Dutton's Nautical Navigation
By Thomas J. Cutler Naval Institute Press
Copyright © 2004 U.S. Naval Institute
All right reserved. ISBN: 1-55750-248-X
Chapter One
Science tells us that our origins are in the sea, and our earliest literature and historical records tell us that we have never strayed very far from it. We have been traveling on the great waters of the world for thousands of years, and from the very beginning we have been concerned with knowing where we are and where we are going on those vast waters. This, of course, is the essence of nautical navigation.
From its earliest days, navigation has been both an art and a science. Much of the navigator's work must be done with precise instruments, exact mathematics, and available technology, yet when the observations have been taken and the calculations made, the seasoned navigator must apply judgment based upon experience to interpret what the science is telling him or her.
TYPES OF NAVIGATION
101 Different schools of thought exist on how to categorize the various forms of nautical navigation. In actuality, these categorizations are not very important but merely serve as a means of helping the neophyte navigator organize the many aspects of a complex subject for learning purposes. The learner will do well to accept from the very beginning that there will be overlap among categories no matter how they are organized.
At one time a fairly simple way to organize the various forms of nautical navigation was to begin with three basic categories: terrestrial, celestial, and electronic. Within the terrestrial (or earthly) forms of navigation are the ancient but still valid methods of piloting and dead reckoning. Celestial (or heavenly) navigation takes advantage of the regularity of the Sun, Moon, stars, and planets to determine position on the surface of the Earth. And electronic navigation began with the invention of radio, which quickly led to radio direction finding, then hyperbolic systems, and eventually satellite and inertial navigation. The problem with this categorization is that there are navigational methods that are not an obvious fit into one of these three categories but are not unique enough to justify their own. Radar navigation, for example, is clearly dependent upon electronics, yet the information it provides yields a form of piloting. Bathymetric navigation is similar. And some have argued that satellite navigation is a form of artificial celestial navigation, while others contend that inertial systems are a form of electronic dead reckoning.
Such confusion need not exist. The modern navigator should simply recognize that there are a great many navigational tools available, know their various capabilities and limitations, and use any or all of them when the appropriate situation arises. With that in mind, categorization will not be attempted, but the various types of navigation are here introduced and explained in greater detail in subsequent chapters.
Piloting
102 Early navigators often stayed close to shore and used familiar sights along the coast as a means of determining where they were. This is piloting in its simplest form. As time and technology advanced, this simple concept of "seeing where you are going" was extrapolated and refined, but the concept remained the same.
Piloting can be thought of as the main form of navigation used when a vessel is in coastal and inland waters. Proximity to land and the shallower depths associated with these waters offer certain advantages that a navigator does not have on the open sea.
Recognizable objects on a nearby shore can be used as a frame of reference to help determine a vessel's position. Early navigators were able to refine this technique by using instruments such as compasses and telescopes to determine their position relative to visible landmarks. Advancing technology improved these instruments and techniques, adding such things as radar to greatly enhance the mariners' ability to measure direction and distance to known objects and thereby fix their position. Some textbooks refer to radar navigation as a separate form of navigation, but because it is used to enhance the navigator's ability to "see" and is used only in inland waters for navigational purposes, it is really an electronically enhanced form of piloting.
In high traffic areas, recognizable objects (such as lighthouses) were added along the shore to serve specifically as navigational references. And because these waters were often shallow enough, markers (such as buoys) were placed in the water at key points (fixed to the bottom to prevent them from drifting away) to serve as virtual "road signs" for the mariner. These became and still are an important element in piloting.
The bottom beneath shallow waters can also be "mapped," providing navigators an additional clue to their position. Depth of water has always been an obvious concern for the navigator because of the fear of running aground, and early navigators dropped weighted lines over the sides of their vessels to determine how much water was beneath them. They eventually realized that there were patterns in the depths, and frequent measurements gave them a "picture" of what the sea bottom looked like. This led to their using bottom contours as another reference for piloting their vessels and is sometimes referred to as "bathymetric navigation." Modern technology has replaced the weighted line with very capable electronic sound transponders that provide a detailed "look" at the bottom, but the technique is essentially the same.
The advantages inherent to inland waters are attended by disadvantages as well. The proximity to land and the shallow waters that aid the navigator also present hazards that are not encountered on the open ocean. Because of the ever-present danger of running aground, there are highly specialized professional navigators in many ports of the world who are intimately familiar with local waters and are paid well to assist visiting ships in safely arriving and departing. Not surprisingly, these expert navigators are known as "pilots."
Dead Reckoning
103 As mariners became bolder in their ventures on the seas, they traveled beyond the range of visual references and into waters too deep to measure with their weighted lines. They then developed procedures to help estimate their position based upon deductive reasoning; in the terminology of the day, this was called "deduced reckoning," which was often abbreviated "ded. reckoning" and thus evolved into dead reckoning (DR), the term we still use today.
In its simplest form, dead reckoning can be thought of as simply keeping track of one's movements. If you proceed from a known location, traveling in a known direction for a known period of time at a known speed, it should be apparent that you will have a much better idea of where you are than if you do not keep track of these things. Of course, the "known" part of this supposition is the potential weak element. Starting from a known location is usually a given, but early navigators had only primitive means of determining direction, time, and distance.
Dead reckoning has another important purpose besides merely keeping track of your movements. DR is used to predict where you think you will be. This is a practice that takes place even in inland waters where there are visual reference points. As a navigator, one of your most important tasks is to prepare for your voyage by laying out your intended trip in advance on a chart (the nautical equivalent of a map). To accomplish this, you begin with your starting point, which is a known position, and then determine a safe and efficient path to follow to get to your destination. The path that you have chosen is marked by intended courses (directions) and speeds as well as predicted times of arrival at designated points (such as time intervals or points at which you intend to change course or speed). This is DR navigation and will be explained in more detail in chapter 9.
It is important to note a generally accepted distinction among navigators. DR navigation is based only upon assumptions relating to course, speed, and time, without taking into account the effects of winds, currents, steering errors, and other forces that might affect the actual travel of a vessel. Correcting a DR position for these additional factors redefines it as an estimated position (EP).
Celestial Navigation
104 Recognizing the deficiencies of dead reckoning when carried on for days without being certain of the effects of wind and current, navigators looked for other means to find their way across the vast sameness of the sea. By carefully studying and recording the visual patterns and regular motions in the heavens above them, mariners long ago learned that they could use these celestial road maps to determine their position at sea.
Celestial navigation is the determination of position by observing the celestial bodies-the Sun, Moon, planets, and stars. In its simplest form, the angle of elevation above the horizon is measured for an identified heavenly body, and a series of mathematical calculations are then performed to compare this information to one's assumed position. Several of these observations and calculations can be used to arrive at a reasonably accurate position. Because the necessary mathematics is complex, navigators have relied for generations on precomputed tables to streamline the process. Today, electronic calculators and computers have almost completely automated the process.
Radio Navigation
105 Radio navigation is the determination of position-and to a lesser extent, direction-using information gained from radio waves received and processed on board a vessel or aircraft. This is accomplished by either radio direction finding or by the use of specially designed hyperbolic navigation systems.
Radio direction finding (RDF) is the oldest form of navigation making use of electronic technology. A vessel equipped with a specially designed directional antenna and receiver can determine with some accuracy the direction from which a radio signal is being broadcast. The navigator can then use this information to help determine the vessel's position.
A hyperbolic navigation system is more complicated but also more accurate. Specially designed radio transmitting stations broadcast signals that appropriately equipped vessels can receive and use to determine their position by comparing the time differences in the received signals. These time differences can be plotted as hyperbolas on charts. There have been a number of different systems created using this principle, but today only a system called "Loran-C" continues in active use in the United States (see chapter 16).
Satellite Navigation
106 As the name implies, satellite navigation involves the use of manmade satellites placed in orbit to aid navigators. While this is the most modern form of navigation, it has been around long enough to have had entire systems created, used, and abandoned in favor of improved methods. Today, the most widely used-and in many ways revolutionary-satellite system is the Global Positioning System (GPS), which has many applications besides nautical navigation. See chapter 17 for a detailed discussion of this and other satellite systems.
THE PROBLEMS OF NAVIGATION
107 Regardless of the specific methods of navigation used by a navigator, the procedures applied must provide a solution to the three basic problems of navigation. These problems are:
1. How to determine position;
2. How to determine the direction in which to proceed to get from one position to another; and
3. How to determine distance, and the related factors of time and speed as progress is made.
Position
108 The most basic problem facing the navigator is that of determining position. Unless you know where you are, you cannot direct the movements of your vessel with accuracy, safety, and efficiency. The term position refers to an identifiable location on the Earth or a point within a manmade system of artificial coordinates. The word position is frequently qualified by such adjectives as "known," "estimated," or "dead reckoning"; these will be further discussed in later chapters, but it is helpful to note at this point that there is a hierarchy of positions as follows: fix-running fix-estimated position-dead reckoning position. Your confidence in your position decreases as you move from left to right in this hierarchy. A fix is based upon very solid information which can be safely relied upon. If you are tied to a pier that shows on your chart, you have an excellent fix on your position. Under way in a narrow channel, as you pass beneath a bridge you again can fix your position on a chart with a great deal of certainty. While piloting, taking visual sightings (see article 109) on several objects can provide a reliable fix of your position. Nearly simultaneous observations of several celestial bodies can provide a fix that is accurate enough for open-ocean navigation, and electronic systems, such as Loran-C and GPS, are capable of providing accurate positioning information as well.
A running fix is similar to an ordinary fix, but because components of information are gathered at different times and artificially moved together, this type of fix is less reliable than a fix in which all the information is obtained simultaneously. Estimated and dead reckoning positions have been discussed above and will be further explained later in this book.
Another positioning term that is often used by modern navigators is waypoint. Defined in some publications as simply "sets of coordinates that describe a location of navigational interest," this term has come to be used primarily with electronic navigation systems and is probably more accurately understood in this context as a specific position that has been programmed into an electronic navigation system. Waypoints may be used to mark points along a planned track (see article 109) or as markers for some particular item of interest, such as a good fishing spot or the entrance to a channel. Modern electronic navigation systems, such as GPS (Global Positioning System) or Loran-C, have features that allow waypoints to be entered into the system and "remembered"; this has major advantages that will become obvious as you read further in this book.
Direction
109 Direction is the orientation of an imaginary line joining one point to another without regard to the distance between them. Direction is measured in angular units called degrees, measured relative to some reference-usually true north-from 0 to 360.
Continues...
Excerpted from Dutton's Nautical Navigation by Thomas J. Cutler Copyright ©2004 by U.S. Naval Institute. Excerpted by permission.
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