279

TS 6 – GNSS

INGEO 2011 – 5th International Conference on Engineering Surveying

Brijuni, Croatia, September 22-24, 2011

The Application of GNSS VRS Service in Industrial Survey

Pavasovi , M., Rezo, M. and Baši , T.

University of Zagreb – Faculty of Geodesy, Fra Andrija Ka i a Mioši a 26, 10000 Zagreb,

Croatia, Web site: www.geof.unizg.hr

E-mail: marko.pavasovic@geof.hr, milan.rezo@geof.hr, tomislav.basic@geof.hr

Abstract

By establishment of CROatian POsitioning System (CROPOS), the network of CORS

GNSS stations, the application of satellite survey methods has been enabled among wide

geodetic population and almost in all geodetic branches. When speaking of GNSS data post-

processing, two aspects are possible: survey data processing at locations (points) with

unknown position (coordinates) and use of CORS (Continuously Operating Reference

Station) data as reference points or VRS (Virtual Reference Station) generation

(interpolation/extrapolation) of measured data (RINEX files) at locations (points) with known

position coordinates and use of CORS data as reference points. In this work, on example of

hydro power plant fundamental geodetic network, the comparison of coordinates determined

by GNSS measurements at network points and coordinates determined by VRS generation

(interpolation/extrapolation) from CROPOS will be given as long as recommendations of

possibility of application of VRS generated data in CROPOS.

Key words: post-processing, CROPOS, VRS, interpolation, extrapolation, hydro power plant,

fundamental network

1 INTRODUCTION

Croatian Positioning System (CROPOS) consists of 30 continuously operating GNSS

reference stations (CORS) distanced approximately 70 km from each other to uniformly cover

the territory of Republic of Croatia (Figure 1). The system was established during 2008 and

put in official use on December, 8th

2008. It provides navigation and positioning to all users

24/7 through three types of services (URL-1):

Differential positioning service (Cro. DSP) – represents networked solution of code

measurements with 0.3 -0.5 m positioning accuracy. Data transfer is realized via

GPRS/UMTS and NTRIP with RTCM 2.3

High-precise real-time positioning service (Cro. VPPS) – represent networked solution

of phase measurements with 2 cm (2D) and 4 cm (3D) accuracy. Data transfer is

realized via GPRS/UMTS and NTRIP with RTCM 2.3 and RTCM 3.1

Geodetic precise positioning service (Cro. GPPS) – that is post-processing by using

RINEX VRS or RINEX CORS data generated and downloaded from official CROPOS

web site

280 INGEO 2011

All of the station coordinates are determined that is adjusted in ITRF2005 (e 2008.83,

GPS week 1503) by using Bernese 5.0 GPS software and transformed to ETRF00 (R05)

reference frame - the realization of ETRS89 which is an official coordinate reference system

of Republic of Croatia (Baši et al. 2004).

Figure 1 CROPOS reference stations

During 2010 Croatian State Geodetic Administration has signed the data exchange

contract with nearby positioning services of neighbor countries (Republic of Slovenia –

SIGNAL – 7 points, Republic of Hungary – GNSSnet.hu and Montenegro – MontePOS – 2

points) (Figure 1) so the quality of CROPOS services, especially in border, areas was

improved. Also, during 2011, Croatian official transformation model called “T7D” and new

geoid model “HRG2009” were implemented in CROPOS VPPS service

(CROPOS_VRS_HTRS96 and CROPOS_VRS_HDKS) to provide real-time online position

and height transformation from inherited compound coordinate reference system of Republic

of Croatia to new, official one and vice versa (Baši 2009).

2 VIRTUAL REFERENCE STATIONS (VRS)

Virtual reference stations (VRS) have significantly changed the approach to geodetic field

operations when speaking of real-time measurements as well as post-processing. The position

of VRS generation according to coordinates and data of nearest CORS stations by

interpolation/extrapolation method that is data generation inside or outside triangle figure

depending on the current position of moving GNSS receiver (real-time measurements) or

required position (post-processing) (Figure 2).

Real-time VRS field application procedure consists of several steps. First step includes

data transfer from CORS stations to data control centre (that is State Geodetic administration

when speaking of CROPOS). Then GNSS receiver through NMEA (National Marine

Electronics Association) protocol sends approximate position to control center where it

allocates the nearest CORS triangle for user and calculates VRS as a function of received

approximate position coordinates. Afterwards, control center transmits VRS data to field

GNSS receiver in form of RTCM (Radio Technical Commission for Maritime Services) 2.3 or

3.1 correction (Figure 2).

Pavasovi , M. et al.: The Application of GNSS VRS Service in Industrial Survey 281

Figure 2 VRS real-time application (left), interpolation (middle) and extrapolation (right)

Post-processing application includes VRS generation for exact known position. Official

CROPOS web site offers so called “RINEX-shop” where registered users can download

CORS data or generate VRS RINEX by entering an arbitrary position coordinates and

ellipsoidal height as well as date and time, duration and interval of measurements (URL-2).

3 THE APPLICATION OF CROPOS VRS IN INDUSTRIAL SURVEY

Selected test area for CROPOS VRS post-processing method was fundamental geodetic

network of dam at hydro power plant akovec on river Drava. Fundamental network consists

of seven points (TM1 – TM7) stabilized with concrete cylindrical columns. On its top, every

column has a screw with thread for setting up the geodetic instruments (GNSS antenna or

total station) as well as reflectors (Figure 3).

Figure 3 Fundamental network of dam at hydro power plant akovec (Google Earth)

Measurements were taken with two different methods:

terrestrial angle and distance measurements (triangulation-trilateration) using Sokkia

SET 1030R total station with angle measurement accuracy of 1 arc second and distance

measurement accuracy of ± (2 + 2 ppm x D) mm when using reflector (prism),

GNSS method using five Trimble R8 GNSS receivers where simultaneous occupation

of points TM7, TM4 and TM3 lasted for 7 hours and 30 min; points TM1, TM2, TM5

and TM6 were alternately occupied for 2 hours; elevation mask was set to 10° with

registration interval of 5 seconds using GPS and GLONASS satellites.

282 INGEO 2011

Terrestrial measurements were adjusted by least square method of indirect 2D

measurements with Best-Fit Columbus 3.5 software (Rezo 2011). GNSS measurements were

adjusted in three separate ways by Trimble Business Center 2.0 software using precise

satellite orbits in actual ITRF realization - ITRF2008 for measurement epoch 2011.36 (GPS

week: 1635, DOY: 131). Coordinate transformation between ITRF2005 and ITRF2008 was

performed by using EUREF official transformation parameters (URL-3) and transformation

procedure (Boucher et al. 2008). Transformation to ETRS89 is performed by official software

for coordinate transformation of Republic of Croatia called “T7D”.

Because of contract terms and data protection policy, further in this work, coordinates of network points won’t be given.

The quality and recommendations for using GNSS VRS method in industrial survey will

be given through comparison of adjusted 2D point distances obtained from terrestrial

measurements with distances calculated from coordinates which are obtained by three types

of GNSS 3D adjustment and transformed to projection coordinate reference system.

3.1 GNSS ADJUSTMENT METHODS AND RESULT COMPARISON

Comparison of point distances obtained from different GNSS adjustment methods with

distances obtained from 2D adjustment of terrestrial measurements are given in table 1 and

shown in Figure 4.

Table 1: Comparison of point distances obtained from different GNSS adjustment methods

Pavasovi , M. et al.: The Application of GNSS VRS Service in Industrial Survey 283

Figure 4 Comparison of point distances obtained from different GNSS adjustment methods

Reference adjustment (marked as adjustment “0” in Table 1) included distance

measurements with ± (2 + 2 ppm x D) mm accuracy and repeat angle measurements in three

cycles with defined deviation of ±5ý from two consecutive cycles at one angle. Thus defined

measurement criteria and force centering of the instrument and reflectors coordinates of

fundamental network points were calculated and used to obtain horizontal distances between

points. Precisely calculated distances were used as reference for comparison with distances

obtained from three types of GNSS measurements adjustment.

First method of GNSS 3D adjustment (marked as adjustment “0” in Table 1) was

performed by fixing three CROPOS CORS stations that form a triangle around fundamental

network: ZABok, BJELovar and AKOvec. ETRS89 coordinates of CROPOS CORS stations

were transformed to actual reference system of precise satellite orbits by above mentioned

procedure. Correlated vectors between known (fixed) points were turned off. Error in height

of GNSS antenna was set to 4 mm and centering error was set to 2 mm. This adjustment used

long baselines from known to unknown points of approximately 54 km. Adjustment

procedure used 95% precision confidential level (2.45 for 2D and 1.96 for 1D). Error

ellipses parameters of adjusted points are given in Table 2.

Statistics of distance comparison with terrestrial method are given through minimal,

maximal, average and standard deviation value of 2D distance differences. Maximum

difference between 2D distances of 25.8 mm (absolute value) is between points TM3 and

TM6. From figure 4 can been seen that those two points do not have a line of sight that is the

distance is just calculated from point coordinates. This also can be attributed to different

epoch of terrestrial due to GNSS measurements of 4-5 months so there can exist actual

displacement of points. Because of this inconsistency it is recommended to do all type of

measurements in close epochs to avoid possible displacements in different epochs. Standard

deviation of ± 9 mm indicates to possibility of usage of this measuring method for concrete

dam deformation monitoring, defining position accuracy of ± 10 mm.

284 INGEO 2011

Table 2 Error ellipses parameters of network points after adjustment “1”

Point

ID

Semi-major

axis

Semi-minor

axis

[m] [m]

TM1 0.005 0.005

TM2 0.004 0.004

TM3 0.004 0.004

TM4 0.004 0.004

TM5 0.004 0.004

TM6 0.004 0.004

TM7 0.004 0.003

In second 3D adjustment (marked as adjustment “2” in Table 1) the goal was to eliminate

errors caused by errors of CROPOS CORS stations ZABok, BJELovar and AKOvec with

defining reference points of fundamental network TM7-TM4-TM3 (see Figure 4). These

points were selected based on criteria of measurements that were taken during the whole time

of campaign duration (7 hours and 30 minutes). Zero adjustment was performed by principle

of free network with fixed 3D coordinates of point TM7 and then the whole network was

readjusted by fixing other two points (TM4 and TM3). Error in height of GNSS antenna was

set to 2 mm and centering error was set to 1 mm. Correlated vectors between known (fixed)

points were turned off again. Adjustment accuracy, analyzed from parameters of error ellipses

of point show 3 mm accuracy in all directions (Table 3)

Table 3 Error ellipses parameters of network points after adjustment “2”

Point

ID

Semi-major

axis

Semi-minor

axis

[m] [m]

TM1 0.003 0.003

TM2 0.003 0.002

TM5 0.003 0.002

TM6 0.003 0.002

The comparison of distances obtained from 2D adjustment of terrestrial measurements

with ones calculates from transformation of adjusted coordinates in this type of adjustment to

projection shows the minimum value of 9.2 mm (between points TM1 and TM2, Table 1),

maximal value of 23.9 mm (absolute value) (between TM3 and TM6) and standard deviation

of ± 8.8 mm.

Pavasovi , M. et al.: The Application of GNSS VRS Service in Industrial Survey 285

Third 3D adjustment (marked as adjustment “3” in Table 1) was performed by using three

generated VRS RINEX point data. VRS were generated on purpose approximately 100 m

away from points TM1, TM3 and TM7 to avoid long baselines. Adjustment accuracy,

analyzed from parameters of error ellipses of point show 2 mm accuracy in all directions

(Table 4)

The comparison of distances obtained from 2D adjustment of terrestrial measurements

with ones calculates from transformation of adjusted coordinates in this type of adjustment to

projection shows the minimum value of 6.1 mm (between points TM6 and TM7, Table 1),

maximal value of 25.9 mm (absolute value) (between TM3 and TM6) and standard deviation

of ± 8.4 mm.

Table 4 Error ellipses parameters of network points after adjustment “3”

Point

ID

Semi-major

axis

Semi-minor

axis

[m] [m]

TM1 0.002 0.002

TM2 0.002 0.002

TM3 0.002 0.002

TM4 0.002 0.002

TM5 0.002 0.002

TM6 0.002 0.002

TM7 0.002 0.002

4 CONCLUSION

According to obtained terrestrial and GNSS measurements at hydro power plant akovec

and statistical indicators it can be concluded that the application of GNSS measurement

method in determining displacements of control points is limited by demanding accuracy of ±

1cm and adjustment types (methods) using commercial software (like TBC). It is necessary to

point out that more precise and reliable measurements as well as adjustments can be

accomplished by detail observation planning, longer observation window and usage of

scientific adjustment software (like for example Bernese). Also, it is important to point out

that the most acceptable adjustment method of GNSS measurements is to VRS stations

(according to statistic indicators) and therefore we recommend using this kind of geodetic

measurement.

REFERENCES

BAŠI , T. (2009): Novi model geoida Republike Hrvatske i poboljšanje T7D modela transformacije. Elaborat za Državnu geodetsku upravu Republike Hrvatske, pp. 1-68, Zagreb.

286 INGEO 2011

BAŠI , T. - FEIL, L. - LAPAINE, M. (2004): Znanstveno-stru no objašnjenje odluke o utvr ivanju službenih geodetskih referentnih koordinatnih sustava Republike Hrvatske. Geodetski fakultet Sveu ilišta u Zagrebu.

BOUCHER, C. - ALTAMIMI, Z (2008): Specifications for reference frame fixing in the analysis of a EUREF GPS campaign. Memo no. 7, IERS Technical Note, October, 24th 2008, Paris.

REZO, M. (2011): Periodi no pra enje horizontalnih i vertikalnih pomaka na objektu HE akovec. Tehni no izvješ e. Sveu ilište u Zagrebu Geodetski fakultet, Sveu ilište u Zagrebu

Geotehni ki fakultet u Varaždinu.

URL-1: http://www.cropos.hr/ (15.06.2011)

URL-2: http://195.29.118.122/ (15.06.2011)

URL-3: http://itrf.ensg.ign.fr/ITRF_solutions/2008/tp_08-05.php (15.06.2011)