An Investigation Into The Accuracy And Performance Of A Lightweight GPS Tag Used To Track Wildlife

An Investigation into the Accuracy and

Performance of a Lightweight GPS Tag

used to Track Wildlife

Paul Duffy

BSc. (Hons) Geomatics

Department of Spatial Information Sciences

Dublin Institute of Technology

January 2010

ii

Declaration

This dissertation is submitted in part fulfilment for the BSc. (Hons) Geomatics of the

Dublin Institute of Technology. It is entirely my own independent work and has

never been submitted as part of any other work.

All secondary sources of information have been acknowledged and referenced in

accordance with the Department of Spatial Information policy on Referencing &

Citation (Behan 2009).

____________________________________

STATEMENT OF WORD LENGTH

c. 12,000 words

iii

Dedication

This work is dedicated to my parents, for their continued support and encouragement

over the last four years.

iv

Abstract

Satellite tags have been used to track wildlife for over thirty years. With the advent

of GPS technology in the mid-1990’s, GPS tags became available. There is a

significant body of research relating to the use of these tags, but comparatively little

work has been carried out on the accuracy and performance of these tags.

This study involved testing of a lightweight GPS tag in field conditions in Counties

Wicklow, Dublin & Kildare. The manufacturers claimed accuracy for the tag is

<15m. The test of the accuracy involved comparing tag locations with contiguous

Differential GPS locations under a range of differing conditions. These tests

included static tests in both open and occluded conditions, tests where the tag

remained static for a long period of time, and tests where the tag was moved to

different locations for each iteration of the duty cycle.

Results found a mean figure for 28.970m Easting and 37.112m Northing RMSE.

Overall tag performance testing resulted in successful GPS fixes being recorded

67% of the time.

v

Acknowledgements

There are many people that I wish to thank for their help during this work.

Dr. Audrey Martin of DIT, my Supervisor, for her guidance and encouragement

during the Project.

Helen Murray-O‟Connor of DIT for her help in writing the funding proposal.

Frank Prendergast, Dr. Paddy Prendergast & Avril Behan of DIT and Gemma Weir

of National Parks and Wildlife Service for their help in attempting to get funding for

the project.

Anselm Griffin and Kevin Mooney of DIT for their invaluable help with the

Statistics element.

Damian Clarke, Project Manager of the Red Kite Re-introduction Project, for his

tree-climbing skills and also for technical advice and suggestions on the scope of the

project.

Avril Behan of DIT and Elaine MacMahon MIARI of Niall D Brennan Associates

for their help with the CAD drawings.

Enda Mullen of the National Parks and Wildlife Service for advice on the project.

vi

The staff of the Wicklow Mountains National Park for allowing me to use their GPS

equipment and also for providing access to test locations for the project, and to the

General Operatives for their help installing the test posts.

Dr. Kendrew Colhoun of the RSPB who advised on the practical use of GPS tags

and on possible tests to be carried out.

The staff of Microwave Telemetry Inc. for answering my many questions with good

grace.

Maura Massana & Cécile Convert of Collecte Localisation Satellites for their help

with getting set up on the Argos system.

vii

Table of Contents

Declaration .................................................................................................................. ii

Dedication ................................................................................................................. iii

Abstract ...................................................................................................................... iv

Acknowledgements ..................................................................................................... v

Table of Contents ..................................................................................................... vii

Table of Figures ......................................................................................................... xi

Table of Tables ....................................................................................................... xiii

Table of Equations ................................................................................................. xiii

Chapter 1 – Introduction ........................................................................................... 1

1.1 General Introduction ...................................................................................... 1

1.2 Project Aims .................................................................................................. 2

Chapter 2 - Literature Review .................................................................................. 4

2.1 Introduction ........................................................................................................ 4

2.2 Main Literary Sources ........................................................................................ 5

2.3 Conclusions ........................................................................................................ 8

Chapter 3 – Materials and Methods ......................................................................... 9

3.1 Instrumentation Overview .................................................................................. 9

3.2 Microwave Telemetry PTT 100 ......................................................................... 9

3.2.1 Argos System Locations ........................................................................... 12

3.2.2 GPS Locations ........................................................................................... 14

viii

3.2.3 Location Classification ............................................................................. 14

3.2.4 Argos Account .......................................................................................... 15

3.2.5 Data Processing ......................................................................................... 16

3.2.6 System Accuracy ....................................................................................... 17

3.3 Trimble Pathfinder ProXH GPS Receiver ....................................................... 20

3.3.1 Differential GPS ........................................................................................ 21

3.3.2 System Software ....................................................................................... 22

3.3.3 Data Processing ......................................................................................... 23

3.3.4 System Accuracy ....................................................................................... 24

3.4 Methodology .................................................................................................... 24

3.4.1 Static Location Test .................................................................................. 25

3.4.2 Canopy Location Test ............................................................................... 29

3.4.3 Post Test .................................................................................................... 34

3.4.4 Moving Location Test ............................................................................... 38

3.4.5 Revisit Test ............................................................................................... 40

Chapter 4: Data Analysis ........................................................................................ 43

4.1. Assessment of Tag Performance ..................................................................... 43

4.2 Assessment of Tag Accuracy ........................................................................... 46

4.3 Static Location Test.......................................................................................... 52

4.4 Static Location Accuracy ................................................................................. 52

4.5 Canopy Location Test Performance Test ......................................................... 54

4.6 Canopy Location Accuracy .............................................................................. 57

4.7 Post Test Performance...................................................................................... 58

ix

4.8 Post Test Accuracy ........................................................................................... 58

4.9 Moving Location Performance ........................................................................ 58

4.10 Moving Location Accuracy............................................................................ 60

4.11 Revisit Test Performance ............................................................................... 60

4.12 Revisit Test Accuracy .................................................................................... 63

Chapter 5 - Conclusions .......................................................................................... 64

Chapter 6 - Bibliography ......................................................................................... 68

x

Appendices

Appendix I – Dissertation Proposal .......................................................................... 75

Appendix II – PTT-100 22g Solar Argos / GPS PTT Tag Specifications ................ 82

Appendix III –GPS PTT Tag Production Form ....................................................... 84

Appendix IV – Argos System User Agreement (SUA) ............................................ 86

Appendix V – Argos ID Number Request Form ...................................................... 95

Appendix VI – Notification of Argos Platform ID ................................................... 97

Appendix VII – PRV/A DS File Format .................................................................. 99

Appendix VIII – Sample PRV/A DS File .............................................................. 102

Appendix IX – Sample Argos Locations File ......................................................... 104

Appendix X – Sample Engineering File ................................................................. 106

Appendix XI – Sample GPS Locations File ........................................................... 108

Appendix XII – Sample Google Earth Locations File ............................................ 111

Appendix XIII – Sample Grid InQuest Conversion File ........................................ 113

Appendix XIV – Communications with Microwave Telemetry, Inc. .................... 117

Appendix XV – GPS Pathfinder ProXH Specifications ......................................... 125

Appendix XVI – Data Dictionary Specifications ................................................... 127

Appendix XVII – Sample OSi RINEX Correction File ......................................... 129

xi

Table of Figures

Figure 1 – Red Kite (Milvus milvus) ........................................................................... 2

Figure 2 – PTT-100 Tag with €1 coin for scale ........................................................ 10

Figure 3 – PTT-100 Layout ...................................................................................... 10

Figure 4 – Argos Location calculations utilising Doppler Effect ............................. 13

Figure 5 – Trimble Pathfinder Pro Series GPS Receiver .......................................... 20

Figure 6 – Location for Static Test............................................................................ 26

Figure 7–Site of Static Location Test ........................................................................ 27

Figure 8 –Installation of Fence-post for Static Location Test ................................... 28

Figure 9 –Tag Installed on post for Static Location Test .......................................... 29

Figure 10 – Location of Canopy Test........................................................................ 30

Figure 11 –Site of Canopy Location Test with test tree ........................................... 31

Figure 12 –Site of Canopy Location Test with Nest ................................................. 32

Figure 13 –Damian Clarke climbing the Test Location Tree.................................... 33

Figure 14 –Tag in situ in Test Tree ........................................................................... 34

Figure 15 – Location of Post Test ............................................................................. 35

Figure 16 – Detail of Post Test Location .................................................................. 37

Figure 17 –Post Test Location looking south-west ................................................... 38

Figure 18 – Locations of Moving Location Test Fixes ............................................. 39

Figure 19 – Moving Test Location Set-up in the Wicklow Mountains .................... 40

Figure 20 – Location of Revisit Test ......................................................................... 42

Figure 21 – Χ2 distribution ........................................................................................ 48

xii

Figure 22 – Two-tailed Χ2 test .................................................................................. 49

Figure 23 – Static Location Test Results .................................................................. 53

Figure 24 – Breakdown of Static Location Positions ................................................ 54

Figure 25 – Canopy Test Location Results ............................................................... 55

Figure 26 – Tag Performance – Canopy Test ........................................................... 56

Figure 27 – Time of GPS Fixes – Canopy Test ........................................................ 57

Figure 28 – Tag Performance – Moving Location Tests .......................................... 59

Figure 29 – Tag Performance – Revisit Test............................................................. 61

Figure 30 – Revisit Test Location Results ................................................................ 62

xiii

Table of Tables

Table 1 – Tag Data Transmission Duty Cycles ......................................................... 11

Table 2 – Argos Location Classes (CLS 2008) ......................................................... 15

Table 3 – Length of a Degree of Latitude at Manufacturers and Project Locations . 19

Table 4 – Extract of Troubleshooting for Tag ........................................................... 44

Table 5 – Possible outcomes of Test ......................................................................... 45

Table 6 – Canopy Location Hypothesis Test ............................................................ 57

Table 7 – Moving Location Hypothesis Test ............................................................ 60

Table 8 – Revisit Location Hypothesis Test.............................................................. 63

Table of Equations

Equation 1 - Χ2

Test .................................................................................................. 50

Equation 2 – Degrees of Freedom ............................................................................ 50

Equation 3 – Root Mean Square Error ..................................................................... 51

1

Chapter 1 – Introduction

1.1 General Introduction

Satellite tags have been used in the tracking of wildlife since the 1970‟s with the first

Global Positioning System (GPS) based tags introduced in 1994 (Mech & Barber

2002). The development of GPS tags has been described by Mech & Barber (2002,

p. 23) as “the latest major development in wildlife tracking”.

However, while a large volume of work has been carried out utilising the tags, there

is a relative dearth of knowledge as to the accuracy of these systems, especially in

the Irish context. Estimates of the accuracy of the tags are provided by the

manufacturers.

In the Irish context, the tags have been attached to some of the birds that have been

re-introduced into Ireland under the auspices of the Golden Eagle Trust (GET),

namely the Golden Eagle (Aquila chrysaetos) White-tailed Sea Eagle (Haliaeetus

albicilla) and the Red Kite (Milvus Milvus) (see Figure 1). The current expense of

GPS tags limits their use for monitoring of large populations of birds of prey

(Hardey et al. 2006).

To date, 9 tags have been attached to the various different species re-introduced by

the GET (GET 2010). They are valuable to the Trust in terms of tracking specific

birds and in increasing public awareness of the trusts work. One of the more sombre

uses of the tags in recent years has been their use in locating the carcasses of dead

birds in order to ascertain their cause of death (Irish Times 2009).

2

Figure 1 – Red Kite (Milvus milvus) (Source - Tony Cross, Welsh Kite Trust)

1.2 Project Aims

The aim of this project was to investigate the accuracy and performance of a PTT-

100 22g Solar Argos / GPS tag in field conditions, under two sets of conditions,

optimal and sub-optimal.

The optimal test was carried out to test the accuracy of the system in field conditions

with good GPS visibility. First the precise co-ordinates of a point in open land was

accurately surveyed using high accuracy GPS. This point was marked with a post

and the GPS tag fixed to this point for approximately 30 days. The locations

calculated for the tag by the service provider were then compared with its known

location.

The sub-optimal test was carried out in a forest situation. Shielding of GPS signals

by trees results in less satellites being available for the calculation of a position, and

3

thus, a less accurate fix (Lekkerkerk 2007). The tag was then installed in the canopy

of a tree in order to test the performance of the tag under the tree canopy.

Statistical analysis of the results was carried out and an assessment of the

effectiveness of the tag in both optimal and sub-optimal conditions will be made.

Data on the performance of the tag in terms of success rate in obtaining GPS fixes

was also be collected and analysed during the course of the project.

4

Chapter 2 - Literature Review

2.1 Introduction

The purpose of this section is to review past scientific papers that are relevant to the

research topic, in this case the use of GPS tags in tracking wildlife. As has been

stated in the introduction, electronic tagging of wildlife has been carried out for the

last thirty years. The majority of this work has been carried out on the Continental

United States.

Although there is a wealth of scientific papers relating to the use of the tags, very

few concentrate on the performance of the tags. This is understandable in that the

primary focus of these tags is to investigate some aspect of the biology or

movements of the species in question. The literature that deals with the tags can be

split into three main categories:

Literary sources that are solely based on biological concerns rather than the

performance of the tag,

Literary sources that focus solely on the Argos system (a method of

calculating position based on the Doppler effect),

and

Literary sources whose primary focus is on the operation and performance of

the tag.

5

2.2 Main Literary Sources

There are numerous literary sources relating to biological studies. Satellite tags have

been used to tag a myriad of species of animals and birds spanning all continents and

species. These range from studies of the movements marine species such as

Loggerhead Turtle (Caretta caretta) where the movements of the species across the

breadth of the Pacific Ocean were correlated with remotely sensed environmental

data (Kobayashi et al. 2008), to the study of the migration patterns of Siberian

Cranes (Grus leucogeranus) between China and Siberia (Kanai et al. 2002). The tags

have also been used on small scale studies of mammal movements such as those of

Roe Deer (Capreolus capreolus) over home ranges of few hectares in France

(Richard et al. 2008), to large scale studies such as those of Snow Leopard (Uncia

uncia) with a home range of 4500km2 in Mongolia (McCarthy, Fuller & Munkhtsog

2005).

Literature in the second category also has some degree of overlap with the first. As it

is a cheaper system to employ, tags based on the Argos system are more commonly

used than GPS tags. The Argos system is a method of position calculation based on a

phenomenon known as the Doppler Effect. The Argos-enabled platform transmits

signals to a satellite and a method of calculation that relies on comparison of the

frequency of the transmitted signals is used to estimate a location for the platform. It

results in locations that fall into various accuracy classes, depending on the number

of signals received. A higher number of signals, results in a more accurate location

Due to accuracy considerations of the system, they are generally used on large scale

studies, especially for tracking the migrations of bird species over long distances.

These include studies on whether raptor species use geographical or geomagnetic

courses while migrating from North to South America (Thorup et al. 2006), analysis

of spatial and temporal patterns of Osprey (Pandion haliaetus) migrating between

Sweden and West Africa (Alerstam, Hake & Kjellén 2006).

6

In Europe, significant research was undertaken by Soutollo et al. (2009).This study

examined the accuracy of various Argos locations in comparison with their true

positions. They found that less than 10% of the positions recorded were in the

highest accuracy class and that the Argos system failed to provide any location in

45% of cases.

Those literary sources that fall into the third category, ones that focus on the

operation and performance of the tags are the ones with the most relevance to this

study. However, these are the fewest in number. There are several key books which

deal with the subject, notably those by Kenward (2001), Braun (2005) and Hardey et

al. (2006).

Kenward (2001) offers an overview of all of the systems available for tracking

wildlife, from radio transmitters to GPS tags. In it he details the pros and cons of the

use of each of the systems, and offers advice on choosing the appropriate system

depending on the type of study.

Braun (2005) has a section entitled “Wildlife Radiotelemetry” which also offers a

comprehensive review of current systems including radio-tracking, Argos tags and

GPS tags and provides information on errors within each system. There is also a

section entitled “Application of Spatial Technologies in Wildlife Biology” which

details with uses and processing of data acquired by such systems.

The seminal work on Raptor surveying in the British Isles is that by Hardey et al.

(2006). In it the authors provide some information on the available systems including

GPS tags, but ultimately conclude that:

“The current expense of satellite tracking means that it is most likely to be used in

studies of the long-distance movements of relatively small numbers of individuals,

rather than the survey and monitoring of larger samples of birds of prey.” (Hardey et

al. 2006, p. 24)

7

The majority of papers that focus on the accuracy of the system deal with cases in

North America. They also deal with GPS collars that store data on the tag which can

be post-processed for greater accuracy.

Graves and Wallers‟ (2006) tested the accuracy of GPS tags by placing the tags in

stationary locations and comparing the resultant tag fixes to those of differentially

corrected GPS locations. They found that differentially corrected tag locations had a

95% circular error probability (CEP) of 22.4m. CEP is described by Van Sickle

(2008, p. 304) as “A description of two-dimensional precision”.

In other words, 95% of the GPS fixes would be located within a circle of radius

22.4m. The paper also examined the performance of the tag when placed on a

Grizzly Bear (Ursus arctos horribilis) with a neck-collar. The authors found that

there was fix success rate of 72%. However, they also found that fix success rate was

dependant on the size of the animal in that in smaller animals the collar could move

around on the neck, thus leading to lost fixes. This shows that there are unforeseen

complications involved in attaching a sophisticated GPS tag to a living animal whose

behaviour cannot be predicted.

A second primary source examined in this study was that of Hansen & Riggs‟ (2008)

who studied the operation of GPS collars by positioning them on locations in the

field. The tag locations were post-processed and compared to differentially corrected

GPS locations. They found that the precision of the collar locations was 3.6m.

Observation rates examined by the study found a success rate of 96%. These figures

suggest a system that is very accurate and has a high fix success rate.

Furthermore, Cain et al. (2005) field tested GPS collars in a mountainous area of

Arizona. They found that the fix interval rate had a direct influence on the success

rate of GPS tags. They also found that topography had a major bearing of location

error.

8

Buerkert & Schlecht (2009) found that the number of GPS fixes was significantly

reduced in obstructed, mountainous terrain, whereas there were negligible effects

found when the tag was in open terrain.

Coelho (2007) studied whether the rate of GPS fixes was dependant on the time of

day and found that there were significantly more fixes collected during the night

when the animals were out foraging, while the number of fixes during the day was

significantly reduced due to the animals sleeping in dense vegetation during the day.

It also found a similar correlation in terms of accuracy of fixes recorded by day and

during the night.

2.3 Conclusions

These studies have shown the variability of results that are achieved using GPS

collars. These collars are considerably larger than the PTT-100 system used in this

study (up to 950g), and have on board differential correction and processing systems.

Despite this, it is not unreasonable to infer that the same issues that have been shown

to affect the performance and accuracy of GPS collars namely fix interval, species

behaviour, canopy cover and topography would also affect the PTT-100 system used

in this study.

9

Chapter 3 – Materials and Methods

3.1 Instrumentation Overview

There were two different types of equipment used in this study. Although both

systems utilise GPS technology, they vary greatly both in terms of their physical size

and operating characteristics. The equipment used was as follows:

PTT 100 22g Solar Argos / GPS PTT Tag,

Trimble Pathfinder ProXH GPS Receiver

The data collected by the systems was processed using the following software

systems:

MTI GPS Data Parser,

Grid InQuest,

GPS Pathfinder Office

3.2 Microwave Telemetry PTT 100

The tag used in this study was the PTT-100 22g Solar Argos / GPS PTT Tag

produced by Microwave Telemetry Inc. of United States of America (see Figure 2).

The acronym PTT stands for Platform Transmitter Terminal which is the generic

name given to the specialized transmitters that are tracked using the Argos system

(Kenward 2001).

10

Figure 2 – PTT-100 Tag with €1 coin for scale

The tag consists of a small, hermetically sealed unit 62mm in length, 22mm wide

with a height of 21mm. The unit contains a small battery pack which is charged by

means of a solar array on the housing of the unit. The sixteen channel GPS antenna

is located on the front of the unit. The Argos antenna protrudes from the rear of the

unit. See Figure 3 for a schematic of the PTT-100 layout and Appendix II for full

specifications.

Figure 3 – PTT-100 Layout (Microwave Telemetry 2009a)

11

Each tag is produced to order by the manufacturer. One of the major considerations

is the duty cycle which is to be used. The duty cycle is “a schedule of transmission

times and rest periods used to extend the study interval by budgeting the PTT‟s

battery life.” (Microwave Telemetry 2009a, p. 28)

The duty cycle is a major consideration when the tag is not solar-powered, as the

battery has finite lifespan. For the purposes of this study the maximum amount of

GPS hits was sought, so the duty cycle was programmed accordingly. The user must

also specify the transmission cycle for the tag. This is the interval (in days) at which

the tag transmits its stored information to the Argos satellite. Table 1 shows the

particular specifications for the data transmission for the tag.

Table 1 – Tag Data Transmission Duty Cycles

GPS Receiver Fix Hour (local time) TX Duty Cycle (transmit every “x” days)

08:00, 10:00, 12:00, 14:00, 16:00, 18:00 3

The tag allows for two methods of location calculation, positions calculated using

the Argos system and GPS. The characteristics of the two systems are as follows:

“Argos location: Argos centers [sic] calculate a transmitter‟s location using the

Doppler Effect on transmission frequency.

GPS Positioning: On request from the user, a specific processing module

extracts the GPS positions included in the messages, validates them and

distributes them in the same format as the Argos locations.” (CLS 2008, p. 8)

In both methods, the coordinates are output in latitude and longitude, using the

World Geodetic System 1984 (WGS84) reference system (CLS 2008).

12

3.2.1 Argos System Locations

The Argos system is “a global satellite-based location and data collection system

dedicated to studying and protecting our planet's environment.”(CLS 2008, p. 1)

It is a Franco – American collaboration involving the following organisations:

Centre National d‟Etudes Spatiales (CNES) - The French Space Agency

National Oceanic and Atmospheric Administration (NOAA)

European Organisation for the Exploitation of Meteorological Satellites

(EUMETSAT)

Collecte Localisation Satellites (CLS) – the Operator of the System

The system has three main components, a satellite-based instrument, a ground

processing component and an Argos-compatible platform (in this case the PTT-100).

The satellite sensor is part of the payloads on the Polar Orbiting Environmental

Satellite (POES) operated by NOAA and also on the MetOp satellite operated by

EUMETSAT. These satellites are in polar orbits 850km above the earth and these

orbits take them within the visibility of any given transmitter at approximately the

same local time each day (CLS 2008). The Argos messages are received by the

satellite and stored onboard for retransmitting to one of the three main ground

receiving stations and also to regional reception stations within the satellites field of

view.

The results are then transmitted to one of two processing stations operated by CLS.

These processing stations calculate locations, process the received data, and forward

it to system users.

13

As has been stated, the Argos system has two independent means of location

calculation. The first of these is what is known as an Argos location. These are

calculated utilising a phenomenon known as the Doppler Effect (see Figure 4). The

Doppler Effect can be defined as “the change in frequency of a sound wave or

electromagnetic wave that occurs when the source of vibration and observer are

moving relative to each other.” (CLS 2008, p. 8)

In the case of Argos locations, when the satellite approaches an Argos-compatible

platform, the frequency of the transmitted signal is lower or higher depending on

whether the platform is moving towards, or away from, the Satellite (see Figure 4).

Figure 4 – Argos Location calculations utilising Doppler Effect (CLS 2008)

The location is calculated using messages transmitted to the satellite by the receiver.

At least four messages are needed to give an accuracy estimate for the position.

Comparison of the first and last messages collected results in two possible locations

14

for the tag. A least-squares analysis is carried out on these two positions, and the

location with the minimal residual error is chosen (CLS 2008).

3.2.2 GPS Locations

The second method of location calculation is GPS. This is more straightforward,

with locations recorded by the GPS antenna in the tag, and then transmitted along

with the Argos messages. In effect, the Argos system acts purely as a carrier for the

GPS positions that have been independently calculated by the tag.

CLS (2008, p. 11) state that

“The Argos system can be used to transmit GPS positions. The advantages are:

Positions are more accurate and do not depend on transmitter quality,

Positions can be collected more regularly over the day.”

The user must declare to CLS that GPS signals are included in the Argos messages

in order to allow the GPS positions to be decoded from the message stream.

3.2.3 Location Classification

CLS provide locations which are classified according to the following criteria:

“type of location (Argos or GPS),

estimated error,

number of messages received during the pass” (CLS 2008, p. 11)

15

Table 2 shows the various location classes and corresponding estimated error values.

Table 2 – Argos Location Classes (CLS 2008)

Class Type Estimated Error Number of messages

received per satellite pass

G GPS <100m 1 message or more

3 Argos <150m 4 messages or more

2 Argos 150m << 350m 4 messages or more

1 Argos 350m << 1000m 4 messages or more

0 Argos >1000m 4 messages or more

A Argos No accuracy estimation 3 messages

B Argos No accuracy estimation 2 messages

Z Argos Invalid Location -

3.2.4 Argos Account

The user can access data in a number of different ways. All of these require a unique

Argos ID number. The user obtains an ID number by entering into a System User

Agreement (SUA) with Argos (see Appendix IV). The completed SUA form is

assessed by the Argos Operations Committee to “check that your program is

compatible with the environmental mission of the Argos system.” (CLS 2009)

The user also submits an ID Number Request Form (see Appendix V). This form

must also specify the type of tag to be used and any tag-specific information (i.e.

16

whether the tag will transmit GPS positions) which may be required to process the

data. Once approval had been granted the user receives notification of the Platform

ID which has been created for the project (see Appendix VI). This information must

then be sent to the tag manufacturer that the correct configuration is incorporated in

the tag. Data from the tag can then be accessed by the user.

3.2.5 Data Processing

Argos provide data to the user in a number of ways including text message, fax,

monthly direct mailing of data on CD-ROM and their recommended method, via

download from their website.

When the user logs on to the website, data is available on the platform for that day,

and the previous nine days. It is important that the user downloads data in the correct

format, in this case a format called PRV/A DS. Each message consists of a string of

32 numbers, each relating to a different parameter of the message (see Appendix

VII). This data contains “locations and sensor data + transmitter diagnostic

information.” (CLS 2008, p. 34)

This data is output in a text file (see Appendix VIII), and requires processing in order

to extract the GPS data. This is done by parsing the data using a program provided

by the manufacturers of the tag called MTI GPS Data Parser (Microwave Telemetry

2009c). This software organises the raw data in a user-friendly format.” (Microwave

Telemetry 2009a)

Various options can be selected when parsing the data, including showing failed

attempts to get a GPS fix. These are useful options for analysis of the data.

Once the data has been processed through the software, the output consists of four

separate files. These are:

17

1. Argos Locations (in a *a.txt file) (See Appendix IX)

2. Engineering Data (in a *e.txt file) (See Appendix X)

3. GPS Locations (in a *g.txt file) (See Appendix XI)

4. Google Earth Locations (in a *.kml file) (See Appendix XII)

The Argos locations are calculated using the method described in Section 3.2.2. The

Engineering file is composed of data about the performance of the tag including

“data about temperature, battery voltage, satellite use and internal clock readings.”

(Microwave Telemetry 2009a, p. 28) This engineering data is sent in every eighth

message transmitted by the tag to the satellite.

The GPS locations are output in Latitude and Longitude. The Google Earth files

show both the Argos calculated and GPS locations, and also the paths between fixes.

As the GPS locations recorded by the tag for this study were compared with readings

from both Autonomous and Differential GPS, one further processing stage was

required. This involved the conversion of the GPS text files from Latitude /

Longitude format to Irish Transverse Mercator (ITM). This conversion was carried

out using a program called Grid InQuest (QGS 2004b). This program is available as

freeware from the Ordnance Survey Ireland (OSi). Batch conversion was used to

convert the GPS files into ITM. This conversion results in a text file containing the

co-ordinates in ITM format (see Appendix XIII)

3.2.6 System Accuracy

As has been stated above, the tag allows for two methods of location calculation,

positions calculated using the Argos system and GPS. For the purposes of this study,

18

only the GPS locations were of interest. There are varying figures given for the

accuracy of the GPS positions, even within the literature of the manufacturer.

The company website states that the PTT-100 has GPS accuracy ± 15m (Microwave

Telemetry 2009b). However, the Field Manual supplied with the tag cites the “GPS

accuracy with “Selective Availability” switched off: Lat / Long ± 18m.” (Microwave

Telemetry 2009a)

Selective Availability (SA) was a method of degrading the accuracy of GPS signals

employed by the United States Department of Defence. As it was finally turned off

on May 2, 2000 (Van Sickle 2008) it would appear that this accuracy figure predates

the year 2000.

Personal communications with the company (see Appendix XIV) in October 2009

state resulted in a stated accuracy of approximately +/-15m in Latitude and

Longitude. (Microwave Telemetry 2009d) and also state that positions are recorded

by the tag to the nearest decimal minute, which equates to 1/6000 of a degree

(Microwave Telemetry 2009d). They go on to state that:

“At our latitude, this corresponds to 18.5 m latitude and 14.6 m longitude--world wide

average is about 15 m.” (Microwave Telemetry 2009d)

The approximate latitude of the manufacturer (at Columbia, Maryland, USA) is 39

N, while the latitude of the project area (in Co. Wicklow, Eire) is 53 N. Through the

use of an online calculator (NGA 2010), the length of a degree of Latitude was

calculated (see Table 3).

19

Table 3 – Length of a Degree of Latitude at Manufacturers and Project Locations

Location Latitude Length of Degree

of Latitude (m)

1/6000th

of a Degree of

Latitude (to nearest

decimal minute)

Microwave

Telemetry,

Columbia,

Maryland, USA.

39.23085 N 111020 18.5033m

Kilafin, Wicklow,

Eire

53.01367 N 111286 18.5477m

As can be seen from Table 3, the difference between the figures at the relevant

Latitudes is negligible in terms of the stated accuracy of the system. Therefore, for

the purposes of this project, it was assumed that the accuracy of the system was in

line with the manufacturers claims in 2009 (i.e. ±15m) (Microwave Telemetry

2009d).

20

3.3 Trimble Pathfinder ProXH GPS Receiver

In order to test the accuracy of the PTT-100 in the field, a system with a higher

accuracy was required for comparison. The system that was chosen for this purpose

was the Trimble Pathfinder ProXH GPS Receiver (see Figure 5).

Figure 5 – Trimble Pathfinder Pro Series GPS Receiver (Trimble 2005a, p. 5)

The system contains an integrated GPS receiver and antenna. Data is recorded using

an accompanying data-logger and can later be post-processed for greater accuracy.

Full specifications for the system are given in Appendix XV.

The unit operates by recording GPS measurements using what is known as Code

Phase measurement. This can be defined as:

“GPS measurements based on the pseudo random code (C/A or P) as opposed to the

carrier of that code.” (Lekkerkerk 2007, p. 187)

21

The measurements are based on the Coarse Acquisition (C/A) code. The unit also

utilises high performance processing software to improve the quality of GPS

positions, mainly by utilising modelling to reduce some of the errors caused by

effects caused by ionosphere and troposphere conditions.

3.3.1 Differential GPS

Differential GPS (DGPS) is a method that can be used to increase the accuracy of

positions recorded by the receiver. It can be defined as:

“The simultaneous reception of satellite signals by two receivers, one of which is in a

known position. The position of the other receiver can then be calculated to high

accuracy.” (Johnson 2004, p. 183)

All GPS signals are subject to errors of varying types. DGPS functions as:

“A process for cancelling out man-made and natural errors in the GPS signal.”

(Lekkerkerk 2007, p. 189)

Errors which most affect the quality of GPS signals include, amongst others:

“..errors in the positions of the satellites, errors in the satellite clocks and the effects of

the earth‟s atmosphere on the speed at which the satellite signals travel.” (Johnson

2004, p. 63)

DGPS works by calculating GPS positions as normal at a Base Station for which the

co-ordinates are exactly known. By knowing its exact position, it can measure

accurately the errors in the position, and either applies corrections in real-time, or

record them in a corrections file for later post-processing.

22

The system can apply real-time corrections using Satellite Based Augmentation

System (SBAS) either from the American Wide Area Augmentation System

(WAAS) or the European Geostationary Navigation Overlay Service (EGNOS)

depending on which is visible.

Post-processing of the data involves recording positions in the field as normal, and

the subsequent use of a corrections file from a Base Station to correct the positions.

In this case, the Base Stations in question are part of the Ordnance Survey Ireland

(OSi) Active GPS Network. Correction Data from this network can be downloaded

free from their website (OSi 2010).

3.3.2 System Software

The system software can be categorised into two types, the software which runs on

the data-logger and the software that runs on a PC for download and processing of

the data. The data-logger operates a software package called Terrasync (Trimble

2009a). This controls both the collection settings of the GPS receiver and acts as an

interface for data entry. The PC software is called GPS Pathfinder Office (Trimble

2008a). This software is used to download and process data collected in the field. It

also allows for the creation of project-specific data dictionaries. A Data Dictionary

can be defined as:

“..a description of the features and attributes relevant to a particular project or job.”

(Trimble 2008b, p. 73)

In the case of this project, a data dictionary was created to record the following

details (see Appendix XVI):

23

Location Number (a unique number for each location)

Location Date

Location Time

Autonomous GPS Fix (whether an Autonomous GPS fix had been recorded

for the position)

Differential GPS Fix (whether a Differential GPS fix had been recorded for

the position)

Location Comment

The data dictionary editor also allows the user to specify a logging interval for the

feature in question. The logging interval is the interval between each GPS reading

that is recorded in the fieldwork file. For the purposes of this project, a logging

interval of three seconds between readings was selected.

3.3.3 Data Processing

To obtain the most accurate results from the system, the data was post-processed.

The first stage in the post-processing was to download the correction files from the

OSi website. These files are in Receiver Independent Exchange Format (RINEX).

RINEX was developed by the Astronomical Institute which...

“..allows different receivers and post-processing software to work together.” (Van

Sickle 2008, p. 109)

Co-ordinates for the survey area were entered into the OSi website, in order that data

from the closest of the sixteen operating stations in the OSi Active GPS Network

could be accessed. Data from a minimum of the three closest stations was then

24

downloaded. The processing was then carried out within the GPS Pathfinder Office.

The field data file (with an .ssf file extension) was corrected using the OSi Base

Station correction files. This resulted in the output of a corrected set of co-ordinates

for the recorded positions (with a .cor file extension). There is also a text file

outputted which gives details of the correction process (see Appendix XVII).

Amongst the summary data are estimated accuracy figures for the corrected co-

ordinates. This was vital in assessing the accuracy of the differential correction

process.

3.3.4 System Accuracy

The accuracy of GPS positions depends on the type of system used, and the

processing which is applied to it. As has been stated above, in order to achieve a high

degree of accuracy, the system which was used in this case was post-processed

Differential GPS. The manufacturers of the system claim it delivers real-time sub-

meter accuracy and provides post-processed sub-foot accuracies. (Trimble 2005a)

The GPS Pathfinder Office outputs a horizontal precision figure as part of its post-

processing routine.

3.4 Methodology

When the project was in its initial planning stages two distinct tests were identified.

These were planned with the overall objectives of the study in mind (see Section 1.2

for details). The ultimate aim was to achieve an outcome that was as scientifically

robust as possible. Due to idiosyncrasies with the performance of the tag, the

methodology evolved over the course of the study.

25

3.4.1 Static Location Test

For the initial test the tag was installed on a static location. This phase of the project

was designed to test the performance of the tag in near-ideal conditions. This in

effect, was the control for the experiment. Thus there were three main considerations

when selecting the site for the Static Location Test.

Firstly to maximize the tags‟ performance in terms of the successful collection of

GPS locations, the location had to have a clear view of the sky to reduce the effects

of multipath and shielding. Multipath is a caused by the reflection of the GPS radio

signal off reflective surfaces such as vehicles, metal buildings etc. Thus

“When the signal reaches the receiver by two or more different paths, the reflected

paths are longer and cause…subsequent positioning errors.” (Van Sickle 2008, p. 315)

Shielding is a simpler concept. It is basically caused when the GPS receiver cannot

“see” some or all of the GPS satellites due to physical obstructions. Lekkerkerk

(2007, p. 11) states that “even a small object blocking the path of the satellite signal

will prevent it from reaching the antenna.”

The second consideration was to maximize the tag performance in terms of

maintaining a charge on the battery via the solar panel. The conditions for this were

similar to those for multipath and shielding, i.e. clear view of the sky.

The final consideration was security. As the tag is an expensive item, a secure

location was required in order to prevent theft. The location that was chosen was a

field owned by the National Parks and Wildlife Service located in Trooperstown,

Laragh Co. Wicklow (see Figures 6 and 7 for details).

26

Figure 6 – Location for Static Test

27

Figure 7–Site of Static Location Test

Once the location was chosen, a 3m post was installed on the 10th

of July 2009 with

the help of NPWS staff (see Figure 8). The top of the post was located 2.5m above

ground level. The ground position of the post was recorded for later comparison with

the tag locations.

28

Figure 8 –Installation of Fence-post for Static Location Test

The tag was mounted on the top of the post for thirty days (see Figure 9). The post

was secured with cable-ties on the mounting loops with care taken to keep both the

GPS antenna and solar panel clear of obstruction.

Data was collected for this site from 1400 hours on the 10th

of July 2009 to 1000

hours on the 10th

of August 2009. The data was downloaded every ten days and

locations processed using the methodology described in Section 3.2.6 above.

29

Figure 9 –Tag Installed on post for Static Location Test

3.4.2 Canopy Location Test

The second test was to determine if the tag would be of use in locating the nests of

wild birds. This involved testing how the tag performed under the tree canopy. This

tested how the tag reacted under conditions where shielding was a factor. As has

been stated above, shielding refers to the physical blocking of the GPS signal. The

problem this causes can be described as follows:

Kenward (2001, p. 70) suggests that “In experiments with GPS tags, canopy closure

reduced location rates but not accuracy, whereas increase in tree density reduced

both location rate and accuracy.”

The location which was chosen was in Killeagh, Shelton Woods, Arklow, Co.

Wicklow (see Figure 10).

30

Figure 10 – Location of Canopy Test

31

The test location was in a Scots Pine (Pinus sylvestris) tree within relatively open

part of the wood (See Figure 11). The tree was 19.5m tall. The test location itself

was a disused Buzzard (Buteo buteo) nest located 14m from the base of the tree (see

Figure 12). These heights were measured with a Blume-Leiss Hypsometer, an

instrument used in forestry to measure tree height.

Figure 11 –Site of Canopy Location Test with test tree (Arrowed)

32

Figure 12 –Site of Canopy Location Test with Nest (Arrowed)

To install the tag in the nest the tree was climbed by Damian Clarke, Project

Manager of the Red Kite Re-introduction Project and a qualified tree-climber (see

Figure 13). This tag was installed on the 10th

of August 2009.

33

Figure 13 –Damian Clarke climbing the Test Location Tree

The tag was attached to a block of wood to raise it slightly from the base of the nest.

This was done to mimic where the tag would sit were it to be attached to an

incubating female bird of prey. The tag was secured around the trunk of the tree with

straps to prevent it from falling (see Figure 14). A secondary reason for this was to

keep the tag in the right orientation, i.e. with the solar panel and GPS antenna facing

towards the sky.


of August 2009 to 1000

hours on the 9th

of September 2009. The data was downloaded every ten days and

locations processed using the methodology described in Section 3.2.6. A GPS

reading was also taken at the base of the tree for comparison with the tag result.

34

Figure 14 –Tag in situ in Test Tree (Source – D. Clarke)

3.4.3 Post Test

The third test, the Post Test was designed to overcome problems found with the

Static Location Test. The test required similar ground conditions to those for the

Static Test, i.e. a secure location with a relatively unobstructed view of the sky.

The location that was chosen for this test was on lands in Oldcourt, Co. Wicklow

(see Figure 15).

35

Figure 15 – Location of Post Test

36

The test involved the installation of seven collinear fence posts 5m apart from each

other (See Figures 16 and 17). The posts were installed using a tape measure, and the

positions later recorded by GPS.

This test was used to check the relative accuracy of the system. Relative accuracy is

defined by IIS (2008, p. vi) as the “Proximity of the relative recorded positions of

features to one another to their true values (internal consistency).”

This test was carried out by leaving the tag on the first post for one duty cycle, then

moving it to a second post for the next duty cycle. The test continued by moving the

tag from the first post to each of the remaining posts in order.

37

Figure 16 – Detail of Post Test Location

38

Figure 17 –Post Test Location looking south-west

3.4.4 Moving Location Test

A moving location test was also designed to test the accuracy of the tag which more

closely mimicked the conditions that the tag would operate in were it to be attached

to a live animal.

The test involved moving the tag constantly between different locations. These

locations were all located within Counties Dublin, Wicklow and Kildare. The only

common factor of all of these locations was that they have good GPS conditions, i.e.

not close to buildings or under tree canopy etc. Figure 18 below show those

locations.

39

Figure 18 – Locations of Moving Location Test Fixes

40

The tests at these locations were carried out as follows. Arrival at the test location

was to be a least 15 minutes prior to the programmed time for the GPS fix. The tag

was placed on a tripod for 30 minutes (see Figure 19) to allow the tag to turn on, get

a GPS fix, and if it was the correct day for it, transmit the collected data to the Argos

satellite. After this had occurred both an Autonomous and a Differential GPS

position were recorded for comparison purposes.

Figure 19 – Moving Test Location Set-up in the Wicklow Mountains

3.4.5 Revisit Test

The final test carried out was a revisit test. This test was deemed necessary in order

to provide more statistical data for the analysis of how the tag operated. The test

required similar ground conditions to those for the Static Test, i.e. a secure location

with a relatively unobstructed view of the sky.

The location that was chosen for this test was also that used for the Post test location,

in Oldcourt, Co. Wicklow (see Figure 20).

41

The test required the tag to be placed on the same location on every second time of

the duty cycle. For the intervening stage of the duty cycle the tag was moved to a

completely separate location. The intention here was to analyse the accuracy by

seeing how accurate the tag was when moved from a known location and then

returned to it. The location of the post that the tag was returned to was recorded by

GPS for comparison.

42

Figure 20 – Location of Revisit Test

43

Chapter 4: Data Analysis

The aim of this study was to investigate the accuracy and performance of the PTT-

100 22g Solar Argos / GPS PTT Tag. As has been seen in Section 3, the methods

used to test the accuracy evolved upon analysis of the initial results.

The analysis of the data will be presented for each test in two distinct sections,

Tag Performance

Tag Accuracy

4.1. Assessment of Tag Performance

Tag performance was assessed by analysing how many successful tag fixes were

recorded as opposed to how many were expected during the time period of the test.

During a full day, the duty cycle of the tag would attempt to get a GPS fix on 6

occasions, at 0800, 1000, 1200, 1400, 1600, 1800 hours.

In the troubleshooting section of the Field Manual for the tag (Microwave Telemetry

2009a), problems regarding the operation of the tag are explained (see Table 4).

44

Table 4 – Extract of Troubleshooting for Tag

Problem Explanation Action

GPS receiver did not get

a GPS fix for an hour

when it should have

The receiver is

turned off because

the battery voltage is

too low.

No Action required, GPS

fixes will resume when

there is sufficient power.

The GPS antenna on

the PTT is obscured

from the sky.

If this occurs during

testing, move the PTT to

an open area, otherwise

wait for the bird to move.

Source: (Microwave Telemetry 2009a, p. 22)

When contacted regarding the performance of the tag, the manufacturers recommend

selecting one of the options on the MTI GPS Parsing Software, to „Output Converted

Values‟ when processing the data. (Microwave Telemetry 2009e) This option gives

one of three reasons why a GPS fix was not recorded.

1. No Fix. i.e. “the PTT's receiver stayed on the full 2 minutes but could not lock

up a fix. This indicates that the satellites were not visible to the PTT.”

(Microwave Telemetry 2009e)

2. Battery drain which is indicated by the engineering data by the message “Batt

Drain”. This is described by the manufacturer as being…

45

“..recorded in the data when the PTT's battery voltage was high enough that it was

able to activate the GPS receiver and try to take a fix, but the voltage dropped below

the threshold necessary for fix acquisition before it could lock up a fix.” (Microwave

Telemetry 2009e)

As the tag is solar-powered, it would not be unexpected for this reason to

prevent a GPS fix from being taken.

3. Low Voltage as indicated by the message “Low Volt”. The manufacturer states

the Low Voltage message indicates that…

“...at that hour the battery voltage was below the threshold necessary for fix

acquisition. (The PTT does not attempt a fix if the battery voltage is below the

required threshold.).” (Microwave Telemetry 2009e)

During the course of this project, there were numerous occasions when there was no

GPS fix recorded at despite none of the three previously listed problems affecting the

tag. This resulted in the author assigning a further category, “Unknown”. Thus an

attempted tag fix could result in one of five possible outcomes (see Table 5):

Table 5 – Possible outcomes of Test

1 GPS Fix Successful GPS Position recorded

2 Low Voltage Failure to record GPS Position

3 Battery Drain Failure to record GPS Position

4 No Fix Failure to record GPS Position

5 Unknown Failure to record GPS Position

46

These are the criteria which were used to assess the performance of the tag in each of

the different tests.

4.2 Assessment of Tag Accuracy

The assessment of the GPS locations returned by the tag is the major part of this

investigation. All of the tests that were carried out on the tag involved at least two

elements, a tag GPS location (i.e. the test position) and a corresponding differential

GPS location (the assumed true position).

The first action carried out was to remove outliers. Outliers are defined as

“...observations that lie unusually far from the bulk of the data” (Montgomery,

Runger & Hubele 1998, p. 35). For any set of data, the presence of outliers can

negatively influence the quality of the data set. There are many criteria which can be

used for the removal of outliers. Ghilani & Wolf (2006, p. 44) state that:

“Generally, any data that differ from the mean by more than 3σ (Standard Deviations)

can be considered as blunders and removed from a data set”.

As each point had both an Easting and Northing value, it follows that the rejection of

either the Northing or Easting value for each point would lead to the rejection of the

entire observation. This was the criteria adopted for this study.

Statistical hypothesis testing was used as a measure to compare the measured

positions against the true values.

Ghilani & Wolf (2006, p. 68) describe hypothesis testing as:

47

“The procedures used to test the validity of a statistic.”

There are two elements to a hypothesis test:

1. The null hypothesis (H0)

2. The alternative hypothesis (Ha)

The null hypothesis is defined as...

“..the theory to be tested.” (Reilly 2006, p. 66)

The alternative hypothesis is defined as...

“..what is accepted when a decision is made to reject the null hypothesis..” (Ghilani &

Wolf 2006, p. 68)

The null hypothesis (H0) is the theory that tag position is statistically the same as the

DGPS position. The alternative hypothesis (Ha) is therefore, that the tag position is

statistically different to the DGPS position.

1. The null hypothesis (H0) Tag Easting and Northing Value = Ground Easting

and Northing Values

2. The alternative hypothesis (Ha) Tag Easting and Northing Value ≠ Ground

Easting and Northing Values

48

The statistical distribution most appropriate for this test was the chi square (Χ2)

distribution (see Figure 21). Ghilani & Wolf (2006, pp. 74 - 5) suggest this method

as a means of checking “if an instrument is measuring at its published precision.”

In this distribution, the solid area to the right of the graph represents the rejection

area (α). The size of the solid area depends on the confidence level to be used. The

standard level to test at is 95%. This results in the solid area containing 5% of the

total area of the graph. The Χ2 value is the figure above which there is strong

evidence to reject the null hypothesis.

Figure 21 – Χ2 distribution

There is also a question of which form of test to use, one-tailed or two-tailed. A one-

tailed test tests whether...

“...the sample mean is statistically greater or less than the population mean.” (Ghilani

& Wolf 2006, p. 73)

49

In the one-tailed test, the entire area rejection area is at one end of the graph as in

Figure 21 above.

In a two-tailed test, the rejection area is split equally at both ends of the graph (see

Figure 22 below).

Figure 22 – Two-tailed Χ2 test

In this particular case, a one-tailed test is more appropriate as we are testing whether

the test value (the tag easting and northing) is within the precision limit specified by

the manufacturer (±15m).

The next stage is to decide on a significance level. This is the level at which we

reject the null hypothesis. In the example shown in Figure 21 above, the rejection

level is 5%. This is the level of significance that was used for this investigation.

The test was then carried out on the data using the following formula:

50

Equation 1 - Χ2 Test (Ghilani & Wolf 2006, p. 75)

22

2

vS

Where v = Degrees of Freedom

S = Observed Value

= Expected Value

Equation 1 returned a figure for both Easting and Northing. The critical value was

then calculated by looking up the Critical Value in the Χ2 distribution table. The

Critical Value “identifies the rejection region.” (Reilly 2006, p. 67)

It is calculated by finding the Degrees of Freedom (df) as follows:

Equation 2 – Degrees of Freedom

1df n

Where n = Total Sample Size

51

There are some drawbacks to the utilisation of this test in this case as it is comparing

the Standard Deviation of the difference in the Easting and Northing value between

the tag value and the ground value, with the claimed accuracy figure of ±15m. The

company state that positions are recorded by the tag to the nearest decimal minute,

which equates to 1/6000 of a degree. The worldwide average for this figure is 15m.

Thus this statistical test is not comparing like with like, but it is the best test that is

available for this study.

To determine the accuracy of the tag, the only method available was Root Mean

Square Error.

Root Mean Square Error (RMSE) also known as Root Mean Square, is a method

used to quantify the errors in a measurement, typically used to give an estimate of

the accuracy of a GPS position.

It was calculated using Equation 3, by operating on the error, or difference between

the sample location (the tag) and the “true” location (in this case assumed to be the

DGPS position).

Equation 3 – Root Mean Square Error (Lekkerkerk 2007, p. 156)

2d

X

Where Σd2 is the sum of the square of the difference between Tag & DGPS Location

X is the Mean

52

This then is the figure used to describe the accuracy of the system for this particular

investigation. The figure that is quoted is therefore in terms of a given distance

RMSE.

4.3 Static Location Test

Data was collected at the static location test site (see Figure 6) from 1400 hours on

the 10th

of July 2009 to 1000 hours on the 10th

of August 2009. With a full duty

cycle of 6 fixes (see Section 3), the total possible number of fixes was 184. The tag

performed exceptionally well, with 183 successful fixes. The occasion that the tag

did not record a successful fix was in the Unknown category.

4.4 Static Location Accuracy

Once the data was downloaded and the analysis begun, one factor that was

immediately apparent. Although the tag had recorded 183 separate fixes, they all

related to only 3 distinct locations (see Figure 23). As can also be seen from Figure

24, 95% of the positions were of the one location. This result was completely

unexpected. The reasons for it were unclear. Communications with manufacturers

did shed any light on the causes why this result was output.

As there were only three separate locations output, the sample size was too small to

perform any statistical hypothesis testing on the data, and subsequent tests were

devised to test the accuracy of the tag.

53

Figure 23 – Static Location Test Results

54

Figure 24 – Breakdown of Static Location Positions

4.5 Canopy Location Test Performance Test


of August 2009 to 1000

hours on the 9th

of September 2009. With the standard duty cycle, the total possible

number of fixes recorded was 181. However, the tag performance under the canopy

proved to be very poor. Out of a total of 181 possible fixes, only thirteen resulted in

the recording of a successful GPS fix. The location of these fixes is shown in Figure

25. Of the 168 occasions when no fix was recorded, only 33 resulted in failure due to

one of the known reasons listed in Table 5. This meant that 135 of the possible fixes

resulted in a failure for unknown reasons. The breakdown of the performance of the

tag under the canopy is shown in Figure 26.

55

Figure 25 – Canopy Test Location Results

56

Figure 26 – Tag Performance – Canopy Test

Under canopy conditions the thirteen fixes that were recorded related to eleven

distinct locations. This is in contrast to the static test where 183 fixes resulted in

three distinct locations.

As can be seen from Figure 27, all of the successful GPS fixes were recorded at only

three of the times in the duty cycle at 10:00, 14:00 and 16:00. This was most

probably due to shielding by either the trunk or branches of the test tree, or of

neighbouring trees. It proved to be impossible to measure in any significant way

what level of shielding was being caused by the canopy of the test tree, and other

surrounding ones. It would appear that there was a gap in the canopy that allowed for

successful fixes at those particular times.

57

Figure 27 – Time of GPS Fixes – Canopy Test

4.6 Canopy Location Accuracy

Although the sample size of successful GPS fixes was small (13), the hypothesis

testing as described in Section 4.2 was carried out. The results are shown in Table 6

below:

Table 6 – Canopy Location Hypothesis Test

Easting Northing

Test Statistic 37.96876258 89.56272552

Degrees of Freedom 12 12

Critical Value 21.02606982 21.02606982

As can be seen from Table 6, both the Easting and Northing test values lie outside

the Χ2 Critical Value, thus there is evidence to reject the Null Hypothesis i.e. the tag

has not performed to the manufacturer‟s specifications under the conditions of this

test.

58

The Root Mean Square Test was also carried out on the data from the Canopy

Location Test. The RMSE for the tag Easting is 25.989m while that for the Northing

value is 39.907m. These results show that under the specific conditions that exist

under the canopy of a tree, 67% of the tag locations will be within these limits. These

results lie outside the manufacturers claimed accuracy limits of ±15m.

4.7 Post Test Performance

Data was collected for this test on the 27th

September 2009. It involved moving of

the tag each post to a post 10m away from the previous. Only one full days‟ duty

cycle was carried out, and this resulted in a full set GPS fixes.

4.8 Post Test Accuracy

For the six locations recorded, only four distinct locations were returned. However, it

was found that the tag operated on Greenwich Mean Time (GMT), while in mid-

September, British Summer Time (BST) was still in operation. BST is equal to GMT

+ 1 hour. Thus the tag was recording positions one hour earlier than was anticipated.

As the tag was still operating in an unexpected way, it was decided to abandon this

particular test, and to attempt a test that more closely resembled how the tag would

operate when it was attached to a live bird.

4.9 Moving Location Performance

Data was collected for Moving Test on varying dates between 7th

October and the 3rd

December 2009.

There were a total of 65 attempts to acquire GPS tag fixes. The tag was set-up 15

minutes before the duty cycle time in GMT and left in situ for 15 minutes after to

59

ensure that the duty cycle time was not missed. This required a total of 32.5 hours of

fieldwork. The 65 attempted fixes resulted in 49 successful fixes and 16 failed

attempts. The results are shown in Figure 28.

Figure 28 – Tag Performance – Moving Location Tests

As can be seen from Figure 28, the tag was successful in recording a fix on 75% of

occasions. The “Unknown” category still makes up a significant portion (15%) of the

times when the tag failed to record a fix. The “No Fix” category does not appear in

this set of results. Although the figure for failed fixes is still quite high at 25%, this

may be explained by the colder conditions which would be expected during these

months in the test locality, as opposed to the generally sunnier and warmer

conditions of the earlier static test, which had an almost 100% success rate. As the

tag is powered by a battery which is charged via a solar panel, it would be expected

that weather conditions would have an effect on power supply.

60

4.10 Moving Location Accuracy

The hypothesis testing was carried out as described in 4.2. The results are shown in

Table 7 below:

Table 7 – Moving Location Hypothesis Test

Easting Northing

Test Statistic 117.0566231 212.1922274


Critical Value 60.48088667 60.48088667

As can be seen from Table 7, both the Easting and Northing test values lie well

outside the Χ2 Critical Value, i.e. thus there is evidence to reject the Null Hypothesis,

i.e. the tag has not performed to the manufacturers specifications.

The Root Mean Square Test was also carried out on the data from the Moving

Location Test. The RMSE determined for the tag in Easting was 24.518m while that

for the Northing was 37.478m. These results show that under as the conditions that

most closely mimic normal operating conditions, 67% of tag locations in Easting and

Northing should fall within these limits.

4.11 Revisit Test Performance

The final test was carried out on various dates between 26th

October and 3rd

December 2009. There were a total of 48 attempts to obtain a successful GPS fix.

The tag was set-up 15 minutes before the duty cycle time and left up for 15 minutes

after to ensure that the duty cycle time was not missed. This required a total of 24

61

hours of fieldwork which resulted in 23 successful and 25 failed attempts to record a

position. The breakdown of these attempts is shown in Figure 29.

Figure 29 – Tag Performance – Revisit Test

As can be seen from Figure 29, over 52% of the attempts to record a GPS fix were

unsuccessful. Again, the “No Fix” category does not appears in these results. The

proportion of “Unknown” attempts to record a fix has increased by 14% from the last

test. This is perhaps unsurprising as this test was carried out during the darkest and

coldest period of the year. Again, this relates to hoe the tag is powered by the battery

and how it is recharged via the solar panel.

Although there were 22 successful locations processed (1 outlier removed), these

corresponded to only 12 distinct locations (see Figure 30). The reasons for this are

unknown.

.

62

Figure 30 – Revisit Test Location Results

63

4.12 Revisit Test Accuracy

The hypothesis testing was carried out. The results are shown in Table 8:

Table 8 – Revisit Location Hypothesis Test

Easting Northing

Test Statistic 122.3373 124.3496


Critical Value 33.92444 33.92444

As can be seen from Table 8, both the Easting and Northing test values lie well outside the Χ2

Critical Value, thus there is evidence to reject the Null Hypothesis i.e. the tag has not

performed to the manufacturer‟s specifications under the conditions of this test.

The Root Mean Square was also calculated for the Revisit Location Test. The RMSE for the

tag Easting is 36.462m while that for the Northing value is 34.850m. These results show that

67% of the tag locations will be within these limits.

64

Chapter 5 - Conclusions

One of the first issues raised during this research was that of the tag recording precisely the

same location when it was not moved. It would not be expected that a system with a much

higher accuracy would record the exact same position if left stationary, let alone one with a

claimed accuracy of ±15m. Variations in the number of satellites visible at any one time, and

ionosphere and troposphere effects change over time, which are some of the reasons for

variation in GPS performance. It is not known how this Autonomous GPS receiver can record

precisely the same position during each stage of the duty cycle. It is possible that there is

some element in the software of the tag that returns the same position if the GPS does not

detect any gross change in position. During the final test, where the tag was moved from a set

location and then moved back, the tag recorded only 12 distinct positions out of a total of 22

successful fix locations.

It is not known how the tag can be placed on one spot, moved to two separate locations over

17km away, brought back to the same spot and then record precisely the same location again.

Anecdotal evidence suggests that this is a common occurrence with GPS tags of this type

(Colhoun 2009). This evidence suggests that users of the tags have found in the past that GPS

tags of this type returned the same location when attached to birds that were found to have

died, or who had shed the tag.

As regards the accuracy of the system, it was possible to draw several conclusions. Tests in

varying different conditions and times of the year disproved the manufacturers claimed

accuracy of ±15m was statistically valid in field conditions. One of the major factors when

considering this result is that the manufacturer quoted figures expected in ideal conditions.

This was not the case in this project.

One interesting fact to emerge from the RMSE tests was that the tag appeared to perform to

the same level of accuracy irrespective of physical conditions, i.e. levels of shielding by tree

canopy.

The average RMSE for the three tests was 28.970m Easting and 37.112m Northing. This is

almost twice the manufacturers‟ claimed accuracy.

65

The purpose of the tag must be considered. It is intended to be used for biological studies,

and it is quite unlikely that accuracies greater than the average RMSE found would ever be

required. This is due to the fact that most studies that use these tags are concerned with gross

movements of species over large ranges (see Section 2.2).

The difference between the Easting and Northing would not be unexpected, as the majority of

GPS satellites are over the equatorial regions, and thus, Northing accuracy decreases the

farther north is travelled.

The investigation into whether the tag performed under the tree canopy, the following

findings were made. Of a total of 181 attempted fixes, only 7% were successful. This figure

is not unexpected, as shielding of the GPS signal is one of the major factors affecting

accuracy. However, the statistic that only 1% of the attempted fixes failed due to “No GPS”

indicates that power issues were a major factor. As the tag is solar powered, it relies on direct

sunlight to power the internal battery. “Low Voltage” and “Battery Drain” accounted for 17%

of the attempts to acquire a fix. However, the majority of the failed fixes (75%) fell into the

“Unknown” category. It is the author‟s contention that these failures were also due to power

issues. In all likelihood, these were occasions when the tag did not even have sufficient

power to turn on and register “Low Voltage” or “Battery Drain” categories.

The tag did record fixes during the month it was in the canopy (7% of possible number of

fixes), and these were within 25.930m Easting and 39.008m Northing of the true location.

This was a better result than had been anticipated. On reflection, it may have been better to

attempt this part of the experiment under more empirical conditions. The use of a moveable

filter to control which directions shielding was a factor may have yielded better results.

However this study has shown that the tag could be used to find a nesting bird. Although the

location was not totally precise (with and RMSE of 25.930m Easting and 39.008m Northing),

it would enable the possible nest site to be located by an experienced field-worker. Whether

the cost of the tags (c, €3,500) makes this cost effective is not known, but it appears unlikely.

If the price of the tags continue to come down, then it may become cost effective to tag all

female birds in an attempt make finding nests a less time-intensive activity.

66

As regards the overall performance of the tag, 658 fixes were attempted in total. There was a

large degree of variation in the conditions under which these were attempted. In terms of

geographical location, fixes were attempted across three counties, in conditions varying from

rural to mountain to urban, at varying altitudes. There were also temporal variances from high

summer to mid-winter and in open and shielded conditions. The total success rate in all

conditions was 67%.

From this research, the following conclusions can be drawn:

1. The tags are most suited for studies of large scale movements such as migrations over

long distances rather than fine-scale movements.

2. Some knowledge of geodetic surveying techniques is an advantage to anyone utilising

the tags, both when processing the data and also in interpreting the results.

3. It is possible that the tag can be used to locate nests in the field.

4. Although the tag accuracy was not found to be within the manufacturers‟ specifications

(±15m Easting and Northing), it is sufficiently accurate (28.970m Easting and 37.112m

Northing RMSE) for the type of work intended.

5. Some method of post-processing of the data would be valuable, both in terms of

increasing the accuracy of the data, and allowing for better interpretation of the data.

6. The tag will, in all likelihood, not return GPS fixes at every point in the duty cycle. A

person utilising the tags should be aware that they will, not obtain the amount of GPS

returns that are stated by the manufacturers. This is due to the natural consequences of

placing a tag on an animal in the field.

7. Although the tag is supposed to operate to operate in condition ranging from -15 to 45

C, the tag did not operate as well in the winter.

8. Further work needs to be carried out on the tags in field conditions in order to more fully

assess their performance and accuracy. This could be carried out under the following

headings:

Shielding – as stated above, more empirical methods of testing the effects of shielding

could be carried out. This may involve a moveable plastic shield that would block off

different quadrants of the sky in order at different time periods.

67

Seasonal Performance – carrying out of the Moving location test throughout varying

temperatures and daylight lengths would provide more information on how the tag

would perform in real-world conditions.

Nest finding performance – the tag could be attached to a bird such as Red Kite and at

attempt made to find a nest in real-world conditions.

68

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Appendices

75

Appendix I – Dissertation Proposal

76

School of Spatial Planning

Department of Spatial Information Sciences

Dissertation Proposal Form

Student’s Name Paul Duffy

Programme / Stage DT112 / 4

77

1: Research Field:

The research field of the project is Global Navigation Satellite Systems, specifically, the use

of GPS tags to track wildlife.

2: Research Question:

The aim of this project is twofold:

1. To test the claimed accuracy of GPS tags used for the tracking of wildlife.

2. To ascertain whether it is possible to locate nests of wild bird species in the field

using these GPS tags.

Statistical analysis of the results will be carried out and an assessment of the effectiveness of

the tag in both optimal and sub-optimal conditions will be made.

3: Review of Relevant Publications:

To date the method of monitoring of wild birds is to fit each with tail-mounted radio packs.

However, Mech & Barber (2002) describe an alternative and in their opinion superior method

– the use of satellite tagging as “the latest major development in wildlife tracking”. To date,

the prohibitive cost of these Global Positioning System (GPS) tags has limited their use for

monitoring of large populations of birds of prey (Hardey et.al. 2006). These small (<30g),

solar-powered tags (see Figure 1) are fitted to the birds using a “back-pack” arrangement.

The tags record data at set periods of the day, and the data are transmitted to satellites in low

polar orbit. The data is transmitted to ground stations operated by the Argos program. This

program was founded in 1978 as joint venture of the French Space Agency (CNES) and the

American National Oceanic and Atmospheric Administration (NOAA).

The ground stations calculate the position of the tag and processes the data recorded by the

tag. The data is then delivered to the end-user. Meyburg & Meyburg (2008) state that the

GPS tags provide “fixes precise to within a few dozen metres.” There does not appear to have

78

been any empirical research carried out on the actual accuracy of the system, nor whether the

tags will work in the canopy of a tree, and if so, how accurate the fixes are.

Figure 1 - PTT-100 30 gram Solar Argos/GPS PTT produced by Microwave Telemetry

Inc, USA

Source: Microwave Telemetry Inc.

4: Methodology

The purpose of the project is to test the accuracy of the GPS tags in both ideal (optimal)

conditions and in marginal (sub-optimal) conditions.

The optimal test will be carried out to test the accuracy of the system in field conditions. First

the precise co-ordinates of a point in open land will be accurately surveyed using GPS. This

point will be marked with a post and the GPS tag fixed to this point for approximately 30

days. The locations calculated for the tag by the service provider will be compared with its

known location.

The sub-optimal test will be carried out in a forest situation. Shielding of the GPS signal by

trees results in less satellites being available for the calculation of a position, and thus, a less

accurate fix (Lekkerkerk, 2008). The tag will be installed in the canopy of a tree in order to

test the performance of the tag under the tree canopy. This will assess how effective the tags

would be in locating actual nest sites.

After these tests have been carried out, the tag will be fitted to a Red Kite. The tag will then

be used by the Red Kite Reintroduction Project.

This project will benefit from the expertise available within the Department of Spatial

Information Sciences, Department of Spatial Information Sciences, Dublin Institute of

Technology, Bolton Street, Dublin 1, from whom the author has sought guidance on the

project.

79

5: Project Outcomes:

The anticipated outcomes of this project

1. An assessment of the accuracy of the GPS tags in field conditions.

2. Promotion of the use of GPS as a technology in the monitoring of endangered species

in Ireland.

3. Promotion of the work of the Red Kite Reintroduction Project & other re-introduction

projects in Ireland, namely the Golden Eagle Reintroduction Project and the White-

tailed Sea Eagle Reintroduction Project.

Project Costs:

Project costs have been based on figures supplied by Microwave Telemetry Inc. and Argos

(figures are approximate and based on exchange rates available at the time of writing (2nd

March 2009)).

No. Item Cost €

1 30g Argos Tag €3,099

1 Ground Track Option for

Tag

€156

1 Argos Satellite Monitoring

(4 months @ 69.75)

€279

1 Delivery of data on CD-

ROM

€150

Total €3,684

Table 1 – Estimated Project Costs

6: Conclusions:

This dissertation project will be carried out during 2009. It is hoped that the project will lead

to further studies of the topic by the author.

80

7: Bibliography:

Argos (2008) Argos User’s Manual [WWW] CLS Group – Available from http://www.argos-

system.org/html/userarea/manual_en.html [Accessed 13-11-08]

Hardey, J., Crick, H., Wernham, C., Riley, H., Ethridge, B. & Thompson, D. (2006) Raptors:

a field guide to survey and monitoring Edinburgh, The Stationary Office

Lekkerkerk, H-J. (2008) GPS Handbook for Professional Surveyors The Netherlands,

CMedia Productions

Mech, L. D. & Barber, S.M. (2002) A Critique of Wildlife Radio Tracking and its use in

National Parks, [WWW] U.S. National Parks Service – Available from

www.npwrc.usgs.gov/resource/wildlife/radiotrk/radiotrk.pdf [Accessed 13-11-08]

Meyburg, B.-U., Eichaker, X., Meyburg, C. & Paillat, P. (1995) Migrations of an adult

Spotted Eagle tracked by satellite. [WWW] British Birds 88: 357-361 –Available from

http://www.raptor-research.de/a_sp100.html. [Accessed 5-5-09]

Meyburg, B.-U. & Meyburg, C. (2008). Satellite tracking of raptors - How PTTs changed

our lives. [WWW] Tracker News 9: 2-5 - Available from http://www.raptor-

research.de/a_sp100.html. [Accessed 5-5-09]

Natural Research Ltd. (-) Satellite Tracking Golden Eagles in Scotland – Sponsors

Information Pack [WWW] Natural Research Ltd. – Available from http://www.natural-

research.org/documents/sattagfundersinformationpack_2009.pdf [Accessed 10-5-09]

Pierre, J. P. & Higuchi, H. (2003) Satellite tracking in avian conservation: Applications and

results from Asia – [WWW] National Institute of Polar Research, Tokyo - Available from

http://www.nipr.ac.jp/~penguin/oogataHP/pdfarticles/10p101-109.pdf

http://www.argos-system.org/html/userarea/manual_en.html

http://www.argos-system.org/html/userarea/manual_en.html

http://www.npwrc.usgs.gov/resource/wildlife/radiotrk/radiotrk.pdf

http://www.raptor-research.de/a_sp100.html



http://www.natural-research.org/documents/sattagfundersinformationpack_2009.pdf

http://www.natural-research.org/documents/sattagfundersinformationpack_2009.pdf

http://www.nipr.ac.jp/~penguin/oogataHP/pdfarticles/10p101-109.pdf

81

8: Websites:

Argos http://www.argos-system.org/html/userarea/welcome_en.html [Accessed 13-11-08]

Golden Eagle Trust http://www.goldeneagle.ie/portal.php [Accessed 2-3-09]

Microwave Telemetry Inc. http://www.microwavetelemetry.com/ [Accessed 13-11-08]

http://www.argos-system.org/html/userarea/welcome_en.html

http://www.goldeneagle.ie/portal.php

http://www.microwavetelemetry.com/

82

Appendix II – PTT-100 22g Solar Argos / GPS PTT Tag Specifications

83

Source: (Microwave Telemetry 2009a, p. 6)

84

Appendix III –GPS PTT Tag Production Form

86

Appendix IV – Argos System User Agreement (SUA)

95

Appendix V – Argos ID Number Request Form

97

Appendix VI – Notification of Argos Platform ID

99

Appendix VII – PRV/A DS File Format

101

Source: (Microwave Telemetry 2009a, pp. 18-9)

102

Appendix VIII – Sample PRV/A DS File

103

03892 92605 9 29 K

2009-10-16 06:53:26 1 10 15 12 00

53 822 353 4144

00 00 00 15

8096 00 53 1374

353 2157 00 53

1068 353 2748 00

53 1035 353 2873

6683

39768

003 msgs 000>-120dB Best: -126 Freq: 681208.5 IQ : 00

Lat1: 53.447N Lon1: 6.974W Lat2: 53.103N Lon2: 5.021W

104

Appendix IX – Sample Argos Locations File

105

Date Time Fix Lat1(N) Long1(E) Lat2(N) Long2(E) MsgCount Frequency Average TI Satellite

Max Str

2009-10-16 06:53:26 1 K

2009-10-16 07:12:18 4 60.0 L

2009-10-16 08:51:34 1 L

2009-10-16 09:54:21 1 A

2009-10-16 10:28:00 A 53.447 -6.974 53.103 -5.021 3 681208.5 61.0 M -126

2009-10-16 13:44:55 4 M

2009-10-16 13:50:27 2 53.157 -6.829 51.086 -17.486 8 681208.5 60.1 N -127

2009-10-16 13:58:16 17 57.8 P

2009-10-16 16:39:12 B 53.144 -6.992 51.749 -14.736 2 681208.5 118.0 K -130

2009-10-16 17:09:37 B 53.269 -6.012 55.664 6.480 2 681208.5 424.0 L -125

2009-10-19 13:16:26 5 N

2009-10-19 13:25:53 2 58.0 P

2009-10-19 14:59:04 B 53.129 -6.572 44.957 -49.240 2 681208.5 61.0 N -134

2009-10-19 20:45:41 3 M

2009-10-22 03:01:43 2 61.0 P

106

Appendix X – Sample Engineering File

107

Tx Date Tx Time Int Date Int Time Satellite ID Activity Tx Count Temperature (°C) Battery

Voltage (V) GPS fix time Satellite Count Hours Since Reset Hours since GPS fix Season Shunt

Mortality GT Season GT Latest Latitude(N) Latest Longitude(E) Passed Checksum

2009-10-16 07:14:18 2009-10-16 07:14:21 L 111 18 13.2 3.85 16 3 255 13 4 0 0

0 53.17517 -6.42900 0

2009-10-16 13:51:59 2009-10-16 13:52:00 N 145 9 17.0 3.95 16 3 255 1 4 0 0

0 53.21017 -6.67717 1

2009-10-16 14:00:39 2009-10-16 14:00:38 P 145 10 17.6 3.93 35 11 255 0 4 0 0

0 53.15850 -6.82000 1

2009-10-19 13:26:22 2009-10-19 13:26:22 P 195 3 19.2 3.85 17 3 255 1 4 0 0

0 53.28767 -4.39600 0

108

Appendix XI – Sample GPS Locations File

109

Date Time Latitude(N) Longitude(E)

2009-10-11 12:00 53.17250 -6.52117

2009-10-11 14:00 53.17517 -6.47117

2009-10-11 16:00 53.17517 -6.47117

2009-10-11 18:00 53.17517 -6.47117

2009-10-12 08:00 53.17517 -6.47117

2009-10-12 10:00 53.17517 -6.47100

2009-10-12 12:00 53.17800 -6.54200

2009-10-12 14:00 53.23083 -6.68417

2009-10-12 16:00 53.15833 -6.81983

2009-10-12 18:00 53.17517 -6.47117

2009-10-13 08:00 53.21433 -6.39950

2009-10-13 10:00 53.23533 -6.33267

2009-10-13 12:00 53.22900 -6.64050

2009-10-13 14:00 53.16467 -6.88333

2009-10-13 16:00 53.19750 -6.46900

2009-10-13 18:00 3.81 no fix

2009-10-14 08:00 53.17800 -6.54217

2009-10-14 10:00 53.30817 -6.42233

2009-10-14 12:00 3.83 no fix

2009-10-14 14:00 53.34750 -6.31883

2009-10-14 16:00 53.32983 -6.44167

2009-10-14 18:00 3.81 no fix

2009-10-15 08:00 3.79 no fix

2009-10-15 10:00 3.79 no fix

2009-10-15 12:00 53.13700 -6.30933

2009-10-15 14:00 53.30550 -6.41333

2009-10-15 16:00 53.18450 -6.49650

110

2009-10-15 18:00 53.17517 -6.47167

2009-10-16 08:00 53.13867 -6.43883

2009-10-16 10:00 53.35050 -6.33183

2009-10-16 12:00 53.21017 -6.67717

2009-10-16 14:00 53.15850 -6.82000

2009-10-16 16:00 53.14650 -7.03000

2009-10-17 12:00 3.77 no fix

2009-10-17 14:00 53.17433 -6.46717

2009-10-17 16:00 3.77 no fix

2009-10-17 18:00 3.75 no fix

2009-10-18 12:00 3.75 no fix

2009-10-18 14:00 53.17433 -6.46600

2009-10-18 16:00 53.17533 -6.46867

2009-10-18 18:00 3.74 no fix

2009-10-19 12:00 53.28767 -6.35333

2009-10-19 14:00 53.17000 -6.81150

2009-10-19 18:00 3.68 no fix

2009-10-20 12:00 3.63 low volt

2009-10-20 14:00 53.21367 -6.30617

2009-10-20 18:00 3.77 no fix

2009-10-21 12:00 3.77 no fix

2009-10-21 14:00 53.35617 -6.31367

2009-10-21 18:00 53.17533 -6.47117

111

Appendix XII – Sample Google Earth Locations File

113

Appendix XIII – Sample Grid InQuest Conversion File

114

2009-10-07 10:00 53.16333 -6.30200 525 W of Liffey Head Bridge

2009-10-08 10:00 53.29033 -6.31467 63 Dodder Valley Park

2009-10-08 14:00 53.01267 -6.27317 160 WMNP Office, Kilafin

2009-10-08 16:00 53.03283 -6.37017 326 Ruined Farm Buildings, Wicklow Gap

2009-10-08 18:00 53.16967 -6.51517 192 Blessington Bridge Car-Park

2009-10-09 16:00 53.35450 -6.30917 41 Lords Walk Car-Park

2009-10-09 18:00 53.34917 -6.45233 52 Newcastle Shopping Centre

2009-10-12 12:00 53.17800 -6.54200 211 Blessington Inner Relief Road

2009-10-12 14:00 53.23083 -6.68417 83 Naas Outer Relief Road

2009-10-12 16:00 53.15833 -6.81983 102 Curragh E of M7

2009-10-13 08:00 53.21433 -6.39950 457 NW of Kilbride Army Camp

2009-10-13 10:00 53.23533 -6.33267 395 Kilakee Road

2009-10-13 12:00 53.22900 -6.64050 94 Fishery Lane, Naas

2009-10-13 14:00 53.16467 -6.88333 101 Curragh E of Kildare Town

2009-10-13 16:00 53.19750 -6.46900 202 Kilbride GAA Pitch

2009-10-14 10:00 53.30817 -6.42233 81 Corkagh Park West

2009-10-14 14:00 53.34750 -6.31883 29 Magazine Fort, Phoenix Park

2009-10-14 16:00 53.32983 -6.44167 61 Grange Castle Business Park

115

2009-10-15 12:00 53.13700 -6.30933 495 SE of Sally Gap

2009-10-15 14:00 53.30550 -6.41333 91 Corkagh Park East

2009-10-15 16:00 53.18450 -6.49650 188 Threecastles

2009-10-16 08:00 53.13867 -6.43883 455 Sorrel Hill

2009-10-16 10:00 53.35050 -6.33183 34 W of Acres Road, Phoenix Park

2009-10-16 12:00 53.21017 -6.67717 103 Naas Outer Relief Road (Blessington Section)

2009-10-16 16:00 53.14650 -7.03000 64 E of Monasterevin

2009-10-19 12:00 53.28767 -6.35333 89 S of LIDL Car-Park, Tallaght

2009-10-20 14:00 53.21367 -6.30617 488 Main Barrier, Featherbeds

2009-10-21 14:00 53.35617 -6.31367 40 W of Polo Grounds, Phoenix Park

2009-10-23 12:00 53.35067 -6.31033 34 W of Army Road, Phoenix Park

2009-10-23 16:00 53.04117 -6.39800 480 Wicklow Gap Car-Park

2009-10-26 14:00 53.18800 -6.46900 197 N of Ballyward Bridge

2009-10-26 16:00 53.16067 -6.36117 318 Joseph’s Cottage

2009-10-27 08:00 53.17033 -6.40300 342 Athdown Forest

2009-10-27 10:00 53.35083 -6.26750 34 Rooftop, Loftus Lane

2009-10-27 14:00 53.35200 -6.31367 34 S of Camogie Road, Phoenix Park

2009-10-28 08:00 53.21600 -6.39650 471 Seahan Forest Entrance

116

2009-10-28 12:00 53.15700 -6.82700 110 W of Curragh Camp Road

2009-10-28 16:00 53.21283 -6.30400 473 Middle Barrier, Featherbeds

2009-10-29 10:00 53.29250 -6.31983 69 Spawell Car-Park

2009-11-08 14:00 53.18783 -6.63600 145 Punchestown Location 1

2009-11-09 12:00 53.14400 -6.86183 103 S of Curragh Camp

2009-11-09 16:00 53.07333 -6.35417 413 E of Sheep-pens, Military Road


2009-11-10 16:00 53.16583 -6.29467 523 Kippure Gates

2009-11-11 08:00 53.20633 -6.29350 437 Lower Barrier, Featherbed’s

2009-11-11 12:00 53.15117 -6.80717 108 S of Graveyard, Curragh


2009-11-12 14:00 53.27533 -6.32333 75 Ballyboden Park


117

Appendix XIV – Communications with Microwave Telemetry, Inc.

118

Ireland.com Mail Ireland.com Mail [email protected]

Re: FAO of Cathy 05 October 2009 22:06:00

From: [email protected]

To: [email protected]

Dear Paul:

Thank you for your email.

What is "Code only GPS"? We are not familiar with the expression.

The lat/lon GPS locations provided by our GPS PTTs are accurate to approximately +/-

15 m. Positions are recorded in the GPS PTT to the nearest decimal minute (1/6000 of a

degree) as detailed in our manual. At our latitude, this corresponds to 18.5 m latitude and

14.6 m longitude--world wide average is about 15 m. (Information on the accuracy of

the PTT is posted on our website, and included in the user manual we shipped with your

PTT. ) A 5 m change in position would not necessarily result in a different location reading

as it is still within the unit's margin of error.

Here is a brief description of your 2D PTTs' DS format sensor message:

Sensor # Length Description

1 4 bit month

2 5 bit day

3 8 bit hour of the day

4 1 bit lat hemisphere (0-N, 1-S)

5 7 bit lat deg

119

6 13 bit lat mins/decimal mins

7 9 bit long deg

8 13 bit long mins/decimal mins


10 7 bit lat deg


12 9 bit long deg



15 7 bit lat deg


17 9 bit long deg



20 7 bit lat deg


22 9 bit long deg



25 7 bit lat deg


27 9 bit long deg


29 16 bit Checksum (crc-16)

120

Note: the first location given in the message (sensors 4 through 8) is the most recent

location.

I hope this helps.

Sincerely,

Cathy

Microwave Telemetry, Inc.

8835 Columbia 100 Parkway

Suites K & L

Columbia, MD 21045

USA

Tel: 410-715-5292

Fax: 410-715-5295

www.microwavetelemetry.com

In a message dated 10/4/2009 1:43:21 P.M. Eastern Daylight Time, [email protected]

writes:

Hi Cathy,

My name is Paul Duffy. I currently studying for an Honours Degree in Geomatics in

the Department of Spatial Information Sciences, Dublin Institute of Technology,

Bolton Street, Dublin 1, Eire. As part of my final year dissertation I am testing the

accuracy of GPS tags in field conditions.


121

I purchased a 22g Solar GPS tag from your company in June of this year for the

purpose of carrying out this research. However, I am having problems with the

results I am receiving from the tag.

I assume that the GPS itself returns data is Code only GPS signals, and thus would

only be accurate to within ±10m. Could you confirm that the tag does indeed use

Code only GPS?

One of the first tests I conducted was to leave the tag in a stationary location for 30

days. I expected a spread of various Latitude and Longitude readings around the

known co-ordinates of the point. However, what the data returned was the same

Latitude and Longitude for the majority of the 30 days. I subsequently moved the tag

to 7 locations 5m apart, with no apparent change in the tag location. I also did this

with an autonomous GPS (of the type for recreational use) for comparison purposes

and this did show a change.

I was wondering whether there was a software or hardware setting within the tag that

would explain this result?

122

I was also wondering if you could tell me what is the raw data message format is

(and is it possible to access it) as this may help me understand the raw data messages

that I am currently receiving from the ARGOS web service.

I would be very grateful for any help you could give me in answering these queries,

Regards,

Paul Duffy

Ireland.com Mail Ireland.com Mail [email protected]

Re: FAO Cathy 11 November 2009 21:31:08

From: [email protected]

To: [email protected]

Dear Paul:

If you had selected the parser option to output the converted values, instead of a "15" in your

message you would see "No Fix". "No fix" means the PTT's receiver stayed on the full 2

minutes but could not lock up a fix. this indicates that the satellites were not visible to the

PTT.

123

You shouldn't need to assume why the PTT didn't get a fix at a particular hour as the

explanation is in the message. The other codes for hours without fixes are:

"Batt drain" -- recorded in the data when the PTT's battery voltage was high enough that it

was able to activate the GPS receiver and try to take a fix, but the voltage dropped below

the threshold necessary for fix acquisition before it could lock up a fix. The raw value for

"battery drain" is a number between 30 and 40.

"Low volt" --which indicates that at that hour the battery voltage was below the threshold

necessary for fix acquisition. (The PTT does not attempt a fix if the battery voltage is below

the required threshold.) The raw value for "low voltage" is 27.

Sincerely,

Cathy

Microwave Telemetry, Inc.

8835 Columbia 100 Parkway

Suites K & L

Columbia, MD 21045

USA

Tel: 410-715-5292

Fax: 410-715-5295

www.microwavetelemetry.com

In a message dated 11/11/2009 3:57:12 P.M. Eastern Standard Time,

[email protected] writes:

Hi Cathy,

Paul Duffy here again. I had a further question for you regarding the parsed

GPS data.

As in the case below, I can see that the 16:00 duty time managed to get a fix.


124

Obviously the 18:00 did not. Can you tell me whether this is due to not

enough satellites being visible or could you give some other explanantion?

2009-11-08 16:00 53.17467 -6.47200

2009-11-08 18:00 140 15

Could I also assume that, for example if there is no output for a given time

(i.e no reading at 14:00 on a particular day), that the unit did not attempt to

get a GPS reading at all (possibly due to power).

Thanks,

Paul Duffy

125

Appendix XV – GPS Pathfinder ProXH Specifications

126

Source: (Trimble 2009b, p. 2)

127

Appendix XVI – Data Dictionary Specifications

129

Appendix XVII – Sample OSi RINEX Correction File

130

Searching for base files...

Search complete.

--------Base Data Details:--------------------

Using reference position from base data

Source: C:\DIT\Dissertation\Final_RINEX_Files

arkl280k.09o

Local time: 07/10/2009 10:59:46 to 07/10/2009 11:59:41

Position: 52°47'49.12799"N, 6°09'27.98562"W, 91.68 m, 0.00 m Antenna height

arkl280s.09o

Local time: 07/10/2009 18:59:46 to 07/10/2009 19:59:41


arkl281i.09o

Local time: 08/10/2009 08:59:46 to 08/10/2009 09:59:41


"W, 163.75 m, 0.00 m Antenna height

--------Coverage Details:--------------------

Rover file: A_TAG_TEST_2.SSF

Local time: 07/10/2009 10:45:56 to 12/10/2009 17:20:23

100% total coverage

1% coverage by arkl280k.09o

1% coverage by PRTL280k.09o

1% coverage by TLLG280k.09o

Differentially correcting...

Differential correction settings:

Use data collection filter settings: On

Correct velocity records: On

Re-correct real-time positions: On

131

Velocity filtering: Off

Output positions: Corrected only

--------------------------------------------------

Processing rover file, A_TAG_TEST_2.SSF ...

...to output file, C:\DIT\Dissertation\A_TAG_TEST_2_1.cor

Carrier processing...

Selected 746 positions for post-processing

Corrected 696 positions

Failed to correct 50 positions

Code processing...

Selected 746 positions for post-processing

Corrected 745 positions

Failed to correct 1 positions

1 of these were missing SuperCorrect data

--------------------------------------------------

Differential Correction Summary:

1 file processed. In this file:

745 (99.9%) of 746 selected positions were code corrected by post-processing

696 (93.3%) of 746 selected positions were carrier corrected by post-

processing

Estimated accuracies for 746 corrected positions are as follows:

Range Percentage

0-15cm 60.7%

15-30cm 32.6%

30-50cm -

0.5-1m 6.7%

1-2m -

2-5m -

>5m -

Differential correction complete.

An Investigation Into The Accuracy And Performance Of A Lightweight GPS Tag Used To Track Wildlife

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Transcript of An Investigation Into The Accuracy And Performance Of A Lightweight GPS Tag Used To Track Wildlife