FiBRESERIESFINDINGS IN BUILT AND RURAL ENVIRONMENTS OCTOBER 2011

ASSESSMENT OF ATMOSPHERIC ERRORS ON GPS IN THE ARCTICPeter Dare and Reza Ghoddousi-Fard

Department of Geodesy and Geomatics Engineering

University of New Brunswick (UNB), Canada

GPS IN THE ARCTIC

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GPS IN THE ARCTIC

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About the authors Contents

Dr Peter Dare

Dr Peter Dare is the Chair of the Department of Geodesy and Geomatics Engineering at

the University of New Brunswick (UNB), Canada, a position he obtained in 2002 having

joined UNB in 2000. He joined UNB after 14 years of teaching and research at a university

in England. His main areas of expertise are in Geodesy and GPS but in addition he has

experience in the broad field of Geomatics.

Peter chaired RICS’ regional university partnership and accreditation board from 2002 when

RICS first expanded into North America and Africa. He is a member of RICS’ Geomatics

Global Professional Group Board, and he also interviews candidates at the final stage of

the Assessment of Professional Competence.

Peter was a Visiting Professor in the Department of Surveying and Land Information at the

University of the West Indies in Trinidad during 2007 and 2008 where he collaborated with

other researchers on topics relevant to the Caribbean, focusing on integrated monitoring

of the Montserrat volcano.

Dr Reza Ghoddousi-Fard

Dr Reza Ghoddousi-Fard currently is a Research Scientist in the Geodetic Survey Division

(GSD), Natural Resources Canada (NRCan), Ottawa, Ontario, Canada. He first joined NRCan

as a post doctoral fellow in early 2009 supported by both GSD and their Geomagnetic

Laboratory. He obtained his PhD from the Dept of Geodesy and Geomatics Engineering at

UNB and it was during his time as a PhD student that the work described here was carried out.

Before joining UNB, Reza obtained both his BScE and MScE from the, K.N.Toosi University

of Technology, Tehran, Iran. His main area of expertise is in atmospheric modelling in

GPS. He has also taught a number of Geomatics courses as a university lecturer and has

experience in geodetic deformation monitoring projects.

©RICS – October 2011

ISBN: 978-1-84219-712-7

Published by:

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The views expressed by the author(s)

are not necessarily those of RICS

nor any body connected with RICS.

Neither the authors, nor RICS accept

any liability arising from the use of

this publication.

This work was funded by the

RICS Education Trust, a registered

charity established by RICS in 1955

to support research and education

in the field of surveying.

Key findings 04

GPS signal refraction 05

Why this research is important 06

GPS positioning in the Arctic 07

Analysis of refraction dependency on azimuth 08

Conclusions 10

About the study 10

Further information 11

02

050550500

GPS IN THE ARCTIC

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Key findings GPS signal refraction

If you want the

best accuracy,

you have to

really study what

influences the

measurements

‘‘

‘‘GPS is used throughout the world for navigation and positioning.

Growing interest in the Arctic region predominantly due to a

reduction in ice coverage suggested that an investigation should

be carried out on region-specific errors in GPS for high-

accuracy users.

With funding from the RICS Education Trust, Peter Dare and

Reza Ghoddousi-Fard of the University of New Brunswick have

explored how to improve positions determined from GPS in the

Arctic region of Canada. There are many factors that have an

influence on the accuracy obtained from GPS, especially for

high-accuracy users. The influence of the Earth’s atmosphere

on the GPS signals as they pass through the atmosphere is

potentially a major source of error in the derived position of

the GPS receiver.

In this work, enhanced algorithms for dealing with the effect

of the lower part of the atmosphere have been developed to

improve GPS results for high accuracy users in the Canadian

Arctic. GPS software developed by researchers at the University

of Berne in Switzerland has been modified to handle atmospheric

parameters calculated from Environment Canada’s weather

models. A field experiment on board the Canadian research

icebreaker CCGS Amundsen was carried out to test the weather

model in the data sparse regions of the Canadian Arctic.

The developed procedures improve the latitude component

of GPS positions.

The key findings are:

can affect the estimated latitude component by more

than 2mm. There was no significant systematic affect

on the longitude component

with either gradient component was not as clear as the

correlation with latitude

be important in applications such as long term geodynamics

studies, maintenance of geodetic control networks, and

climatic studies.

Radio signals emitted by GPS satellites orbiting the Earth

at an altitude of about 20 000 km can be used to determine

positions by GPS receivers. On their way to the receivers on

the Earth’s surface, or at sea, or even on other satellites, the

radio signals have to pass through the Earth’s atmosphere

and the atmosphere causes the signals to be refracted.

When considering refraction of GPS signals, the atmosphere

is usually divided into an upper part (ionosphere) and a lower

part (neutral atmosphere) which extends upto an altitude of

about 80 km. The part of the neutral atmosphere closest to

the Earth (troposphere) is where most of the refraction of the

neutral atmosphere occurs – the troposphere extends upto a

height of about 10–15 km. In this region, water vapour in the

atmosphere can have a significant impact on GPS positioning,

as it is one of the most difficult aspects of refraction to deal

with. It is the neutral atmosphere that is the focus of this study,

and in particular its composition in the Arctic and how this can

affect coordinates of points in the Arctic region determined by

the use of GPS.

When dealing with refraction in the neutral atmosphere, the total

refraction is usually spilt into two components: a ‘dry’ part and

a ‘wet’ part (referred to as hydrostatic and non-hydrostatic in

the scientific literature). The dry component is usually relatively

easy to estimate from surface pressure measurements, while

the wet component is harder to estimate as it is affected by

variations in water vapour.

GPS IN THE ARCTIC

06

The Earth’s polar regions play key roles in our global

environment and are highly sensitive to climate change.

Apart from the importance of the Arctic in global climate

studies, global warming has increased the importance of

high latitude studies. In addition to increasing the amount

of precipitation in the polar regions, global warming is also

likely to increase sea level both from melting of ice caps

and glaciers but also from warming (and hence expansion)

of the waters. Enhanced approaches for environmental

monitoring as well as increased atmospheric data sources

are of great importance for research on polar regions.

The Global Positioning System (GPS) is a well-established

positioning and navigation system, and is also being used

for applications such as sea level monitoring, navigation of

hydrographic vessels and many other positioning projects.

GPS can also be used to determine the amount of water vapour

in the atmosphere to assist weather prediction and atmospheric

and climate research. Procedures capable of improving the

aforementioned GPS applications can assist research related

to monitoring of environmentally sensitive regions such as

Arctic. Canada is one of the countries that borders the Arctic

region and so it is especially interested in research based in

the Arctic as this may assist in future sovereignty claims.

Why this research is important

The design of the GPS ‘system’ results in satellites in the Arctic

only rising to an altitude of 55 degrees, whereas they can go

close to overhead at lower latitudes. This means that more

tracking of satellites at low elevation angles will take place in

the Arctic. This has implications when considering the refraction

effect. Let us consider two signals, each with an elevation angle

of just 3 degrees, one looking North (azimuth of zero degrees),

and the other looking South (azimuth of 180 degrees, so the

signals are 180 degrees apart). Due to their low elevation

angles, these signals will leave the neutral atmosphere about

1500 km away from each other. It is quite likely that the

atmospheric conditions at these locations could be very

different, and so the refraction for both these signals would

be different. In effect, the research notes that the refraction

depends upon azimuth. This variation in refraction, as a function

of azimuth, is sometimes ignored when processing GPS data.

Therefore, this study investigated the refraction variation, and

quantified its effect.

GPS positioning in the Arctic

07

GPS IN THE ARCTIC

09

Initially, a field test was carried out in the Arctic by estimating

the refraction onboard the Canadian research vessel CCGS

Amundsen as it traversed the North West Passage in 2005.

On the vessel a GPS receiver was placed, as well as a precise

barometer, and a Water Vapour Radiometer (WVR) – an instrument

used to directly measure the amount of water vapour in the

atmosphere (see Figure 1). One of the conclusions of this study

was that Environment Canada’s numerical weather prediction

(NWP) model could be used to determine reasonable estimates

of water vapour. This was deduced by estimating the amount

of water vapour from their model, and then comparing that with

the measurements of water vapour made by our WVR. In Figure 2,

the days of 231-248 in 2005 was plotted against the amount of

wet refraction in the “up” direction on board CCGS Amundsen

as determined by four approaches:

(UNB3m) developed at UNB.

Overall, there is good agreement between WVR and NWP

estimates of wet refractivity over the studied period and region.

However, the agreement is degraded during time of rapid

changes in wet refractivity. This could have been due to local

effects on the WVR measurements and/or small scale weather

phenomena which might not be detectable by the NWP model.

For example, over a rather short period around start of day 241

there is a difference of about 4 cm between the NWP and WVR

values, while shortly after there is no noticeable difference.

The variability in accuracy of the NWP derived estimates of wet

refraction seen here (and by many other researchers using other

data) has led many scientists to try to improve NWP models by

assimilating GPS derived values of water vapour into the NWP

model computation.

To determine the amount of refraction variation due to azimuth,

the data from over 70 radiosondes covering Canada and the

northern USA were analyzed. A radiosonde is a weather balloon

that carries with it equipment to measure various parameters of

the atmosphere (including pressure, temperature, and humidity),

and transmit them to a receiver on the Earth.

Figure 1: Water Vapour Radiometer (WVP) onboard CCGS Amundsen

Analysis of refraction dependency on azimuth

The analysis showed a systematic decrease in the dry refraction

in the northerly direction and an increase in the southerly

direction (there is effectively a dry refraction gradient in the

North-South direction). As a result, when averaged over a year,

differences of about 4 cm in the estimated distance to the

satellites in the North-South direction at a 3 degree elevation

angle could be seen. The analysis also showed no significant

systematic gradient in the East-West direction. It was deduced

that the decrease in dry refraction towards the North is due to

a thinning of the atmosphere as you move north. Figure 3 shows

the variation in the thickness of the atmosphere in Canada and

the northern USA. It is clear from this figure that the atmosphere

is about 700 m thinner in the Arctic at the plotted epoch (9 p.m.

GMT on September 5 2007).

In addition, the temperature in the Arctic is clearly generally

lower than temperatures to its south. This in itself makes the

Arctic air drier (and so less humid) since cold air is unable to

hold as much water vapour as warm air. However, there is still

enough water vapour in the atmosphere to cause significant

refraction to the GPS signals.

Figure 2: Wet refraction results from UNB3m, NWP, and WVR (raw and smoothed values) onboard the CCGS Amundsen

Figure 3: Thickness of the atmosphere at 9 p.m. GMT,

September 5 2007

08

GPS IN THE ARCTICGPS IN THE ARCTIC

The study was carried out by Reza Ghoddousi-Fard under

the supervision of Professor Peter Dare (University of New

Brunswick), with funding from the RICS Education Trust,

Canada’s Natural Science and Engineering Research Council,

New Brunswick Innovation Foundation, Northern Scientific

Training Program of Indian and Northern Affairs Canada,

and Canada Foundation for Innovation. Data access was

provided by Environment Canada’s Meteorological Service

of Canada, and The U.S. National Oceanic and Atmospheric

Administration. Computing resources were provided

by Atlantic Computational Excellence Network.

The Arctic region is under pressure. Sea levels are changing,

ice is melting, navigational routes are opening up, resource

exploration is becoming a possibility, and international

boundaries are becoming a focus for countries. One of the

technologies that can have an impact in all these fields is GPS,

and as such it is important to be aware of the accuracy

achievable and region-specific error sources, when operating

in the Arctic region. Positioning results from GPS can be

impacted my many error sources, and it is important to reduce

their effect to obtain high accuracy results. Minimising the effect

of atmospheric refraction is one of the most challenging tasks

for high accuracy users. It was one component of refraction

(the dry component of neutral atmosphere) that was the focus

of the research carried out. This neutral atmosphere rises to

a height of about 80 km from the Earth’s surface, and above

that is the ionosphere. The research has determined that for

high accuracy users of GPS, a modified processing strategy

for dealing with the neutral atmosphere can lead to

improved results.

for dry gradients in the investigated area

were evaluated using Environment Canada’s weather models

by modifying the scientific processing software “Bernese”,

developed at the University of Berne

refraction can affect the estimated latitude component by

more than 2 mm. There was no significant systematic affect

on the longitude component

either gradient component was not as clear as the correlation

with latitude

be important in applications such as long term geodynamics

studies, maintenance of geodetic control networks, and

climatic studies.

Conclusions

About the study

10 11

The results published here are abbreviated results from

Reza Ghoddousi-Fard’s PhD thesis:

Ghoddousi-Fard, Reza (2009). Modelling Tropospheric Gradients

and Parameters from NWP Models: Effects on GPS Estimates.

Ph.D. dissertation, Department of Geodesy and Geomatics

Engineering, Technical Report No. 264, University of New

Brunswick, Fredericton, New Brunswick, Canada, 216 pp.

Other contributions resulting from the above work are:

Ghoddousi-Fard, R. P. Dare and R. B. Langley. (2009).

“Tropospheric Delay Gradients from Numerical Weather

Prediction Models: Effects on GPS Estimated Parameters.”

GPS Solutions, Vol. 13, No. 4, September, pp. 281-291.

Ghoddousi-Fard, R. and P. Dare (2007) “A Climatic Based

Asymmetric Mapping Function Using a Dual Radiosonde

Raytracing Approach”. Proceedings of the 20th International

Technical Meeting of the Satellite Division of the Institute of

Navigation “ION GNSS 2007”, Fort Worth, Texas, USA, 25-28

September , pp 2870-2879.

Ghoddousi-Fard, R. and P. Dare (2006) “Comparing Various

GPS Neutral Atmospheric Delay Mitigation Strategies: A High

Latitude Experiment”. Proceedings of the 19th International

Technical Meeting of the Satellite Division of the Institute of

Navigation “ION GNSS 2006”, Fort Worth, Texas, USA,

September, pp. 1945-1953.

Contact

Professor Peter Dare

e dare@unb.ca

Reza Ghoddousi-Fard

e Reza.Ghoddousi-Fard@NRCan-RNCan.gc.ca

Department of Geodesy & Geomatics Engineering

University of New Brunswick

PO Box 4400

Fredericton

New Brunswick

Canada, E3B 5A3

http://gge.unb.ca

Further information

‘‘ ‘‘ By understanding the

refraction better, the

estimated latitude

was improved

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