GPS Field guide © WHO 2002
09/25/02 GPS_field_guide 8:19 A9/P9 CONTENTS CHAPTER 1: INTRODUCTION TO GPS 1.1 What is GPS ?…………………………………………………………...2 1.2 How does GPS work ?…………………………………………………..3 1.3 Source of GPS signal errors……………………………………………...7 CHAPTER 2: WHY USING GPS IN THE CONTEXT OF THE WHO WHS ? CHAPTER 3: PRESENTATION OF THE GARMIN ETREX GPS UNIT 3.1 Overview of the eTrex unit……………………………………………..10 3.2 Utilisation of the eTrex unit…………………………………………….13 3.2.1 Installation of the batteries………………………………………….13 3.2.
09/25/02 GPS_field_guide 8:19 A9/P9 CHAPTER 1: INTRODUCTION TO GPS 1.1 What is GPS? The Global Positioning System (GPS) is a system allowing to precisely identify locations on the earth's surface. The GPS system has 3 parts (Figure 1): · · · The Space segment: a network of 24 satellites placed into orbit (Figure 2). The first GPS satellite was launched in 1978 and a full constellation of 24 satellites was achieved in 1994. Each satellite is built to last about 10 years.
09/25/02 GPS_field_guide 8:19 A9/P9 1.2 How does GPS work? GPS satellites circle the Earth twice a day in a very precise orbit at an altitude of around 19 000 Kilometres (Figure 2). This constellation allows any user to access between five and eight satellites from any point on the Earth. Figure 2: The GPS Satellite network Each satellite transmits radio signal information which is tracked and used by the GPS receiver to calculate the user's exact location.
09/25/02 GPS_field_guide 8:19 A9/P9 The receiver: 1. Calculates the distance to the first satellite he is able to catch. Let's suppose that the receiver calculates a distance of 17'000 km between this first satellite and the receiver. This will mean that the receiver is located somewhere on a sphere that is centred on this first satellite and that has a radius of 17'000 km (Figure 3). Figure 3 - Sphere indicating the potential location of the GPS receiver with one satellite signal 2.
09/25/02 GPS_field_guide 8:19 A9/P9 3. Repeats the operation mentioned under point 2 with a third satellite. In our example the receiver finds out that it is 20,000 km from the third satellite which narrows its position down even further, to the two points (Figure 5 point b) where the 20,000 km sphere cuts through the circle reported on Figure 4 (corresponding to the intersection of the first two spheres).
09/25/02 GPS_field_guide 8:19 A9/P9 1.3 Sources of GPS signal errors Even if today's GPS receivers are extremely accurate, certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. If most of the sources of error are unavoidable, it is important for the user to be aware of the ones that he can influence and be prepared to take steps to reduce their impact.
09/25/02 GPS_field_guide 8:19 A9/P9 There may also be sources of signal multipath (i.e on Figure 8) occurring when the GPS signal is reflected off these objects before reaching the receiver. This increases the travel time of the signal and creates errors of distance estimation between the satellite and the receiver.
09/25/02 GPS_field_guide 8:19 A9/P9 CHAPTER 2: WHY USING GPS IN THE CONTEXT OF THE WHO WHS ? The first reason for using GPS in the WHS context is the added value it gives to the survey by allowing the geographical dimension to be taken into account without requiring important investment (many GPS receivers are inexpensive, GPS data is free, and survey staff can be trained quickly to their use).
09/25/02 GPS_field_guide 8:19 A9/P9 CHAPTER 3: THE GARMIN ETREX UNIT The Garmin eTrex GPS receiver has been chosen for the WHO WHS because it is easy to use and inexpensive. Its utilisation in the context of different surveys operated in the field has proven its suitability for this kind of work. This device is completely waterproof, runs for approximately 22 hours on two AA alkaline batteries and can operate under dense tree canopy.
09/25/02 GPS_field_guide 8:19 A9/P9 Figure 10 - Back view of the Garmin eTrex device Table 1 describes the elements reported on the Figures 9 and 10 Inside the waterproof case are located a set of electronic components that allow the receiver to perform different operations and to display the final result on the LCD screen through a specific operating system.
09/25/02 GPS_field_guide 1. Internal GPS Antenna 8:19 A9/P9 This antenna allows the receiver to track satellites signals. Thus, when using the GPS, the unit should be parallel to the ground and facing upwards. Apart from the cover of the holster nothing should be put on this part of the unit as this may stop satellite signal. 2. LCD Display Screen Screen where all the information is displayed. The LCD Display is one fragile part of the eTRex it may be injured and it must be well protected. 3.
09/25/02 GPS_field_guide 8:19 A9/P9 3.2 Utilisation of the eTrex unit This section presents the major elements that will be used in the context of the WHO WHS. It introduces you to the basic features of the eTrex as well as some basic definitions of terms in a more detailed way than presented on the Quick Reference guide. 3.2.1 Installation of the batteries To install the batteries, remove the battery cover (Figure 10) by turning the D-ring at the back of the receiver 1/4 turn counter-clockwise.
09/25/02 GPS_field_guide 8:19 A9/P9 3.2.3 The Garmin eTrex Pages The eTrex operating system is composed of different pages organised in a hierarchical way starting from 4 main pages (Figure 12). The 'NORMAL SKYVIEW SATELLITE' page is one of them. To Switch from one of these main page to the other you have to use the 'PAGE' button. In the context of the WHS we will only need to use two of them: - The 'NORMAL SKYVIEW SATELLITE' page - The 'MENU' page.
09/25/02 GPS_field_guide 8:19 A9/P9 This information concerns: - The Status of the device. Two type of messages can appear: a) "WAIT…LOCATING SATELLITES" indicating that the receiver is tracking satellite signals. b) "READY TO NAVIGATE" indicating that the receiver has found satellite signals strong enough to know its location and that you can start to use it. - The location accuracy message that is displayed when the device is ready to be used ("ACCURACY").
09/25/02 GPS_field_guide 8:19 A9/P9 You can access to these two pages by pressing the ENTER button from the 'NORMAL SKYVIEW SATELLITE' page in order to open the 'OPTIONS' window. From there you can use the UP and DOWN buttons to highlight the page you want to access and press the ENTER button. Once you have finished working on one of these pages you can come back to the 'NORMAL SKYVIEW SATELLITE' page, pressing the PAGE button again (Figure 14).
09/25/02 GPS_field_guide 8:19 A9/P9 The 'ADVANCED SKYVIEW SATELLITE' page This page contains the same elements as the 'NORMAL SKYVIEW SATELLITE' page but presented in a different graphical way (Figure 16). This display gives more information about the location of the satellites in the sky (represented as a numbered square) around your location (the point at the centre of the screen).
09/25/02 GPS_field_guide 8:19 A9/P9 Figure 17 - The 'MENU' page There are two options of interest for the WHS on the 'MENU' page: the 'MARK' and 'SETUP' ones. The Figure 18 allows you to visualise how to access to the structure of these two options. Figure 18 - Scheme to access the 'MARK' and 'SETUP' option pages To access to the selected option you have to highlight it in the menu using the 'UP' or 'DOWN' button (on Figure 16 the 'MARK' option is highlighted ) and press the ENTER button.
09/25/02 GPS_field_guide 8:19 A9/P9 The 'MARK WAYPOINT' option page Once the GPS receiver has been set-up and that the user has found an open area for the reading, this page will allow the user to visualise his location (Figure 19). The other information or options reported on this page will not be used in the context of the WHS. Figure 19 - the 'MARK WAYPOINT' option page The 'SETUP' option page The 'SETUP' option page (Figure 20) allows the user to customize the eTrex.
09/25/02 GPS_field_guide 8:19 A9/P9 The 'UNITS' setup page This page (Figure 21) allows the user to setup the units of the different parameters. It is important to make sure that this page contains the setting specific to the WHS as indicated in the Figure 21 a) and in the Quick Reference Guide. b) a) Figure 21 - The 'UNITS' page (a) with the scroll down list (b) To change the value or format in a specific field you have to first highlight it using the UP or DOWN button (e.g.
09/25/02 GPS_field_guide 8:19 A9/P9 CHAPTER 4: TROUBLESHOOTING 4. 1 eTrex does not turn on: 1. 2. 3. Check to see if the batteries are installed correctly and if the battery terminals are clean. If the eTrex does not turn on after point 1: place a fresh set of AA batteries in the unit (refer to Quick Reference Guide point n°3 ' Installing the Batteries' and use Alkalyne batteries as they have a longer life).
09/25/02 GPS_field_guide 8:19 A9/P9 - eTrex then asks: 'IS TODAY dd-mmm-yy?'. Two possibilities: a) The date indicated matches the current date : · Press UP/ DOWN buttons · Highlight 'YES' and press the ENTER button, the eTRex will continue tracking satellites and switch to the Satellite page. b) The date is incorrect: · Press UP/ DOWN buttons · Highlight 'NO' and press the ENTER button, the eTRex will do an 'AUTOLOCATE' and switch to the Satellite page.
09/25/02 GPS_field_guide 8:19 A9/P9 4.6 eTrex does not display the local time in the 'SETUP' page The eTrex operates on universal time (or GMT, Greenwich Mean Time ). It is not necessary to have your local time displayed. Do not worry about a wrong displayed time. 4.7. The geographic coordinates I get are outside the range indicated in the 'Country information' document : 1. Make sure the eTrex is configured as indicated in point 7 of the Quick Reference Guide and redo the reading correctly. 2.
09/25/02 GPS_field_guide 8:19 A9/P9 GLOSSARY Accuracy : How close to the real value a measurement is. Backlight: Selectable on/off illumination that lights the display for enhanced screen at night and in low light. Contrast: The difference between the lightest and darkest areas on a display screen. Coordinates : The unique description of a geographic position using numeric characters.
09/25/02 GPS_field_guide 8:19 A9/P9 Variance: In this context correspond to the errors in magnetic compass readings caused by variance in the earth's magnetic field at different locations on the planet. Waypoint: Current location on the face of the Earth, in terms of the specific latitude/longitude coordinates. BIBLIOGRAPHY / LINKS This document has been created using information coming from the following publications or Web Sites: Garmin International, Inc.
