Navigation

E

VHF Omnidirectional Range

VHF omnidirectional range (VOR or omnirange) is an electronic navigation system used mainly by airplanes and air traffic control. A VOR station located at an airport broadcasts a very high frequency (VHF) radio signal in all directions. The radio signal has a different phase, or characteristic, in each direction. (An RDF signal, by contrast, is the same in all directions.) Airplanes have a special receiver that interprets the signal’s unique phase and displays the compass direction to or from the station on an electronic display in the cockpit. Stations that have distance-measuring equipment also send a signal showing the distance to or from the antenna. This enables the navigator to determine both distance and bearing, which together produce a reliable position. The military uses a specialized version of the VOR called tactical air navigation (TACAN) or a combination VOR/TAC system.

F

Inertial Navigation

In inertial navigation, a set of highly sophisticated electronic instruments uses the principles of dead reckoning navigation to track a craft’s changing position and maintain its course. In traditional dead reckoning, human error is unavoidable. Steering errors, currents, gusts of wind, and other external forces can drive a ship or an aircraft off course, unbeknownst to even the most skilled human navigator. An inertial guidance system automatically identifies forces that cause steering errors or speed errors and makes the adjustments necessary to bring the craft back on course. Once the craft leaves the starting point, the inertial navigator automatically measures, calculates, and records the craft’s speed and direction, and the changes to them (called accelerations).

An inertial navigation system has three components: a gyroscope that responds to forces that might throw the craft off course; an accelerometer that interprets the gyroscope’s responses, and a computer that makes necessary adjustments. A gyroscope may be oriented in any direction. When oriented to true north, it serves as a compass that is completely unaffected by Earth’s magnetic field or by the iron and steel on the craft. A gyroscope consists of a spinning wheel balanced in a set of rings, called gimbals, that prevent it from wobbling. The wheel spins in the same direction at a constant speed until an outside force acts on it. It reacts to that outside force in a predictable way that can be detected and interpreted by the accelerometer. Data collected by the gyroscope and accelerometer are sent to the computer, which calculates the craft's position and makes necessary steering adjustments.

Inertial guidance also provides information to the human navigator and acts as an automatic pilot. It is so accurate that it enables a pilot to land an aircraft in poor visibility conditions. Many types of military craft, including guided missiles, submarines, and army tanks, employ inertial navigation systems. The military uses inertial navigation because the system is impossible to jam or sabotage from outside using existing military technology. Inertial navigation is also required safety equipment in passenger aircraft making international flights. Aerospace companies launch telecommunications satellites for private companies aboard rockets equipped with sophisticated inertial guidance systems. However, inertial guidance systems are too expensive for widespread civilian use.

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VI

History of Navigation

The early seafarers who first started plying the water about 3500 bc rarely left sight of land. At night they pulled their reed boats up on beaches or anchored in safe harbors identified by landmarks, such as hills, tall trees, or partially submerged rocks. They gave these landmarks colorful names, many of which persist to this day. For example, a sailor may come across rocks or islands named Lone Rock or Sow and Pigs (a big rock surrounded by many little ones).

From the time they took to the sea, mariners developed devices to help them navigate safely at increasingly long distances from land. One of the first was the sounding pole, carried on board to gauge water depth. They also made the first aids to navigation, which consisted of piles of rocks on shore or stakes driven into the sea bottom. Because few maps were available at the time, early navigators relied heavily on memory and on written guides called pilot books. Pilot books provided information about coastal piloting in ancient Greece, Rome, and Egypt—for example, how long it took to sail from one busy port to another, and then how to get into that port without running aground. But pilot books provided little information about sailing offshore and out of sight of land.

A

Ancient Offshore Navigation

Ancient navigators learned to study patterns in the world around them. They followed the schedule of the Sun, which they knew always rose in the east and set in the west, but that the exact direction varied with the seasons. They found patterns in the direction and velocity of the wind. In the Mediterranean Sea, for example, some winds were warm (these were from the south), others cold (from the north), others dry (from the east), still others wet (from the west down the length of the Mediterranean Sea). Mediterranean navigators represented these patterns in a picture called a wind rose, which resembles the dial on a modern compass showing the cardinal points of north, east, south, and west. Much later, when sailors began to travel offshore, they discovered predictable seasonal winds. They called these winds trade winds because commercial sailing ships took advantage of them to carry their cargoes to their destinations as fast as possible.

Ancient mariners also studied the sea itself and the waves on it. In the Pacific Ocean, Polynesians navigated accurately over distances as great as 3,200 km (2,000 mi) using the wind and waves as references. Waves have different shapes in shallow water and near islands than in deep water offshore. When big, stable ocean swells turn into many stubby breakers, the water is becoming shallower, which almost always indicates that land is nearby.

B

Development of Celestial Navigation

Early sailors also followed the stars. Stars and constellations held such importance that ancient Greeks and Romans named them after the great gods and goddesses of their cultures. In the 2nd century bc, the Greek astronomer Hipparchus identified and cataloged more than 850 stars and may have used a device to measure the altitude of celestial bodies. He also believed that the world was round and that, like all spheres, it could be better understood with a grid of lines dividing it up into 360 degrees (see Trigonometry: History). Within 200 years these lines had come to be known as latitude and longitude. The Greek mathematician Ptolemy declared that each degree had 60 minutes, and each minute consisted of 60 seconds.

For many centuries sailors planned their courses, sailed, and recorded their progress, all in reference to a single star over the top of the globe, the North Star. Also called the polestar because it lies almost directly over the North Pole, this star never seemed to waver in the northern hemisphere. In reality, it rotates in a very small circle in the sky because it lies about 2.5° to the side of the North Pole. However, few sailors can steer to within 3 degrees of a course, even with modern instruments, and the North Star still provides a reference point for sailors to this day.

The concept of determining position by measuring the altitude of a celestial body traces its roots to ancient times. Polynesians used a device called a latitude hook to measure the distance between a star and the horizon. They had different hooks for different latitudes. Whichever hook fit indicated how far north or south the navigator was (in other words, the hook determined the latitude). The ancient Arabs used a tool called a kamal (“guide”), which worked somewhat like the latitude hook, to measure the North Star.