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TS 3 – Poster Presentation

INGEO 2011 – 5th International Conference on Engineering Surveying Brijuni, Croatia, September 22-24, 2011

Railway Track Deformation Surveying Using GNSS Realand Virtual Reference

Švábenský, O.1, Plášek, O.2 and Bureš, J.1

1 Brno University of Technology, Faculty of Civil Engineering, Institute of Geodesy, Veve í331/95, 602 00 Brno, Czech Republic, Web site: www.fce.vutbr.cz/ Tel.: +420 541 147 211, E-mail: svabensky.o@fce.vutbr.cz, bures.j@fce.vutbr.cz 2 Brno University of Technology, Faculty of Civil Engineering Department of Railway Structures and Constructions, Veve í 331/95, 602 00 Brno, Czech Republic Tel.: +420 541 147 320, E-mail: plasek.o@fce.vutbr.cz

Abstract

Comparison of real and virtual reference station performance in post-processed kinematic GNSS deformation surveying of a railway track superstructure. Subject of measurement is a section of the railway line Znojmo-Retz within the newly reconstructed viaduct over Thaya valley. Surveying of track deformations using GNSS and other measuring techniques started in November 2009 and continues in regular three month intervals. The originally designed mobile carrier of the GNSS antenna and the reflecting EDM prisms enables simultaneous satellite and classical distance/angle measurements is deployed. Results show covariance with the physical parameters of the bridge construction, mainly with the temperature.

Key words: deformation surveying, railway track, GNSS, virtual reference station

1 INTRODUCTION

One of the ways to use the VRS concept is an a-posteriori generation of virtual data for post-processed GNSS applications. This approach has its undeniable advantages, but the user must also reckon with some adverse circumstances. The main benefits include mainly economic savings - all receivers that are available for the task can be used as rovers. The biggest drawback to the use of a-posteriori generated VRS data is the uncertainty whether the data actually becomes available (e.g. failure of network solutions in extreme weather conditions, strong ionospheric activity or other reasons). Another fact is that the VRS data is generally "thinner" than the real data from local reference station (LR) and also have a slightly larger variance, which is due to the technology of VRS data generation, e.g. (Wanninger, 2002). Analyses showed that the VRS data contain in average 80 – 90% records compared with LR data, with 0.6 to 1.1 lower number of satellites per epoch (Bureš and Švábenský, 2010).

Institute of Geodesy of BUT in the last few years dealt with practical problems of the postprocessing VRS use in several GNSS deformation surveys. One of the last is the application of the VRS concept in the deformation monitoring of the reconstructed railway

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viaduct near Znojmo (Southern Moravia). Basic information concerning the reconstructed bridge structure is presented in the first part of the paper. The paper is further focused on long-term monitoring which is aimed to the assessment of an interaction of the continuous welded rail and the bridge structure with large expansion length. Parts of the monitoring are permanent measurements of the bridge structure expansion, displacement of rails, temperatures, ballast pressure, stresses in the bridge end, and also the displacements of the track by geodetic surveying methods. On base of the data collected in several realized measuring epochs it was possible to compare the real and virtual reference deployment.

2 SUBJECT OF DEFORMATION MONITORING

The reconstruction of the Znojmo viaduct included removal of existing bridge structure, consolidation of abutments, construction of new supporting sills and the bridge wings on the reorganized substructure, and installation of the new steel structure with upper orthotropic deck. Abutment sills and wing walls were both completely newly built. Both abutments were also reinforced by vertical micropiles. The remediation injections of the bridge abutments have been necessary

Expansion length of the new bridge structure is over 220 m, which is much more than the RIA regulation allows for bridge with continuous welded rail (which is 80 m for a given arrangement). Because of the track dilation the reinforced track was designed, consisting of more rigid crossing bearers which are connected by the secondary rails in the regions of greatest expansion. The rails in these regions are fastened by fastenings with reduced clamp force that allows for free sliding of rails. Because of minimization of ballast dilution due to the bridge dilation the track superstructure on the bridge is closed by the bridge partition. The transition from continuous welded rail bridge to the rail body went handled on both sides of the bridge with the rail expansion facilities to protect the bridge from longitudinal shifts caused by temperature changes and operational effects. Main rail expansion joint (EJ 2) is located on the moving bridge bearings, supplemented by another three rail expansion joints (EJ 1, 3, 4) located on both sides of the bridge.

The monitoring of Znojmo viaduct structure includes the continuous pressure and strain measurement on the bridge partition, monitoring of the bridge expansion, dilation of rails, strain in rails along the bridge length, temperature of the bridge, rails and air. The foil resistance strain gauge transducers, displacement sensors, and temperature sensors are used. Measurements are performed continuously, data are transfered on-line through GSM to recording computer. Geodetic measuring methods are used in epoch deformation surveying of the bridge and railway track.

3 SURVEYING OF TRACK ALIGNEMENT CHANGES

Continuous railway track and bridge monitoring covers the whole bridge (220.97 m). Geodetic measuring methods are applied epoch-wise, covering the bridge and adjacent track sections in north-east and south-west foreground, including three points P1, P2, P3 in the bridge deck. Control points are established at places above the bridge supports and on the two abutments (JT, JG, ST, SG). Another control point TARS is set up outside the bridge structure - on top of the retaining wall in lateral distance of 17 m from the axis of the bridge. The layout of control and measured points is shown in Fig. 1.

Švábenský, O. et al.: Railway Track Deformation Surveying Using GNSS … 87

Figure 1 Znojmo viaduct – layout scheme of control and measured points

The deformation surveying uses a combination of polar method with total stations, and GNSS kinematic Stop & Go method. It means that the displacements of measured rail points are determined by polar method from two different stations (JT, ST) and by GNSS survey.

To implement the chosen methodology an original measuring carriage was designed. The mobile rail carriage is equipped with a precise adjustment and readers for both the rail marks, and it also serves as a carrier of EDM reflecting prisms and a carrier of GNSS antenna for satellite measurement (Fig. 2). In each transversal profile the position of one rail point (in this case the right) is determined by direct measurement, position of second rail point (the left) is determined relative to the right rail point from changes in reading of the auxiliary millimeter scale attached to a rectangular arm (accuracy of reading is 0.1 mm). If the measurement is interrupted due to a passing train, the measuring carriage is moved off the track (without interrupting the reception of satellite signals) and then re-deployed with repeated check surveying of the last two track points.

To determine the location of points on the rail strip the GNSS kinematic method with static intervals (Stop & Go) had been used. The individual rail points were observed in intervals of the average length of 45 seconds, using the data recording frequency 1 Hz. Localreferences were positioned at the control stations JG and TARS. The GNSS measurements were also referenced to the nearest CZEPOS permanent station CZNO in Znojmo, at a distance of 2 km from the bridge.

Stability of the control points JG, SG on the bridge abutments had been verified in each measuring epoch by static GNSS measurement in respect to stations TARS, CZNO, and also in respect to fixed Virtual Reference Station (VRS) position in the centre of the bridge deck.

Expected standard deviation of the one relative horizontal position component was less than 3 mm for static GNSS measurements, of about 5 mm for the GNSS Stop & Go measurements, and of about 3 mm for the polar measurements using a total station

Figure 2 Rail measuring carriage

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4 DATA PROCESSING AND EVALUATION

The results of the evaluation of GNSS phase measurements are the coordinate differences of the observed rail points in respect to the position of the local reference station (JG) in the global geocentric coordinate system. The calculations were performed using the Leica LGO software. Basic processing parameters in all epochs were: the elevation mask angle 10 °, the precise satellite ephemeris from IGS resources, the standard model of ionosphere, the Hopfield troposphere model, the final solution using the combination of L1 + L2 frequencies.

In further processing steps the calculated geocentric coordinate differences dX, dY, dZwere transformed into a local Cartesian horizontal system NEU. This system has its origin in station JG, N and E axes lie in the local horizontal plane and the U axis direction coincides with the local vertical. The N axis is oriented positively to the northern branch of the local meridian, the E axis is directed to the east. The transformation equations are e.g. (Hefty, 2004):

dZ

dY

dX

BLBLB

LL

BLBLB

dZ

dY

dX

LBR

u

e

n

sinsincoscoscos

0cossin

cossinsincossin

, ..., (1)

where B , L are the ellipsoidal latitude and longitude of the station JG.

The values dn, de obtained as differences between the epochs compared are then converted to the displacement components ds, dq using the relations

de

dn

dq

ds

cossin

sincos ......, (2)

where is the track axis azimuth, in arc it is the tangential azimuth in a particular point.

One of the components (ds) is in the axial direction of the track and the second component (dq) is in the transverse direction.

Analysis showed that the track longitudinal displacements determined by single geodetic methods well correspond. In Fig. 3 the dilation of the track bridge section in epoch E3 is shown as an example, as resulting from GNSS and polar surveys.

The GNSS monitoring has proved that so far the both bridge abutments are stable in time – the evaluated differences in the horizontal position of points TARS, JG, SG in respect to control stations CZNO and VRS in all epochs were within twice the horizontal standard error in determining these differences (assumed standard deviation of one horizontal coordinate component determined by GNSS relative static positioning: e,n = 0,003m + 1ppm) – for actual values of the differences between epochs see Table 1.

Švábenský, O. et al.: Railway Track Deformation Surveying Using GNSS … 89

Figure 3 Measured dilation of the track bridge section in epoch E3

In Table 2 the accuracy ratios of kinematic Stop & Go using the VRS and LRS surveys are assessed from the results of several epochs, that illustrate the accuracy lowering by VRS use for the single coordinate components and the baseline length.

Table 1 – Differences in Relative Position of Control Points (static GPS) difference [m] difference [m] difference [m] difference [m]

baseline dn(1-2) de(1-2) dn(1-3) de(1-3) dn(1-4) de(1-4) dn(1-5) de(1-5)

VRS - JG -0,0023 -0,0013 -0,0029 0,0015 -0,0023 -0,0015 0,0003 -0,0033

TARS -JG -0,0028 -0,0035 -0,0036 0,0031 -0,0022 0,0002 0,0002 0,0014

CZNO - JG -0,0045 -0,0021 -0,0034 0,0028 0,0003 -0,0002 -0,0014 0,0004

TARS - SG -0,0022 0,0025 -0,0005 0,0013 0,0014 -0,0020 0,0010 -0,0017

CZNO - TARS -0,0005 0,0000 0,0009 0,0005 -0,0025 -0,0003 -0,0014 -0,0013

Table 2 – S&G Accuracy Ratios VRS/LR deform. st.dev. VRS / st.dev. LR

epoch dX dY dZ d SD

E1 2,1 1,8 1,6 1,8

E2 1,4 1,7 2,1 1,9

E3 1,7 2,3 2,6 2,0

E4 3,0 2,3 2,3 1,8

Special care must be paid to the reference frame integrity between epochs in case of VRS deployment, i.e. to any modifications of permanent network coordinates in course of time. In this case such problem appeared at beginning of the 2011 year when the coordinates of the CZEPOS stations were officially changed. In consequence of this the VRS coordinates generated for the original position also have changed. Therefore for continuation of the deformation monitoring using the VRS it was necessary to introduce corrections to evaluated coordinate differences according to the change of the nearest CZEPOS station CZNO coordinates.

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5 CONCLUSIONS

The results of epoch and continuous deformation monitoring of the Znojmo viaduct indicate that the seasonal and daily dilation changes of the track and bridge construction are clearly related to changes in temperature. The monitoring also fully proved the proper function of installed rail expansion joints. On base of the measured displacements an equivalent coefficient of thermal expansion could be determined, which reflects the real behavior of the bridge structure. Linear regression using LS estimation gave current equivalent expansion coefficient value m = 9,0.10

-6 K-1. The data of geodetic and other measuring methods were used for the estimation.

It has been proved that it is possible to perform GNSS deformation surveying employing the VRS, but somewhat lower horizontal and vertical accuracy (in average by a factor 2 to 3) is to be awaited according to the way of VRS data generation within the CZEPOS permanent network. Similar findings are described e.g. in (Švábenský, 2008) (Pesci et al., 2008), (Bureš and Švábenský, 2010). The GNSS static method proved to be fully reliable for stability monitoring of the control points using longer observation intervals, which is valid for post-processing VRS use too. The VRS employment in the GNSS kinematic methods with short observing times is more sensitive to disturbing factors influencing the signal reception, e.g. multipath and diffraction effects.

In present time there is going on gradual hardware modification at all the CZEPOS stations (antenna changes, firmware upgrade) which will enable inclusion of the GLONASS data into the network solution in near future. This will have an impact also on VRS service as to the enhancement and better reliability.

Acknowledgement

The paper was elaborated with support of the project MSM 0021630519, and with support of the EU project reg. nr. CZ 1.05/2.1.00/03.0097 within the regional centre “AdMaS”.

REFERENCES

BUREŠ, J., ŠVÁBENSKÝ O.: 2010, Applications of CZEPOS Network Products in Engineering Geodesy. In: Proceedings of the Seminar “Satellite Methods in Geodesy and Cadastre”, Brno, pp.95-103 (in Czech).

HEFTY, J.: 2004, Global Positioning System in Four-Dimensional Geodesy, Bratislava, STU. (in Slovak).

PESCI, A., LODDO, F., CENNI, N., TEZA, G., CASULA, G.: 2008, Analyzing Virtual Reference Station for GPS Surveying: Experiments and Applications in a Test Site of the Northern Appenines (Italy). Annals of Geophysics, Vol. 51, N. 4, pp. 619-631.

ŠVÁBENSKÝ, O.: 2008, Employment of Virtual Reference Station in Deformation Surveys of Railway Track. In: Proceedings of 4th International Conference INGEO 2008, TS4, Bratislava (CD ROM).

WANNINGER, L.: 2002, Virtual Reference Stations for Centimetre-Level Kinematic Positioning. In: Proceedings of the ION GPS 2002, Portland, pp. 1400-1407.