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
03
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
04
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
Reza Ghoddousi-Fard
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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