Garmin GPS User Manual

GPS Field guide

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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

3.2.2 The eTrex operating system………………………………………...13

3.2.3 The eTrex pages……………………………………………………..14

3.2.3.1 The 'NORMAL SKYVIEW SATELLITE' page………………...14 The 'DISPLAY' page………………………………………………..16 The 'ADVANCED SKYVIEW SATELLITE' page………………...17

3.2.3.2 The 'MENU' page………………………………………………..17

The 'MARK WAYPOINT' option page…………………………….19

The 'SETUP' option page……………………………………………19

The 'UNITS' setup page…………………………………………….20 The 'SYSTEM' setup page………………………………………….20

CHAPTER 4: TROUBLESHOOTING

4.1 eTrex doesn't turn on…………………………………………………….21

4.2 The message "READY TO NAVIGATE" doesn't appears on the screen 21

4.3 The message "TROUBLE TRACKING SATELLITES. ARE YOU INDOOR NOW ?" appears on the screen…………………………..21

4.4 I am not getting an accuracy higher (better) than 20 meters…………….22

4.5 eTrex does not display the desired units when making the accuracy or

coordinates reading………………………………………………….22

4.6 eTrex does not display the local time on the 'SETUP' page…………….23

4.7 The geographic coordinates I get are outside the ranges given in the protocol form……………………………………………………….23

GLOSSARY………………………………………………………………………….24

BIBLIOGRAPHY / LINKS………………………………………………………….25

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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. Replacements are constantly being built and launched into orbit.

· The Control segment which consists of ground stations, located around the world that make sure the satellites are working properly.

· The User segment: the GPS receivers used by the community (eg. ETrex device).

Figure 1 : The 3 GPS segments

This satellite-based system offers highly precise location data for any point on the planet, in any weather conditions, 24 hours a day. It is mainly used for navigation, positioning and other research applications.

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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.

The satellite signal:

This constantly transmitted radio signal passes through clouds, glass and plastic but not through most solid objects such as buildings and mountains.

A GPS signal contains three kinds of coded information essential for determining a position:

· An I.D. that identifies each of the 24 satellites of the network

· The almanac data that contains the orbital information for all satellites in the system.

· The ephemeris data, which contains important information about the status of the

satellite (healthy or unhealthy), and the current date and time.

To determinate precise latitude and longitude/position, the receiver measures the travelling time of the signal between the satellites and itself and transform it into a distance.

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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. Calculates the distance to a second satellite for which it is able to catch a signal

(19'000 km for example). This tells the receiver that it is not only located on the first sphere (Figure 3) but also on a second sphere centred on the second satellite and that has a radius of 19'000 km. Or in other words, that the receiver is somewhere on the circle where these two spheres intersect (Figure 4 point a).

Figure 4 - Circle indicating the potential location of the GPS with two satellite signals

(a)

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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).

Figure 5 - Potential locations of the GPS receiver with 3 satellites signals (b)

Most of the time one of these two points is absurd and is rejected by the receiver that is then able to give the exact location. In some cases, a fourth satellite is necessary to know which of the two points is the correct one. In any case, a configuration with 4 satellites is better in order to increase the accuracy and/or reduce the sources of error.

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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.

The greatest source of error is connected to the position of the satellite in the sky when taking the measurement. The spread of the satellites in the sky is called the Positional Dilution of Precision (PDOP). A good PDOP is obtained when the satellites are located at wide angles relative to each other (Figure 6 a). In the contrary, a poor PDOP result from satellites being located in a line or in a tight grouping (Figure 6 b).

Figure 6: a) Good PDOP b) Poor PDOP

The second source of error comes from the infrastructure (buildings, bridges) and particular landform (mountains) that are located around the receiver. These objects can block the reception of the signal (Figure 7), causing position errors or possibly no position reading at all.

)

Figure 7 - Example of good visibility (a)

and bad visibility (b) of satellites due to obstacles

GPS units typically will not work indoors, underwater or underground.

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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.

1 2 1 2

Figure 6 Examples of signal multipath

Figure 8 - Example of multipath GPS signal connected to buildings or mountains

This underlines the importance for the users to be located in the most open area as possible before taking the measurement.

There are other sources of error over which the user does not have control, including:

· Atmosphere delays — The satellite signal slows as it passes through the

atmosphere. The GPS system uses a model that calculates an average amount of delay to correct for this type of error.

· Receiver clock errors — A receiver's built-in clock is not as accurate as the

atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.

· Orbital errors — Also known as ephemeris errors, these are inaccuracies of the

satellite's reported location.

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