountains and islands, and of such artificial landmarks as buildings, buoys, and lighthouses. Piloting involves determining a landmark's bearing (direction) and distance from the vehicle. A bearing is determined by using a compass or a pelorus. A pelorus is a compass-like device that measures the bearing of a landmark in relation to a ship. It has a circular face marked off in degrees and a sight vane that rotates above it. The navigator aims the sight vane at a landmark, and the bearing is shown on the face of the pelorus. A device called an azimuth circle can be attached to a compass and rotated like a sight vane to determine bearings. A hand compass can also be used. The simplest method of piloting involves finding the bearings to two or more landmarks. A landmark's bearing is represented on a chart as a line called a line of position. The navigator determines the bearing of one landmark and draws a line of position from the landmark in the direction shown by the compass or pelorus. The vehicle's position is somewhere along this line. The navigator then ascertains the bearing of another landmark and draws a second line of position. The vehicle's location is the point where the two lines cross. Other methods of piloting include (1) observing the bearings of a single landmark at various intervals of time, and (2) observing when two landmarks lie on a single bearing, Piloting can be used to navigate most types of vehicles. Ships use this method when entering or leaving ports, or when sailing close to land. Many small boats are piloted with only a chart and compass. Navigators may also use a depth finder, a device that measures the depth of the water. A navigator compares the depths indicated by this device with the depths on the chart.

Celestial navigation is a method of determining a vehicle’s location by observing certain celestial bodies--the sun, the moon, the stars, and the planets. A publication called an almanac lists the positions of these bodies at all times during the year. Celestial navigation uses Greenwich Mean Time (GMT), the time in Greenwich, England. There are many variations of celestial navigation. In one common method, the navigator finds a vehicle's position in relation to an assumed position (AP). An AP is a point chosen by the navigator that is at or near the vehicle’s dead reckoning position. To determine the position of the vehicle, the navigator uses a sextant. This instrument measures the angular distance of a celestial body above the horizon. Such a measurement is called the observed altitude of the body. The navigator then calculates the computed altitude, which is what the body's altitude would be if the vehicle were at the AP. The navigator finds the computed altitude by solving a mathematical problem involving a spherical triangle. The points of the triangle are (1) the AP, (2) the North Pole or the South Pole, and (3) the geographic position (GP) of the celestial body. The GP is the point on the earth directly beneath the body. The navigator finds the GP of the celestial body in the almanac. A ' special calculator or a navigation aid called a sight reduction table is used to solve the triangle and determine the computed altitude. The navigator then performs certain calculations to compare the computed altitude with the observed altitude.

These calculations place the vehicle on a line of position, A fix is established by repeating the process with one or more celestial bodies. For most ships, celestial navigation can be used only when the sky is clear. Celestial navigation can be used on airplanes and spacecraft at any time these vehicles fly above the clouds. Submarines have a special navigation periscope that enables a navigator to view celestial bodies while the ship is underneath water.

Various navigation systems are based on electronic devices, most of which use radio signals. Radio signals vary in frequency, the rate at which a radio wave vibrates as it travels through air or space. High-frequency navigation signals generally can be used with greater accuracy. However, high frequency signals cannot be transmitted beyond the horizon. Low-frequency signals usually are not as accurate in establishing position as are high-frequency signals, but they can be received thousands of miles away. Common navigation systems that involve reception of radio waves include loran, omega, omnirange, and radio direction finding. In the United States, transmitters for these navigation systems are based on land, and most are operated by the government. Other systems that use radio signals include radar and satellite navigation. Radar equipment is carried aboard a vehicle or based on land. Satellite transmitters are based in space.

Loran is used to guide ships and some airplanes as they approach coastal waters from the sea. The word loran stands for long range navigation. A loran system uses two types of stations called a master station and a secondary station. Special equipment aboard a ship or a plane uses the low- or medium-frequency signals sent by these stations to establish a loran line of position. The location of a vehicle is at the point where the lines of position derived from two pairs of loran stations intersect on a chart. In most cases, a single master station is paired with each of two secondary stations. The type of loran system used in the United States and Canada is loran-C. A loran-C signal has a range of more than 1,000 miles (1,600 kilometers) during the day. Under certain atmospheric conditions, it can be received more than 3,000 miles (4,800 kilometers) away at night, when low-frequency radio waves travel farther. A position can be located within 1/4 mile (0.4 kilometer).

Omega is a worldwide navigation system for planes and ships, The system has eight transmitters located throughout the world and requires international cooperation to operate. Special equipment aboard a vehicle receives and processes the radio signals sent by two pairs of omega transmitters. This equipment establishes two lines of position for the vehicle. The vehicle's position is at or near the point where the lines of position cross.

Omnirange is a short-range navigation system for aircraft flying over land. A special receiver electronically traces the signal of an omnirange transmitter to determine the bearing of the transmitter from a plane. The location of the transmitter is shown on the aeronautical chart. Many omnirange stations also transmit signals for airborne distance-measuring equipment, which calculates a vehicle's distance from a transmitter.

Radio Direction Finding involves locating the bearing of a transmitter called a radio beacon. A radio beacon's signal is received aboard a vehicle by a device called a radio direction finder (RDF). The navigator turns the antenna of the radio direction finder to find the direction of the radio beacon. The RDF shows when the antenna is pointing toward the beacon. Radio direction finding is one of the oldest electronic navigation systems for aircraft and ships. It is generally used as a piloting aid along coastal waters. The range of a radio direction finding signal depends on the type of radio beacon.

Radar is based on the transmission of radio signals toward an object and the reception of the signals that are reflected from the object. The return signals create an image on a radar screen, which shows the direction and distance of the object from the vehicle. Radar has many uses in navigation. It helps a navigator carry out a piloting operation by locating the vehicle's position in relation to landmarks at night or in bad weather. Radar also is used to locate obstructions and to prevent collisions with other vehicles. In addition, navigators use radar to track the positions of airplanes, missiles, ships, and spaceships.

There are several satellite-based navigation systems. Probably the best known is the U.S. Navy's TRANSIT system, which began worldwide operation in 1964. The TRANSIT system consists of seven operating satellites and several spares that circle the earth in widely spaced orbits. A computerized receiver on a vehicle fixes its position by analyzing the Doppler shift of the signals broadcast by any TRANSIT satellite as it passes overhead.

.Another worldwide satellite system, the Navstar Global Positioning System (GPS), is scheduled to become fully operational in 1995. The word Navstar stands for Navigation Satellite Tracking and Ranging. Upon completion, it will consist of 21 satellites and three spares that circle the earth in six different orbits. Each satellite broadcasts its exact position and the time. A computerized receiver on a vehicle analyzes how long it takes the broadcast from at least three different satellites to reach it. The receiver then determines its location by circulating its distance from these satellites. The GPS enables all types of vehicles on the earth and in the air to find their position under all weather conditions.

GLONASS, a satellite navigation system similar to the GPS, was established by the Soviet Union. This system provides positioning information for land navigation across most of Europe. After the break-up of the Soviet Union in 199 1, control of GLONAS S passed to an alliance of former Soviet republics, then to Russia.

Inertial guidance involves the use of a computer and a device called an inertial navigator. An inertial navigator constantly monitors changes in the vehicle's motion and sends this information to the computer, The computer uses these changes, plus the distance and direction traveled, to calculate the vehicle's course and position. Inertial navigation is used to guide a variety of vehicles, including aircraft and submarines. Some guided missiles also use it.

In ancient times, sailors navigated by observing various celestial bodies and constellations and by studying the seasonal directions of the wind. During the Middle Ages (A.D. 400's- 1500's), navigators drew simple charts that included wind directions for different seasons, plus compass directions. Many of today's navigation instruments developed from crude equipment used hundreds of years ago. For example, the first compass consisted of a magnetized piece of metal on a straw. The straw floated in a container of water. The sextant developed from the astrolabe and the octant, two early instruments for measuring the angle between a celestial body and the horizon. An astrolabe consisted of a disk, plus a sight that turned on a pivot. An octant had the same arrangement of mirrors as a sextant.

In the I 700's, James Cook, a British navigator, became the first person to use modern celestial navigation methods. Cook used these techniques in several voyages to the Pacific Ocean. The development of radio, followed by its use on ships and airplanes in the early 1900’s, marked the start of electronic navigation. During World War II (1939-1945), German scientists developed navigation systems to guide V- I and V-2 rockets. On the Allied side, British and American scientists made important advances in radar technology.

Satellite navigation began in 1960, when the United States launched a satellite called TRANSIT I B, but the TR-ANSIT system did not begin daily operation until 1964. In the 1970's, the United States began to launch the first several satellites of the Navstar Global Positioning System (GPS). During the 1980's, development of the GPS continued. The system is scheduled to become fully operational in 1995, when the last of its satellites begins operation.