Practical Navigation for
the Modern Boat Owner
Copyright © 2017 Fernhurst Books Limited
62 Brandon Parade, Holly Walk, Leamington Spa, Warwickshire, CV32 4JE, UK
+44 (0) 1926 337488 | www.ferhurstbooks.com
First published in 2008 by John WIley & Sons Ltd
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British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
ISBN 978-0-470-51613-3 (hardback)
ISBN 978-1-912177-51-6 (eBook) ISBN 978-1-912177-52-3 (Mobi)
Cover Points
Foreword
Introduction
1 The Global Positioning System
How Your GPS Receiver Tells You Which Satellites It Can See
How GPS Works
Accuracy of the Fix
GPS Blackout
Deliberate Interference
GPS Is Line of Sight
Selective Availability
Differential GPS
Wide Area Augmentation Service
Switch-On Delays
Measurement of Speed
Measurement of Course
Measurement of Heading
Errors in COG and SOG
2 Our Address on the Earth’s Surface
The Equator
Latitude
Greenwich Meridian
Longitude
Our Address
International Date Line
Measurement of Latitude and Longitude
Distance and Direction
Direction
The Flat Earth
The Spherical Earth and ‘Map Data’
Chart Errors
Chart Scale
Measuring Latitude and Longitude
Chart Symbols
3 The Magnetic Compass
The Earth’s Magnetic Field
Steering Compasses
Compass Deviation
Compass Correction
DIY Compass ‘Swing’
Fluxgate Compasses
4 Constructing a Route
Using Second-Hand Waypoints
Loading the Route into the GPS
Constructing a Route on an Electronic Chartplotter or PC
5 Tides
Tidal Heights
Tidal Flow
Finding the Value of the Tidal Flow
6 Boat Speed
Speed over the Ground
Speed Through the Water
Measuring Speed Through the Water
Log Errors
7 Depth Sounders
How They Work
Depth Units
Calibration
Depth Alarms
False Echoes
Fishfinders
8 Finding Position
GPS
Other Methods
Position Lines
Fixing Your Position Using Position Lines
Errors in Position Lines
How Far Can You See?
When All Else Fails
Chartplotters
9 Passage Planning
Overview
Detailed Plan
Just Prior to Departure
Passage Planning – Procedure
Preplan
For the Planned Day of Departure
Passage Making
Passage Grid
Approach ‘Spider’s Web’
Compass Rose as a Waypoint
Unmarked Danger as a Waypoint
Clearing Bearing
10 Pilotage
Who Does the Piloting?
Means of Pilotage
International System of Buoyage
International Buoyage – All Areas
Planning
The Basics of Preparing a Pilot Plan
Making a Pilotage Plan
Working as a Team
11 Automatic Identification System
What is Automatic Identification System?
How Does AIS Work?
Class A AIS
Class B AIS
AIS Displays
The AIS Display on a Chartplotter
AIS Class B Transceiver
12 Radar
How Radar Works
Navigation Using Radar
Pilotage Using Radar
Radar Overlay on a Chart Plotter
Setting up Your Radar
Radar Used for Collision Avoidance
13 Autopilots
Types of Autopilot
Using the Autopilot
14 Personal Computers
What Type of PC
What Make of Chart-Plotting Software?
What Type of Electronic Charts?
Electronic Charts for PC Based Chartplotters
Selecting the Software
Constructing a Route
Sailing Yacht Route Planning
Sending the Route to the GPS
AIS on a PC
Radar on a PC
Navtex on a PC
Tides on a PC
Connecting to the Boat’s Systems
Appendix A Deduced Reckoning and Estimated Position
DR Navigation
Estimated Position
Leeway
Error in EP
EP with Multiple Headings
Appendix B Course to Steer
Where Do You Want to Go?
What Time Interval Do You Choose?
Draw in the Tide
Draw in the Boat Speed
Ground Speed
Comparison with EP
Appendix C Tidal heights and Tidal streams
Atmospheric Pressure Corrections
Tide Tables
UKHO Tidal Predictions
SHOM Tidal Predictions
Tidal Streams
Appendix D Tidal Planning and Plotting
A Long Passage Using a Single CTS
Where will the Tide Take You?
Credits
• If you wonder why your in-car navigator shows that you are in a field – Read on
• If you wonder why your in-car navigator takes you on farm tracks – Read on
• If you think that the electronic charts on your chartplotter are accurate – Read on
• If you want to know the depth of water over the rocks – Read on
• If you think pressing ‘GO TO’ will take you safely to your destination – Read on
The methods of navigation used by the modern boat owner have changed quite rapidly from the traditional methods still currently taught. This doesn’t make the old methods wrong; it just means that the emphasis has changed.
With GPS used in many cars, the level of computer skills of the general public being high, and the so-called paperless office, the modern boat owner desires a different approach to navigation.
‘Practical Navigation for the Modern Boat Owner’ will lead you through all aspects of navigation of your boat in a logical order. The pencil and paper chart part of the subject is not introduced until it’s demonstrated that some knowledge of traditional navigation is necessary. This practical approach to the subject will ensure that although the modern electronic methods of navigation remain at the forefront, the reader will never be lacking in sufficient knowledge to navigate his/her boat safely in any circumstance.
Proper passage planning is not only desirable, but it is also a legal requirement. This topic is thoroughly covered in an entirely practical manner.
The boat owner cannot rely entirely on electronic navigation for pilotage. Pilotage will introduce the well-established and practical aspects of entering and leaving a harbour or anchorage.
Radar is another area where legally the boat owner is required to know how to use this valuable tool. Again, this topic is approached using a practical and easily understood approach.
When I gained my Flight Navigator’s License in 1973, other than when I was actually on the ground, I never knew where I was, only where I had been! By the time you had worked out and plotted a fix, you were at least 60 miles further on. Even when I flew Boeing 747s, without a Flight Navigator, the inertial navigation system, which used three onboard gyroscopic platforms to measure acceleration in all three planes to determine where you were, could be 10 miles in error by the time you had flown 12 hours. Incidentally, the Apollo spacecraft to the moon used only one of these inertial systems for navigation!
Modern airliners use a combination of inertial navigation systems continually updated by automatically tuning into ground-based aids to remove any inherent errors. This has the huge benefit of using at least three different types of data on three completely separate systems to continuously monitor each other for errors, which if found are reported to the pilots.
The first time that I ever knew where I was all the time was when I started using GPS on board my own yacht, assuming of course that what it was telling me was correct.
Fortunately for me, I had around 10 million miles of ‘real’ navigation behind me and I knew when I could trust my GPS and when to treat it with a certain amount of suspicion.
My aim in this book is to show you how to use all the navigation tools at your disposal to the best advantage and to be able to weigh up which ones to place more reliance on according to the circumstances.
To me, navigation has always been more than a means to an end, and I hope you will get as much enjoyment out of it as I do.
How Your GPS Receiver Tells You Which Satellites It Can See
How GPS Works
Accuracy of the Fix
GPS Blackout
Deliberate Interference
GPS Is Line of Sight
Selective Availability
Differential GPS
Wide Area Augmentation Service
Switch-On Delays
Measurement of Speed
Measurement of Course
Measurement of Heading
Errors in COG and SOG
The original global positioning system (GPS) consists of 24 satellites orbiting the Earth at a distance of around 11 000 miles. Each orbits once every 12 hours in six orbital plains, so there will be between five and eight satellites in view at any time, from any point on the Earth’s surface. The drawing here shows only three orbital plains for clarity.
There are a number of spare satellites in orbit in case of failure and each satellite has a life expectancy of about 7 years. New satellites are launched by the US military as required.
Fears about the American monopoly of accurate position fixing amongst non-USA countries have lead to the establishment of GLONASS (a Russian system) and the pending establishment of GALLILEO (a European system). They work in a similar manner and new versions of GPS receiver may be able to operate with any system.
On startup, a GPS receiver starts looking for satellites and will display a page showing you its sky view all around the horizon. The outer ring is the horizon, the inner ring is at an elevation of 45 degrees and the centre represents the position in the sky vertically overhead (the zenith). The predicted positions of satellites are shown as empty circles which become coloured when a satisfactory satellite signal is received. The serial number of the satellite is shown in the circle. Alongside, the diagrams are vertical bars representing the signal strength (in fact the signal-to-noise ratio or quality of the signal) and again each bar is numbered. In this way, you can see the number of satellites and the quality of the signals being received in order to form an idea of how good a fix you are likely to get. There’s often a number giving an indication of the fix accuracy, more of which later.
In order to find its position on the Earth’s surface, a GPS receiver needs to find its distances from at least four satellites. Theoretically, it needs only three, but the clock on the receiver is not accurate enough to allow this.
Distance is measured by measuring the time taken for the GPS signal to travel from the satellite to the receiver. As the time taken is only 0.06 second for a satellite immediately overhead, an error of one thousandth of a second would give an error of 200 miles! Each satellite has an onboard ‘Atomic Clock’, which is super accurate, but for each receiver to be similarly equipped, GPS would not be a practical proposition.
Satellites transmit a semi-random signal, which the receiver matches with its own semi-random signal. The distance the receiver has to move its own signal to get a match is a measure of the time difference and a range can then be calculated. It’s a bit like matching continually repeated barcodes in reality. This is accurate enough to get a first guess at the distance.
If the distance to the satellite is calculated by the receiver, it can be plotted as a position line, where any place on the Earth’s surface is the same distance from the satellite. The receiver must lie somewhere on that position line.
If the distances from two more satellites are calculated and plotted, the receiver must lie on all three lines. Normally, this can occur at only one point on the Earth’s surface and so that must indicate the position of the receiver.
Because of small inaccuracies in the receiver’s clock, there will be an error in its position. The position lines will not intersect at the same point and will form what is known as a cocked hat.
A clever trick within the receiver converts the ranges into pseudo ranges, which allows them to be shuffled around within certain limits.
The range from a fourth or even more satellites is calculated and added to the fix.
The extra position line(s) allows the timing error to be determined and this results in a good fix, where all the position lines intersect at only one point.
With range being calculated using the time taken for the signal to travel between the satellite and the receiver, any variation in the speed of the signal and the actual path followed will lead to errors.
Errors due to these effects will normally amount to no more than ±15 metres for 95% of the time, being made up from the following:
• ionospheric effects, ±10 metres;
• ephemeris errors, ±2.5 metres;
• satellite clock errors, ±2 metres;
• multipath distortion, ±1 metre;
• tropospheric effects, ±0.5 metre;
• numerical errors, ±1 metre or less.
With my boat moored in the marina, normal GPS errors were plotted as shown over an 8 hour period. Although most were contained within the 25 metre diameter circle, one was almost 100 metres in error. This is perfectly normal GPS performance.
Solar flares can cause a complete GPS signal blackout on the sunlit side of the Earth’s surface. In 2006 flares on the 5th and 6th of December caused profound and severe effects to GPS receivers causing a large number of them to stop tracking satellites. Professor Dale Gary of the New Jersey Institute of Technology said ‘This solar radio burst occurred during a solar minimum, yet produced as much as 10 times more radio noise than the previous record … at its peak, the burst produced 20 000 times more radio emission than the entire rest of the Sun. This was enough to swamp GPS receivers over the entire sunlit side of the Earth’.
The Solar flare cycle covers a period of 11 years.
The strength of the radio signals carrying the GPS data is very low and can easily be interfered with. Enemies can deliberately try to disrupt signals in a relatively small local area and military agencies regularly deliberately interfere with the signals to judge the results. These tests are promulgated in advance.
A GPS receiver must be able to ‘see’ a satellite in order to receive its signal. If buildings, cliffs or trees obstruct that line of site, the signal from that satellite will not be received and the accuracy of the fix may be degraded. It’s possible that the signal may be received as it bounces off another surface so it will take longer time to arrive and will give an inaccurate range. Again this can degrade the fix accuracy.
The signal can penetrate some solid surfaces, such as glass, GRP and canvas, and it is sometimes possible for a receiver antenna mounted inside the boat to work satisfactorily.
Originally, civilian users had their signals deliberately degraded by the US military inducing a randomly varying error, known as selective availability, ensuring that accuracy was no better than 100 metres for 95% of the time. This selective availability has been switched off, but the US military may reintroduce it, without warning, at any time. This must always be considered a possibility. On the accompanying chart, the error that disappears northward off the chart was over 800 metres.
Errors that occur from a corrupt satellite signal will be incorporated into the fix by a GPS receiver and can lead to very large errors, measured in miles, and will continue until the satellite is switched off by the monitoring team, which could take up to one and a half hours.
A GPS receiver fixed in one place will know exactly where it is. Any position derived from the received GPS signals can be compared with its known position and any error deduced. If this error was transmitted to the nearby GPS receivers, they could take account of this error in deducing their own position to give a much more accurate result, with a 95% probability error of 3 metres. This is known as differential GPS (DGPS).
To take advantage of this, the GPS receiver needs both a separate DGPS receiver and to be within range of a DGPS station, usually about 200 miles. This is commonly used for survey GPS and was beginning to be common for leisure users until selective availability was switched off, when its need for normal leisure use disappeared because of the inherent 15-metre accuracy.
Wide Area Augmentation Service (WAAS) uses a network of ground stations to monitor the GPS position accuracy. The error corrections are sent to two master stations, which in turn send error correction information to the constellation of satellites. The continuously varying error correction information is broadcast by the satellites and is then available to all WAAS compatible GPS receivers. The 95% error is then reduced to 7.5 metres. Manufacturers usually optimistically claim a 3-metre accuracy. Integrity monitoring is part of this system, so anomalous signals from under-performing satellites are automatically discarded.
WAAS is available only in the United States of America, but European Geostationary Navigation Overlay Service (EGNOS) and the Japanese Multi-Functional Satellite Augmentation System (MSAS) provide the same service in areas covered by these. A WAAS compatible receiver will operate with EGNOS and MSAS.
Modern GPS receivers normally have 12 or more channels which can receive data from 12 different satellites simultaneously. Satellites are moving fairly rapidly along their paths and the ability of the receiver to ‘lock’ onto a large number of satellites means that they are always using the best data available. It also means that their ‘startup’ times are very quick.
The oldest receivers have very few channels, so they have to divide their time between using data from only one or a few satellites and searching for new ones. They are inherently slow.
When a new GPS receiver is first switched on, it has no idea of the time, date, where it is or where the satellites are. As the information about the whereabouts of the satellites is transmitted only every 12.5 minutes, it will be some time before the GPS can compute its first fix. This is known as a cold start.
When the GPS is switched in the same geographical position as when it was switched off, it knows where to expect the satellites to be, the date and the time, so modern 12 channel receivers can compute their first fix very quickly.
If the GPS receiver has been moved since it was last switched off, it will take longer time than a hot start but much less than a cold start.
There is nothing inherent in the GPS signals that measure speed. However, the receiver does have a lot of built-in information that it can use to present useful information. Once the GPS receiver has worked out its position, it can use its knowledge of the shape and size of the Earth to determine the distance between any two points, so that once it is in motion it can work out the distance between two fixes, and taking the time taken to travel this distance it can deduce its speed. This speed is the speed over the ground (SOG), not to be confused with the speed through the water.
Boat speed is the speed of the boat through the water and is displayed on the water speed display. Wind, waves and tide will cause the speed over the ground to differ from the water speed.
The GPS signal contains no information on the direction in which the boat is moving. Because the GPS receiver knows the shape of the Earth, it can determine the direction that it has travelled from one fix to another. This course over the ground (COG) is exactly what it says and may not be the same as the course steered by the boat.
