FiBRESERIES ASSESSMENT OF ATMOSPHERIC ERRORS ON GPS IN THE ARCTIC FINDINGS IN BUILT AND RURAL ENVIRONMENTS OCTOBER 2011

Transcription

1 FINDINGS IN BUILT AND RURAL ENVIRONMENTS OCTOBER 2011 FiBRESERIES ASSESSMENT OF ATMOSPHERIC ERRORS ON GPS IN THE ARCTIC Peter Dare and Reza Ghoddousi-Fard Department of Geodesy and Geomatics Engineering University of New Brunswick (UNB), Canada

2 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 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. 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 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: Published by: RICS Parliament Square London SW1P 3AD 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

3 Key findings GPS signal refraction 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 highaccuracy 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 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 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. If you want the best accuracy, you have to really study what influences the measurements 04 05

4 Why this research is important GPS positioning in the Arctic 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. 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

5 Analysis of refraction dependency on azimuth 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 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 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 2: Wet refraction results from UNB3m, NWP, and WVR (raw and smoothed values) onboard the CCGS Amundsen Figure 1: Water Vapour Radiometer (WVP) onboard CCGS Amundsen 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 ). Figure 3: Thickness of the atmosphere at 9 p.m. GMT, September 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

6 Conclusions Further information 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. About the study 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. 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 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, September, pp 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 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. By understanding the refraction better, the estimated latitude was improved 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 5A

7 rics.org/research RICS HQ Parliament Square London SW1P 3AD Worldwide media enquiries: e pressoffice@rics.org Contact Centre: e contactrics@rics.org t +44 (0) f +44 (0) Advancing standards in land, property and construction. RICS is the world s leading qualification when it comes to professional standards in land, property and construction. In a world where more and more people, governments, banks and commercial organisations demand greater certainty of professional standards and ethics, attaining RICS status is the recognised mark of property professionalism. Over property professionals working in the major established and emerging economies of the world have already recognised the importance of securing RICS status by becoming members. RICS is an independent professional body originally established in the UK by Royal Charter. Since 1868, RICS has been committed to setting and upholding the highest standards of excellence and integrity providing impartial, authoritative advice on key issues affecting businesses and society. RICS is a regulator of both its individual members and firms enabling it to maintain the highest standards and providing the basis for unparalleled client confidence in the sector. RICS has a worldwide network. For further information simply contact the relevant RICS office or our Contact Centre. Europe (excluding United Kingdom and Ireland) Rue Ducale Brussels Belgium t f ricseurope@rics.org Asia Room 2203 Hopewell Centre 183 Queen s Road East Wanchai Hong Kong t f ricsasia@rics.org Americas One Grand Central Place 60 East 42nd Street Suite 2810 New York USA t f ricsamericas@rics.org Parliament Square London SW1P 3AD t +44 (0) f +44 (0) contactrics@rics.org Africa PO Box 3400 Witkoppen 2068 South Africa t f ricsafrica@rics.org Ireland 38 Merrion Square Dublin 2 Ireland t f ricsireland@rics.org Oceania Suite 2, Level 16 1 Castlereagh Street Sydney, NSW 2000 Australia t f info@rics.org.au Middle East Office G14, Block 3 Knowledge Village Dubai United Arab Emirates t f ricsmenea@rics.org India 48 & 49 Centrum Plaza Sector Road Sector 53, Gurgaon India t f ricsindia@rics.org OCTOBER 2011/ VP/1012RS