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Navigation

People can only navigate safely, whether in enclosed waters or along a coast, if the hazards that may be encountered on the way are identified and marked in such a manner as to make them obvious and avoidable.
Navigation aids are broadly classed as either local or international. Local aids are those installed by the waterways authority responsible for a paticular area.

Examples of local aids are signs that restrict some actions such as beaching a boat, restrict the speed in the area, or guide boats safely into a harbour entrance or the centre of a channel. Then there are clearance lights on low bridges, special lights indicating an island in the centre of a fairway and lights on floating buoys intended for tying up large ships. The shapes and the light characteristics of these local aids are determined by the local waterways authorities and are not international in configuration. However, they do appear on charts of the area and are also generally described in sailing directions for the area published by the local authorities.

All other navigation aids are designed to be international, so they are the same in any harbour or port in the world. The body responsible for designing this system of universal navigation aids is the International Association of Lighthouse Authorities, known as IALA.

The system in operation in most countries is known as the IALA System A. It is called System A to distinguish it from the IALA System B, which contains some differences designed to cater for paticular difficulties in some ports.
The IALA System A consists of three types of marks: lateral marks, cardinal marks and individual marks. Note that the word "mark" is the general term used to describe any navigation sign intended to give direction or point out a hazard. A mark can be a floating buoy, a beacon or light mounted on the shore, or a pole stuck into the harbour bottom with some identifying shape on top of it.

Charts

There is no subsitute for knowing where you are. As with any other skill, some reference material is required to provide you with the information necessary for you to work effectively.
Charts are the most important of all navigation aids. They should not be confused with waterways maps which, although very useful for short trips in confined waters, do not have the accuracy or the wealth of information charts provide.
Charts not only give the information necessary to navigate safely from one point to another by avoiding hazards, they are also used to record the boat's position and to find the direction to steer so as to arrive at the desired destination.

Basic Navigation Tools

Navigation also relies on having some basic tools, some hardware, to use in conjunction with reference material such as charts.
Here are the important bits of hardware we need:
  • Steering compass that can be fixed onto the boat.
  • Pencil - preferably a soft lead.
  • Eraser - you can carry it in your pocket.
  • Rule - the clear plastic ones are good because you can see the chart through them.
  • Pair of parallel rules or a Douglas protractor-necessary if any extensive coastal navigation is to be undertaken.
  • Pair of dividers to measure distance.
  • Hand bearing compass - held in the hand and used to take bearings of prominent objects.
  • Depth Sounder - Often used in conjunction with dead reckoning as it improves the accuracy of the estiamates of position.

Navigation doesn't have to be complicated to be effective, but it must be accurate as you can make it. Remember that your safety depends on it. Know how to do it before venturing out to sea.

GPS - Satellite Navigation System

The GPS system was designed for and is operated by the U. S. military. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock.
The Space Segment of the system consists of the GPS satellites. These space vehicles (SVs) send radio signals from space.
The nominal GPS Operational Constellation consists of 24 satellites that orbit the earth in 12 hours. There are often more than 24 operational satellites as new ones are launched to replace older satellites. The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each day). There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. This constellation provides the user with between five and eight SVs visible from any point on the earth.

SPS - Standard Positioning Service

Civil users worldwide use the SPS without charge or restrictions. Most receivers are capable of receiving and using the SPS signal. The SPS accuracy is intentionally degraded by the DOD by the use of Selective Availability.

  • SPS Predictable Accuracy
  • 100 meter horizontal accuracy
  • 156 meter vertical accuracy
  • 340 nanoseconds time accuracy
These GPS accuracy figures are from the 1994 Federal Radionavigation Plan. The figures are 95% accuracies, and express the value of two standard deviations of radial error from the actual antenna position to an ensemble of position estimates made under specified satellite elevation angle (five degrees) and PDOP (less than six) conditions.
For horizontal accuracy figures 95% is the equivalent of 2drms (two-distance root-mean-squared), or twice the radial error standard deviation. For vertical and time errors 95% is the value of two-standard deviations of vertical error or time error.
Receiver manufacturers may use other accuracy measures. Root-mean-square (RMS) error is the value of one standard deviation (68%) of the error in one, two or three dimensions. Circular Error Probable (CEP) is the value of the radius of a circle, centered at the actual position that contains 50% of the position estimates. Spherical Error Probable (SEP) is the spherical equivalent of CEP, that is the radius of a sphere, centered at the actual position, that contains 50% of the three dimension position estimates. As opposed to 2drms, drms, or RMS figures, CEP and SEP are not affected by large blunder errors making them an overly optimistic accuracy measure Some receiver specification sheets list horizontal accuracy in RMS or CEP and without Selective Availability, making those receivers appear more accurate than those specified by more responsible vendors using more conservative error measures.

GPS & SPS information courtesy of The Geographer's Craft Project, Department of Geography, The University of Texas at Austin. Copyright 1999 Peter H. Dana.


    Related links
  • Advanced Coastal Navagation Course
  • Australian Search & Rescue
  • AutoNav Marine Systems
  • Canadian Notices to Mariners
  • Captain Brent Forsberg's Piloting Tips
  • Celestaire
  • Celestial Navigation for the Complete Idiot
  • C-MAP Electronic Charting Center
  • C-MAP UK
  • Eldridge TideWare
  • Explorer Chartbooks
  • A Global Positioning System Overview
  • The GPS Nuts
  • Greenwich 2000
  • ICAN
  • International Navigation School
  • Latitude: The Art and Science of Fifteenth-Century Navigation
  • Map Masters
  • Maptech
  • Marine Chart Services
  • Marine Computing Int.
  • MarineNav
  • The National Imagery & Mapping Agency Marine Navigation Department (NIMA)
  • The National Imagery & Mapping Agency's Online Navigation Publications
  • National Ocean Service's MapFinder
  • Nautical Software Inc.
  • Nautisk Forlag
  • Navigate!
  • The Navigation Aid Course Conversion Calculator
  • Navigation Center
  • Navigator Light
  • Navtech
  • NetSea, Inc.
  • Ocean Navigator Magazine
  • The Office of Coast Survey (NOAA)
  • Online NOAA Charts
  • rec.boats.electronics
  • Sail Online
  • Sam Wormley's Global Positioning System (GPS) Resources
  • sci.geo.satellite-nav web resources list
  • Sea Information Systems & Thomas Gunn Navigation Services
  • Service Hydrographique et Océanographique de la Marine (French)
  • Shipsoftware.com
  • South Pacific & Caribbean Charts
  • Starpath School of Navigation
  • United States Inland Navigation Regulations
  • USCG Navigation Rules Q and A Page
  • US Coast Guard Navigation Center
  • Waterproof Charts
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