June 09, 2007

For each journey we make, using our own means of transport, we make at least some sort of preparation. However, in contrast to road travel, preparation for journey by sea or air, before and during travel, is much more detailed because of the higher degree of risks involved, and the technical nature of the means of transport. Navigation and the means of navigation provide the answer to addressing these risks and to manage the journey safely, effectively and efficiently. Therefore navigation is highly important in today’s world as the bulk of air, sea and space travel is dependent on that.

Navigation is the art and science of determining one’s position so as to safely travel to a desired destination. It includes the charting (making a chart) of a course for a ship or aircraft and seeking its guidance from place to place. The word “navigate” is derived from the Latin roots ‘navis’ meaning ‘ship’ and ‘agere’ meaning ‘to move’ or ‘to direct’. Though navigation is divided into branches such as sea, air, land and space, to a large extent, marine (sea) navigation is given more importance because it is in sea travel that navigation has the most use.

Different techniques have evolved over the ages in different cultures, but all involve locating one’s position against known locations or patterns.

For specifying the location of a place on earth, use is made of the equator, the prime (Greenwich) meridian, latitudes and longitudes. The equator is the imaginary horizontal line, along the surface of the earth’s sphere that divides the earth into two equal parts as northern hemisphere and southern hemisphere. Latitudes are the horizontal lines that are drawn parallel to the equator in the northern and southern hemisphere. Each latitude is specified by its angular distance from the equator and this distance is measured in degrees (one degree equals 69 miles). Latitude ranges from 0° at the equator to 90° at the North and the South Pole.

Similarly, longitudes are the vertical lines or meridians, stretching from the North Pole to the South Pole, that divide the earth’s sphere into two equal parts. For reference purposes, the line that passes through Greenwich, England, has been specified as prime (Greenwich) meridian. Each longitude is specified by its angular distance (measured in degrees) from the prime meridian and ranges from 0° at the Greenwich meridian to 180° in both east and west directions.

By making use of latitudes and longitudes, the location of any place on Earth can be specified in terms of degrees and direction. For example when we say that Karachi’s location is 25*N67*E, this means that Karachi is located at latitude of 25° N and a longitude of 67° E. Or, it lies at 25° north of the Equator and 67° east of the Prime Meridian.

History of navigation
The earliest form of navigation was land navigation. Sea navigation began later. People such as the Phoenicians, the Ancient Greeks, the Persians, Arabians, the Norse and the Austronesians have excelled as sea-faring explorers.

Early navigators used celestial bodies (sun, stars, and planets) for navigation. Most sailors have always been able to find absolute north from the stars or by using a dual sundial or diptych (a device that measures time by the position of the sun). Piloting was also common as a method that uses familiar landmarks as guides. Dead Reckoning (estimating position based on a ship’s speed and direction) was used in cases where landmarks were out of sight. With time, inventions were made such as “cross-staff” (later substituted by astrolables) to determine latitude, “precision hour-glasses” for time-keeping, and the magnetic compass to continue sailing when visibility was limited. The navigating equipment carried by Columbus was most probably a compass, a cross-staff, and a table of the sun’s declination (its position north or south of the equator). Vasco de Gama, on his first voyage around the Cape of Good Hope in 1497 used an astrolabe.

After Isaac Newton published the Principia, navigation was transformed. Starting in 1670, the entire world was measured using essentially modern latitude instruments and the best available clocks. A number of scientific journals during this period were started, especially to chronicle geography.

In 1730 the sextant was invented and navigators rapidly replaced their astrolabes. A sextant uses mirrors to measure the height of celestial objects with regard to the horizon. The problem of finding the longitude remained unsolved until the invention of the chronometer (a precision timepiece used aboard ships to provide accurate time for celestial observations). The appearance of the Nautical Almanac (publication containing celestial information) in 1767 was a great step forward in navigation, and the 19th century saw the development of books on navigation that far surpassed any earlier instructions. Subsequent developments included the placing of lighthouses and buoys (a float attached by rope to the seabed) close to shore to act as sea signposts.

In the late 19th century radio was invented and direction-finding was quickly adapted to navigation. Up until 1960 it was commonplace for ships and aircrafts to use radio direction-finding on commercial stations in order to locate islands and cities. Around 1960, LORAN was developed. This used time-of-flight of radio waves from antennas at known locations to locate geographic positions. An analogous system for aircraft, VHF omni-directional range and DME, was developed around the same time.

In 1974, the first GPS (Global Positioning System) satellite was launched. The GPS system now permits accurate geographic location with an error of only a few metres, and precision timing to less than a microsecond. More than two dozen GPS satellites are in medium Earth orbit, transmitting signals allowing GPS receivers to determine the receiver's location, speed and direction.

Methods of navigation
Most modern navigation relies primarily on positions determined electronically by receivers collecting information from satellites. Most other modern techniques rely on crossing lines of position (LOP). An LOP can refer to two different things: a line on a chart and a line between the observer and an object in real life. A bearing or angle is a measure of the direction to an object. In addition to bearings, navigators also often measure distances to objects.

The methods involved in navigation are as follows:

Dead reckoning
Dead reckoning is the process of estimating one’s present position by projecting course and speed from a known past position. It is also used to predict a future position by projecting course and speed from a known present position.

Pilotage
Simple navigation begins with pilotage. This is to know one’s position in familiar territory by familiarising oneself to visual landmarks. One can also use a map, nautical or aeronautical chart to perform pilotage in unfamiliar locations.

Celestial navigation
Celestial navigation is most often used out of sight of land, based on observation of the positions of the Sun, Moon and stars relative to the observer and a known location. In addition to the sextant mentioned above, celestial navigation requires an accurate time source such as a chronometer and a nautical almanac. Some facility for plotting the resultant LOP is also required.

Electronic navigation

It consists of:
• Radio navigation
• Radar navigation
• Satellite navigation

Radio navigation -— Radio navigation uses radio waves to determine position by radio direction finding systems. Radio navigation works by pointing a directional antenna in ‘various directions’ and then listening for the direction from which the signal from a known station comes.

Radar navigation — Marine radar systems can provide very useful navigation information in a variety of situations. When a sea-ship is within radar range of land or special radar aids to navigation, the navigator can take distances and angular bearings to charted objects and use these to establish LOPs on a chart.

Satellite navigation — Satellite navigation uses artificial earth satellite systems, such as GPS, to determine position. Global Navigation Satellite System or GNSS allows small electronic receivers to determine their location to within a few metres using time signals transmitted along a line of sight by radio from satellites.

It is imperative that proper navigation techniques and methods are used for all kinds of travelling and no mistake is made by the person using those techniques and methods. Failing to do this would lead to all kinds of inconveniences for persons making use of sea, air, and space travel particularly, and might well lead to accidents and disasters.