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CHAPTER 1 INTRODUCTION TO MARINE NAVIGATION
DEFINITIONS
Marine navigation is a blend of both science and art. A key union between the knowledge of theory, the application of mathematics and the exercise of seafaring instincts that have proven to be the crucial elements behind successful maritime voyages for millennia.
A good navigator is one who plans each voyage carefully. As the vessel proceeds, he or she gathers navigation information from a variety of sources and then evaluates this information to determine the ship's position. The navigator then compares that position against the voyage plan, operational commitments, and their pre-determined “dead reckoning” position. A good navigator also anticipates dangerous situations well before they arise, and always stays “ahead of the vessel,” ready to address navigational emergencies at any time. The navigator is increasingly a manager of a diverse assortment of resources-electronic, mechanical, and human. Navigation methods and techniques vary with the type of vessel, the conditions, and the navigator's experience. The navigator uses the methods and techniques best suited to the vessel, its equipment, and conditions at hand.
Some important elements of successful navigation cannot be acquired from any book or instructor. The science of navigation can be taught, but the art of navigation must be developed from experience.
Methods of navigation have changed throughout history. New methods often enhance the mariner’s ability to complete their voyage safely and expeditiously, and make the job easier. One of the most important judgments the navigator must make involves choosing the best methods to use. Each method or type has advantages and disadvantages, while none is effective in all situations. Some commonly recognized types of navigation include:
• Bathymetric navigation uses the topography of the sea floor to acquire positioning data. A vessel's position is determined with respects to known locations of geographic features of the ocean bottom.
• Celestial navigation involves reducing celestial measurements taken with a sextant to lines of position using calculators or computer programs, or by hand with almanacs and tables or using spherical trigonometry.
• Dead reckoning (DR) determines a predicted position by advancing a known position for courses and distances. A position so determined is called a dead reckoning (DR) position. It is generally accepted that only course and speed determine the DR position. Correcting the DR position for leeway, current effects, and steering error result in an estimated position (EP).
• Inertial navigation is accomplished by integrating the output of a set of sensors to compute position, velocity, and attitude. These sensors include gyros and accelerometers. Gyros measure angular rate with respect to inertial space, and accelerometers measure linear acceleration with respect to an inertial frame.
• Piloting involves navigating in restricted waters with frequent or constant determination of position relative to nearby geographic and hydrographic features.
• Radio navigation uses radio waves to determine position through a variety of electronic devices.
• Radar navigation uses radar to determine the distance from or bearing to objects whose position is known. This process is separate from radar’s use in collision avoidance.
• Satellite navigation uses radio signals from satellites for determining position.
Electronic systems and integrated bridge concepts are driving navigation system planning. Integrated systems take inputs from various ship sensors, electronically and automatically chart the position and provide control signals required to maintain a vessel on a preset course. The navigator becomes a system manager, choosing system presets, interpreting system output, and monitoring vessel response. In practice, a navigator synthesizes different methodologies into a single integrated system. He or she should never feel comfortable utilizing only one method when others are also available. Since each method has advantages and disadvantages, the navigator must choose methods appropriate to each situation and never rely completely on only one system.
With the advent of automated position fixing and electronic charts, modern navigation has become an almost completely electronic process. The mariner is constantly tempted to rely solely on electronic systems. But electronic navigation systems are always subject to potential failure, and the professional mariner must never forget that the safety of their ship and crew may depend on skills that differ little from those practiced generations ago. Proficiency in conventional piloting and celestial navigation remains essential.