GPS - A practical guide

This practical guide explains how to get the best from your GPS, and takes you through GPS navigation in a clear and structured way. It summarises how to navigate with a map and compass. The guide also looks at digital mapping, and how GPSs and mapping software can be used positively together.

CHAPTER ONE - INTRODUCTION

Satellites and GPS technology

When my grandfather sat watching Neil Armstrong make his historic ‘small step’, the idea that in just a few years’ time we would be using satellites to navigate our way round the world was probably no more than a pipe dream in some scientist’s brain. Those pioneering astronauts got to the moon without satellite navigation – almost unimaginable for those of us who switch on our in-car sat-navs just to get to the shops!

The origin of the current system dates back to the late 1970s when the Americans launched 24 satellites that circle the earth twice a day. These emit a weak radio signal that GPS receivers employ to calculate the user’s exact location. The idea is very simple. By calculating the difference between when the signal was sent and when it was received, the GPS receiver can calculate the satellite’s location.

Once the receiver has signals from three satellites, it can calculate its own location in 2D (latitude and longitude). With four or more satellites it can work out altitude as well. The early receivers could only accept signals from a maximum of six satellites, but modern units can lock onto up to 16 satellites at any one time.

GPS – Global Positioning System – was developed for American military uses. The satellites were programmed to send out a random error to civilian users, which limited the accuracy of the location to around 150m. When America was involved in military conflict, this ‘selective availability’ (SA) was switched off, so their bombers could, presumably, bomb more accurately.

In the year 2000 – following the development of technology with which the Americans could confuse enemy GPSs – SA was permanently switched off; suddenly our GPS receivers were accurate to within 15m. This coincided with a sharp reduction in the price of receivers and they became a far more common sight on the hills and mountains of the world.

GPS accuracy is good, but there are situations where it isn’t good enough. Airline pilots, for example, currently are not allowed to land using GPS in poor visibility, because the system is not precise enough. A secondary system which checks the accuracy of the circling satellites is necessary, and comes in the form of a number of geostationary satellites and a series of ground stations. The American system is called WAAS (Wide Area Augmentation System) and the European EGNOS (European Geostationary Navigation Overlay Service). GPSs within an area covered by these ground stations are accurate to 2 or 3m. Newer GPSs can pick up these signals; the display will indicate in some way that this is happening (Figure 1.3). Even if you’re picking up EGNOS signals, most units will tell you that you’re registering WAAS ones.

Over the next few years – funding permitting – the European ‘GPS’ system, Galileo, will be coming on stream. This will be a series of 30 satellites covering the whole globe. Galileo will work in tandem with the American system and the lesser-known Russian one (GLONASS) and will mean our GPSs will soon be accurate to around 1m.

The technological implications of this are many (and beyond the scope of this book) and will be far more exciting than the navigational kit we currently play with on the hills and in our cars. Expect some stunning developments over the next few years.

Using GPS receivers

There is a huge range of GPS receivers aimed at walkers and other outdoor enthusiasts currently on the market, as well as units aimed at motorists, either built in to the vehicle or removable, and cross-over GPSs that have ‘sat-nav’ programmes for motorists but can also run software more suited for outdoor navigation – on which this book will focus.

GPSs for the outdoor enthusiast (with or without on-screen Ordnance Survey mapping) are designed to help get you from A to B (and perhaps help you identify exactly where A and B are!). In doing so a GPS will bombard you with masses of extra information which you may or may not want. Speed, average speed, altitude, bearing you should be taking, track (the actual bearing you’re walking on), the distance you are from the correct route, and so on. We’ll look at these in greater detail in later chapters.

The added benefit of GPSs is the fact that they can link with digital mapping software on your computer. Used together these become an invaluable planning and analysis tool. You can plan a route in advance, follow the route on the GPS and analyse the actual route you walked later on your computer.

Chapter 2 discusses what the information on a GPS display means, and encourages you to read the instruction manual so you know how your particular machine works. Chapters 4 and 5 go through the essentials of navigating with a GPS, starting with the most elementary skills and building to more complicated routines as you become more confident. There are practical exercises throughout the book, giving you an opportunity to reflect on what you’ve read and put those skills into practice.

GPSs are little computers, and there are occasions when they can fail. Chapter 6 looks at the steps you need to consider should this happen, whether you have to reboot the unit, replace the batteries or resort to traditional navigation skills (the latter are covered in Chapter 3, which also looks at how your GPS can be used to replicate these methods).

The final two chapters introduce digital mapping and look at how these can be used to enhance your developing GPS skills, and how to keep these new-found skills honed.

This is not a ‘quick-read-and-become-an-expert’ book. Take your time to try the exercises practically and develop your skills and you’ll emerge a confident and effective GPS navigator.

First things first: let’s get to grips with what your GPS is telling you.