The analysis of the stability of permanent GPS station VILNIUS (VLNS)/Nuolat veikiancios VILNIAUS (VLNS) GPS stoties stabilumo analize.

Parseliunas, Eimuntas ; Kolosovskis, Ricardas ; Putrimas, Raimundas 等

1. Introduction

GPS station VILNIUS, which is a part of the EUREF permanent network (EPN), is the reference point of ETRS89 (European Terrestrial Reference System of 1989) in Lithuania (Fig. 1) (Investigations ... 1999; GPS station ... 2002; Investigation ... 2004; Development ... 2007, 2008; Parseliunas, Kolosovskis 2005; Parseliunas et al. 2008; Parseliunas et al. 2010a, 2010b; Zakarevicius et al. 2008). The main information on the station and GPS data are available on the Internet <http://www.epncb.oma.be> (EUREF. 2011).

Lithuania has been participating in the activities of the EUREF permanent network since 1996, when GPS station VILNIUS started regular continuous observations of GPS satellites. The GPS station was established with a help of the Onsala Space Observatory of Chalmers University (Sweden). The GPS station was mounted in the territory of Vilnius international airport. This station has been operating since the end of 1996, providing daily files of 24 hours continuous measurements (phase and codes on both frequencies) with a sampling rate of 30 seconds.

IERS four character identifier VLNS and DOMES number 1080M001 were assigned to VILNIUS GPS station by the International Earth Rotation Service in 1999. VILNIUS station is operated and maintained by the Institute of Geodesy, Vilnius Gediminas Technical University. The main task of the VLNS permanent GPS station is to take part in EUREF activities and serve as reference to GPS campaigns in Lithuania.

The measurements of GPS station VLNS are used (together with the measurements performed by other stations) (Investigations. 1999; Development. 2007, 2008; Parseliunas et al. 2010a):

--for determining the coordinates of the VLNS station and their changes in time,

--for studying the movements of Euro-Asian plate and intra-plate movements,

--for defining ETRS (European Terrestrial Reference System) in the Lithuania,

--as reference station data for GPS campaigns both in Lithuania and Europe.

The VLNS station has a category of class A, which means that its positions have 1 cm accuracy during all epochs of the time span of the used observations (Fig. 2).

The process of establishing a permanent GPS Station is very tightly connected with research and educational work of the Department of Geodesy and Cadastre and the Institute of Geodesy of Vilnius Gediminas Technical University (Investigations ... 1999; GPS station. 2002; Investigation ... 2004; Development ... 2007, 2008; Parseliunas, Kolosovskis 2005; Parseliunas et al. 2008, Parseliunas et al. 2010a, 2010b).

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2. Updated technical equipment

Following the standards of International GPS Service (IGS) aiming at precise geodynamical applications, at present, permanent GPS station VILNIUS is equipped with Ashtech GPS receiver Z-12 continuously tracking all visible GPS satellites making measurements using C/A-code, P-code and the carrier phase on both frequencies L1 and L2 and with Ashtech chock ring GPS antenna having Dorne Margolin element (Fig. 3).

[FIGURE 3 OMITTED]

It should be stressed that the GPS antenna with no touching has never been removed from the monument. In the only case, snow protecting radom was removed in 2002 as recommended by EPN CB. Such an issue gives good preconditions for the stability and reliability of researching the station position using records over a long time.

The standard operation of the Ashtech Z-12 GPS receiver is 30 s data sampling. The cut-off angle of the elevation mask was set to 5[degrees], which makes almost a clear view of the sky in all directions down to this cut-off angle (Fig. 4).

It should be mentioned that in the summer of 2010, the receiver gave the failure of the memory card that was replaced by a memory card of the same type of a receiver loaned by BKG Certainly, it was impossible to avoid a long data gap caused by replacing the memory card (Fig. 6).

The GPS reference mark consists of a steel plate with a forced centring hole embedded on the top of a 3.0 m high concrete pier, 30 cm in diameter. The pier is anchored to the ground by a concrete block of 1.5 m in size to a depth of 2.5 m.

The building of the GPS station close to the pier was removed in 2008. Instead, a special metal box was dedicated to put all necessary station equipment (Fig. 5).

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A special box has been already provided with standard electrical power (220 Volts AC). New power points were installed to supply the GPS receiver and modem. The receiver and modem have a backup with a 12V battery and smart UPS to overcome possible 220 V power source failures. The whole system is capable of working up to 12 h during power breaks without the interruption of any satellite observations. In case of longer power failure, the receiver itself stores the observed data into the receiver memory. In this case, an operation up to 5 days is possible. However, downloading manual data is then necessary.

The quality of the observed data is checked at the Institute of Geodesy following routine procedures and converted to RINEX and Hatanaka compressed formats. Then, the compressed data are sent to the centre of EUREF regional data and is available for regional processing of the EUREF permanent network. Next, GPS measurements are checked and written in the RINEX format (Gurtner, Mader 1990; Estey, Meertens 1999; Hatanaka 1996a, 1996b; Carpentier et al. 2004; Takacs, Bruyninx 2002; Kenyeres 2009; Lehmann, Figurski 2000; Kaniuth, Stuber 2005).

VLNS data are processed in four EUREF analysing centres--Nordic Geodetic Commission (NKG) from GPS week 0998, Geodetic Observatory Pecny (GOP) from GPS week 0998, Warsaw University of Technology (WUT) from GPS week 0998 and Military University of Technology (MUT)--from GPS week 1558, leading to a weekly combined solution and time series (Kenyers 2009).

3. Coordinate variations

VLNS data have been processed within the EUREF network since 1999 when VLNS was included in the EUREF permanent network. Regularly updated positions/velocities (see Table 1) are the result of the multi-year adjustment of all weekly combined EPN solutions in which outliers were eliminated and station discontinuities were applied (Kenyers 2009).

Two types of coordinate time series are distinguished:

1. Residual coordinate time series displaying the residuals of Helmert transformation between a cumulative coordinate/velocity solution based on weekly combined EPN solutions and on each of weekly combined EPN solutions:

--raw time series: coordinate outliers are not eliminated, one coordinate/velocity set for each station (Fig. 6),

--cleaned time series: coordinate outliers are eliminated, a new coordinate/velocity is estimated if necessary (Fig. 7).

2. Coordinate time series displaying the evolution of the coordinates the time, included in weekly combined EPN solutions:

--ITRS time series: coordinates of ITRS extracted from a weekly solution as is (Fig. 8),

--ETRS time series: coordinates of ITRS extracted from a weekly solution and transformed to ETRS89 (Fig. 9).

The three plots of each figure represent North-South (N), East-West (E) and Up (V) components showing the position of the GPS station with time.

Variations in a straight line show the uncertainty (or noise) in the measurement of station position from a single day of GPS data. There are no significant departures from the straight line which indicates the absence of any problem of data processing. Short period movements are mainly caused by earth tides, ocean and atmospheric loading effects, ionospheric and tropospheric refraction.

The gap in data between the spring of 1999 and that of 2002 occurred because of the fault of the GPS receiver and due to the fact that the VLNS station was temporarily excluded from the combined EUREF solution.

From the analysis of the above results, the following conclusion can be drawn: the maximum amplitude of the repeatability value is about 5 mm for the North and East component and 10 mm for the Up component.

The evolution of weekly VLNS coordinates in the ITRF system is shown in Fig. 9.

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The north and east horizontal components exhibit a uniform linear trend reflecting the systematic drift of the movement of the Eurasian tectonic plate towards northeast.

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Estimated ETRS89 coordinates (N, E, U) velocities and their uncertainties are calculated as follows (Fig. 10, 11, 12):

[v.sub.N] = -0.04 [+ or -] 0.15 mm/y,

[v.sub.E] = -0.48 [+ or -] 0.09 mm/y,

[v.sub.U] = 1.03 [+ or -] 0.48 mm/y.

The evolution of ETRS89 geocentric coordinates (X, Y, Z) is shown in Fig. 10, 11, 12.

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[FIGURE 11 OMITTED]

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The velocities of ETRS89 geocentric coordinates (X, Y, Z) are as follows:

[v.sub.X] = -20.0 mm/y,

[v.sub.Y] = 13.6 mm/y,

[v.sub.Z] = 8.0 mm/y

4. Conclusions

1. VILNIUS (VLNS) permanent GPS station is reference to GPS activities in Lithuania. The more than ten-year period of observations has proved the good quality and stability of the reference site, GPS antenna and receiver.

2. It should be stressed that the GPS antenna has not been changed during the whole period of observations which gives good premises for the stability and reliability of the station.

3. The estimated ETRS89 coordinates (N, E, U) velocities and their uncertainties are calculated as follows: [v.sub.N]= -0.04 [+ or -] 0.15 mm/y, [v.sub.E] = -0.48 [+ or -] 0.09 mm/y, [v.sub.U] = 1.03 [+ or -] 0.48 mm/y.

4. The range of variability in the jump size is similar to the North and East component (less than 5.0 mm). The range of variability in the jump size is larger for the Up component (up to 10.0 mm) rather than for the North and East ones. The Up component has seasonal changes in the form of a sinusoid.

doi: 10.3846/13921541.2011.626261

Acknowledgements

The authors are grateful for Onsala Space Observatory and BKG for technical support provided for the VLNS station.

The paper was prepared using materials included in the contract No 641-MA between the Institute of Geodesy, VGTU and National Land Service.

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Eimuntas Parseliunas (1), Ricardas Kolosovskis, Raimundas Putrimas, Arunas Buga

Institute of Geodesy, Vilnius Gediminas Technical University, Sauletekio al. 11, LT-10223 Vilnius, Lithuania

E-mail: (1) eimis@vgtu.lt (corresponding author)

Received 05 June 2011; accepted 07 September 2011

Eimuntas PARSELIUNAS. Assoc. Prof., Dr at the Department of Geodesy and Cadastre, Vilnius Gediminas Technical University, Sauletekio al. 11, LT-10223 Vilnius, Lithuania (Ph +370 5 274 4703, Fax +370 5 274 4705), e-mail: eimis@vgtu.lt. Doctor (1992). The author of two teaching books and more than 50 scientific papers. Participated in a number of international conferences. Research interests: graph theory in geodesy, adjustment of geodetic networks, geo-information systems.

Ricardas KOLOSOVSKIS. Assistant at the Department of Geodesy and Cadastre, Vilnius Gediminas Technical University, Sauletekio al. 11, LT-10223 Vilnius, Lithuania (Ph +370 5 274 4703, Fax +370 5 274 4705), e-mail: ricardas.kolo sovskis@vgtu.lt. A graduate from Vilnius Gediminas Technical University, (MSc in 1994). Research training at the GPS centre of Aalborg University, 1997. Research interests: GPS observations and networks.

Raimundas PUTRIMAS. Assoc. Prof., Dr at the Department of Geodesy and Cadastre, Vilnius Gediminas Technical University. Sauletekio al. 11, LT-10223 Vilnius, Lithuania (Ph +370 5 274 4703, Fax +370 5 274 4705), e-mail: RaiPut@vgtu.lt. Author of more than 30 scientific papers. Participated in a number of international conferences. Research interests: calibration of geodetic instruments, adjustment of geodetic networks.

Arunas BUGA. Assistant at the Department of Geodesy and Cadastre, Vilnius Gediminas Technical University, Sauletekio al. 11, LT-10223 Vilnius, Lithuania (Ph +370 5 274 4703, Fax +370 5 274 4705), e-mail: arunas.buga@vgtu.lt. A graduate from Vilnius Gediminas Technical University (MSc in 1991). Research training at the GPS centre of Aalborg University, 1991, 1997, 1998. Research interests: calibration of geodetic instruments, GPS observations and networks.

Table 1. The positions (geocentric coordinates), velocities and
accuracy (cumulative solution of GPS weeks 860-160) of the VLNS
station

X/Vx, m       Y/Vy, m       Z/Vz, m

Reference frame ETRF2000 in the epoch of 2005.0

3343600.971   1580417.554   5179337.122
0.000         0.000         0.000

Reference frame ITRF2005 in the epoch of 2005.0

3343600.621   1580417.740   5179337.298
0.000         0.000         0.000

[sigma]X/[sigma]Vx, m   [sigma]Y/[sigma]Vy, m   [sigma]Z/[sigma]Vz, m

Reference frame ETRF2000 in the epoch of 2005.0

0.0002                  -0.0004                 -0.0001
0.0001                  0.0000                  0.0001

Reference frame ITRF2005 in the epoch of 2005.0

-0.0182                 0.0142                  0.0087
0.0001                  0.0000                  0.0001