INFLUENCE OF IONOSPHERE IN ARCTIC AND ANTARTIC REGIONS ON GPS POSITIONING PRECISION

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1 INFLUENCE OF IONOSPHERE IN ARCTIC AND ANTARTIC REGIONS ON GPS POSITIONING PRECISION A. Krankowski 1, L. W. Baran 1, I. I. Shagimuratov 2, J. Cisak 3 1 Institute of Geodesy, University of Warmia and Mazury in Olsztyn OLSZTYN, POLAND, kand@moskit.uwm.edu.pl; Fax: West Department of the Institute of Geomagnetism, Ionosphere and Radio Wave Propagation (IZMIRAN) of the Russian Academy of Sciences KALININGRAD, RUSSIA 3 Institute of Geodesy and Cartography, Warsaw, POLAND ABSTRACT Results of analysis of the influence of ionosphere over the Arctic and Antarctic regions on positioning precision are presented in the paper. The analysis relies on studying repeatability of vectors co-ordinates. Vectors of different length (130km km) were investigated during the quiet and disturbed ionosphere (ionospheric storms).the GPS observations from following periods of the ionospheric storms: February, September, September and October 1999, were analysed. The IGS and SCAR permanent observations from Onsala (57 0 N, 12 0 E), Kootwijk (52 0 N, 6 0 E), Kiruna (67,8 0 N, 20,9 0 E), Metsahovi (60 0 N, 24 0 E), Tromso (69,6 0 N, 18,9 0 E), O Higgins (-63 0 N, E), Davis ( N, 78 0 E), Mawson (-68 0 N, 63 0 E), East Ongle Island (-69 0 N, 40 0 E) and Arctowski (-62 0 N, 58 0 W) were taken for analysis. Bernese ver.4.2 software was used for the analysis of observational data of the eight and twelve-hour sessions. Results were referred to those obtained from 24-hours sessions from periods of the quiet ionosphere. Strong correlation between TEC changes and all vector components were obtained. The changing conditions of ionosphere mostly affect height component. The height differences obtained from the quiet and disturbed ionosphere reach a few dozen millimeters even for 130km long vectors. INTRODUCTION To determine the ionospheric TEC, a geometry-free linear combination is used. It contains the ionospheric delay and the ambiguities for phase measurements and equipment biases for code measurements. The relationship between ionospheric delay and the TEC, and difference between dual-frequency code (P) and phase () measurements may be written (Baran et al, 1997): P[ m] M TEC [ m] M TEC / cos / cos z A p z A Here TEC is the vertical electron content, M is a scale factor, p, are noise terms, A p and A are equipment biases (A contains the phase ambiguity), z is the zenith angle of the ray at the subionospheric point. p

2 TEC VARIATIONS DURING STORM The magnetic storm under consideration occurred on September Diurnal values of TEC on September 1999 for every single station were estimated. Diurnal variations at different sites are given in Fig.1. At the first day of the storm i.e. 27 September, a significant TEC increase took place. The positive effect of storm during daytime occurred at all sites. On the second day of storm the negative phase only for the high-latitude station Thule was found. On the others stations the positive disturbance lasted through the storm. Fig. 1 Space-time occurrence of storm in Total Electron Content during September 1999 (TEC given in el/m 2 )

3 STUDYING OF THE REPEATABILITY OF THE COORDINATES AND THE LENGTH OF THE VECTOR DURING THE IONOSPHERIC STORMS Analysis relied on studying of repeatability of the co-ordinates and the length of the vectors connecting Onsala with Metsahovi, Hoefn, Thule, Kiruna, Ny-Alesund, Reykjavik and Tromso stations, Davis with Mawson and Casey stations and O Higgins with Arctowski station. Coordinates of the Onsala, Davis and O Higgins stations were fixed. The distances of all mentioned above stations to Onsala, Davis and O Higgins respectively are given in Table 1. Table 1. The lengths of vectors Station Distance (km) Station Distance (km) ONSALA Metsahovi 784 Reykjavik 1956 Kiruna 1250 Ny-Alesund 2387 Tromso 1406 Thule 3622 Hoefn 1640 DAVIS Mawson 636 Casey 1397 O HIGGINS Arctowski 132 Three periods of the ionospheric storms from February, September, September and October of 1999 were analysed. Bernese v.4.2 software was used for the analysis of observational data of the six, eight and twelve-hours sessions. Results were referred to those obtained from 24-hours sessions. In Figures 2-7 the examples of oscillations in N, E and U for chosen vectors are shown. On February 16 th and on September 27 th, 1999, during maximum of ionospheric storm one can see the extreme in determined components. Fig. 2 Day-to-day changes of North, East and Up components obtained from 24-hours sessions The best repeatability was achieved for 24-hours sessions. For Onsala-Metsahovi vector, shown in Fig. 2, maximum discrepancies amount to: N=5mm, E=13mm and U=20mm. Variations vector length reached 12mm. When ionosphere is quiet (TEC amounts to 2-7 TECU) the nighttime observations have the main influence on 24-hours results. This conclusion was confirmed by results obtained from daytime observations carried out between The largest discrepancies occurred for U component. Day-to-day changes achieved 23mm in U, and 13mm in the vector length (Fig. 3).

4 Fig. 3 Day-to-day changes of North, East and Up components obtained from 12-hours sessions As one can see in Fig. 4, the highest influence of ionosphere was observed during processing observations carried out between , when TEC showed largest changes. During ionospheric storm (27 and 28 September) TEC values changed from 6 to 23 TECU for Metsahovi station (Fig. 1). One can also see the largest changes for U component. Day-to-day changes reached 32mm in U, and 19 mm in the vector length (Fig. 3). Fig. 4 Day-to-day changes of North, East and Up components obtained from 8-hours sessions Mentioned above analyses showed, that influence of ionospheric storms can be observed for vectors longer then 650 km. However, period of ionospheric storm between 15 and 18 February 1999, showed the occurrence of unfavorable influence of ionospheric storms on analysed components even for length of vector 132 km (O Higgins-Arctowski). The highest discrepancies amounted for U component too. Day-to-day changes (using observations carried out between ) amounted to 220 mm in U, and 14mm for the vector length (Figures 5-7). Fig. 5 Day-to-day changes of North, East and Up components obtained from 24-hours sessions for Arctowski station and changes of O Higgins-Arctowski vector length

5 Fig. 6 5 Day-to-day changes of North, East and Up components obtained from 12-hours sessions for Arctowski station and changes of O Higgins-Arctowski vector length Fig. 7 5 Day-to-day changes of North, East and Up components obtained from 6-hours sessions for Arctowski station and changes of of O Higgins-Arctowski vector length CONCLUSIONS The ionosphere in Arctic and Antarctic regions has considerable influence on the accuracy determination of the vectors. The periods of the increased ionospheric activity were analysed. Strong correlation between TEC changes and North, East and Up vector components were obtained. The changing conditions of ionosphere mostly affect height component. The height differences obtained from the quiet and disturbed ionosphere reach a few dozen of millimeters even for 132km long vector (O Higgins-Arctowski). Because of the great dynamics of the TEC during the ionospheric storms the special attention should be paid to the vector co-ordinates obtained from semidiurnal GPS sessions. REFERENCES Baran L.W., Shagimuratov I.I., Tepenitsina N.J., 1997, The use of GPS for Ionospheric Studies, Artificial Satellites, Vol.32, No 1, pp Baran L.W., Krankowski A., I.I. Shagimuratov, 2000, Influence of ionospheric storm on positioning precision, Geophysical Research Abstracts of XXV General Assembly of the European Geophysical Society, Nice, France, April 2000, Vol. 2. Schear S., Beutler G., Mervart L., Rothacher M., Wild U., 1995, Global and Regional Ionosphere Models Using the GPS Double Difference Phase Observable, IGS Workshop Proceedings on Special Topics and New Directions, GeoForschungsZentrum, Potsdam, pp