Navigation

• 1. Introduction

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  Navigation is the field of study that focuses on monitoring and controlling the movement of a craft or vehicle from one place to another. Every technique within it shares a single goal: locating the navigator's position against known locations or patterns. From a sailor sighting a star to a smartphone pulling signals from orbit, the act is the same at heart. How did people learn to cross open water with no land in sight? Why did finding longitude defeat mariners for most of recorded history? And what happens to a ship when its satellites go dark? The answers run from the Marshall Islands to the floor of the deep ocean, and from a Latin verb meaning to sail to a navigational computer hardened against radiation.

• 2. Reading The Open Ocean

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  Polynesian navigation is probably the earliest form of open-ocean navigation, built on memory and observation rather than written instruction. Early Pacific Polynesians read the motion of stars, the weather, the position of certain wildlife species, and the size of waves to thread a path from one island to another. Their knowledge was recorded on instruments like the Marshall Islands Stick Charts of Ocean Swells. The compass entered the story far away, in China. One of the oldest compasses is Chinese in origin, traced to the Han dynasty since around 206 BC. The Song dynasty Chinese adapted it for sea navigation during the 11th century, and its first recorded use in Western Europe and the Islamic world came around 1190. The word itself is younger than the practice. The term stems from the 1530s, from the Latin navigare, meaning to sail or steer a ship, drawn from navis, ship, and the root of agere, to drive.

• 3. Instruments Of The Discoveries

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  Ramon Llull, a Spanish astronomer, left the oldest record of a sea astrolabe, dating from 1295. Land astrolabes had existed since the Hellenistic period, but bringing one to sea was a different problem. Portuguese navigators are credited with perfecting the instrument during the early Portuguese discoveries of the Age of Discovery. Martín Cortés de Albacar, a Spanish cosmographer, wrote the earliest known description of how to make and use a sea astrolabe in his Arte de Navegar, published in 1551. The quadrant was the first altitude-measuring instrument used extensively at sea, reintroduced by Leonardo of Pisa in the 13th century and first recorded in use by Diogo Gomes in 1461. The cross-staff followed from the 14th century, believed to have come from early Arab navigators, though it forced the user to squint at the sun. John Davis solved that flaw with the backstaff in 1595. With astrolabe, quadrant, backstaff, and compass together, widespread open-seas navigation began in the 15th century. The Portuguese explored the Atlantic coast of Africa from 1418 under Prince Henry, and Bartolomeu Dias reached the Indian Ocean by that route in 1488.

• 4. The Voyages That Followed

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  In 1492 the Spanish monarchs funded Christopher Columbus's expedition to sail west across the Atlantic toward the Indies, a voyage that resulted in the Discovery of the Americas. Vasco da Gama commanded a Portuguese expedition that reached India in 1498 by sailing around Africa, opening direct trade with Asia. The Portuguese pressed further east, reaching the Spice Islands in 1512 and landing in China the following year. The first circumnavigation of the earth was completed in 1522. The Magellan-Elcano expedition was a Spanish voyage of discovery led by the Portuguese explorer Ferdinand Magellan and finished by the Spanish navigator Juan Sebastián Elcano, after Magellan died in the Philippines in 1521. The charts that guided such voyages were precious objects. One of the oldest surviving marine charts is the Carta Pisana, drawn on a sheepskin and dating to 1275. Because paper charts were expensive and rare in the early days, mariners protected them with tools like the Traverse board and traverse tables, the oldest of which dates back to 1428.

• 5. The Longitude Problem

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  Latitude was the easier of the two coordinates to find. Mariners in the Northern Hemisphere sighted the pole star, Polaris, with a sextant; the star's height in degrees above the horizon equals the observer's latitude, within a degree or so. Longitude defeated them for far longer. For most of history mariners struggled to determine it, because longitude can only be calculated if the precise time of a sighting is known. From about 1767 until about 1850, mariners lacking a chronometer used the method of lunar distances, taking a lunar observation with a sextant and a nautical almanac to calculate the time at zero longitude. Reliable marine chronometers were unavailable until the late 18th century and not affordable until the 19th. Time and longitude are bound tightly together. Each second of chronometer error equals 15 seconds of longitude error, which at the equator is a position error of a quarter of a nautical mile, about the accuracy limit of manual celestial navigation. The marine chronometer was developed under John Harrison and others to keep that time true, and the first proper sextant arrived in 1757, its parts and usage developed by inventors including Pierre Vernier and John Campbell.

• 6. Shooting The Stars

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  A navigator shoots a number of stars in succession to give a series of overlapping lines of position, and where they intersect is the celestial fix. Celestial navigation rests on observing the Sun, Moon, planets, and navigational stars with a sextant. The sextant measures the angle between a celestial body and the sensible horizon, using two mirrors, an index arm, and a graduated arc. One mirror, the index mirror, rotates as the index arm moves; the other, the horizon glass, is half silvered and half clear, so the reflected body can be rested precisely on the visual horizon. Three errors must be corrected before each use: perpendicular error, side error, and index error. The practice of taking celestial observations from the deck of a rolling ship, often through cloud and with a hazy horizon, is described as by far the most challenging part of celestial navigation. Air navigators faced the same sky with a different tool, the bubble octant or bubble sextant, used to fix an aircraft's past position before inertial systems and GNSS arrived. A single day's celestial routine was demanding. Traditional practice called for two or more star observations at morning twilight, with prudence suggesting six stars, plus Sun lines, a noon meridian observation, and compass-error checks by azimuth of the Sun.

• 7. Lines, Ranges, And Radio

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  A fix is the intersection of two or more lines of position, and most modern techniques rely on finding where those lines cross. A line of position can be a bearing to a charted object, a radar range that draws a circle of position, a celestial circle of equal altitude, or even a depth sounding from an echo sounder on certain coastlines. Radar gives ranges and bearings to objects within range of its scanner, and a fix from radar alone is called a radar fix. William Burger described parallel indexing in his 1957 book The Radar Observer's Handbook, a technique that offsets a line parallel to the ship's course to keep a set distance from hazards. Radio carried navigation over the horizon. A radio direction finder rotates a directional antenna and listens for the strongest signal from a known station, a system widely used in the 1930s and 1940s. Hyperbolic systems followed. Decca was first deployed during World War II so Allied forces could achieve accurate landings, and it later guided helicopters to North Sea oil platforms. The OMEGA Navigation System, approved for development in 1968 and run by the United States Navy with six partner nations, reached worldwide coverage with only eight transmitters before it was terminated on the 30th of September 1997. LORAN-C, operating between 90 and 110 kHz, still serves several nations, and Russia runs a nearly identical system called CHAYKA.

• 8. Dead Reckoning To Deep Space

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  Dead reckoning advances a prior position using the ship's course and speed, producing a DR position that course and speed alone determine. Correct it for leeway, current, and steering error and you get an estimated position. An inertial navigator develops an extremely accurate estimated position by computing location from motion sensors instead of outside signals. After alignment, an inertial navigation system reads acceleration along three axes from accelerometers and rate of rotation about three axes from gyroscopes. Its great strength is independence: it needs no outside information, resists weather, and cannot be detected or jammed. Its weakness is drift, since errors accumulate at a rate roughly proportional to the time since the last fix. The first inertial system is considered to be the V-2 guidance system the Germans deployed in 1942, and the U.S. Navy later built the Ships Inertial Navigation System during the Polaris missile program. Satellite navigation changed everything after the first experimental satellite launched in 1978. GPS, officially NAVSTAR GPS, was developed by the United States Department of Defense and costs about US$750 million per year to maintain, yet remains free for civilian use. By 2024 it shared the sky with Russia's GLONASS, the European Union's Galileo, and China's Beidou, with over 100 satellites in medium Earth orbit and stated accuracy between 1 and 10 metres. The rise of GNSS jamming and spoofing has renewed interest in resilient methods, and space navigation pushes further still. Pulsar navigation compares X-ray bursts from known pulsars to fix a spacecraft's position, a method tested by agencies including NASA and ESA.