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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: [email protected], [email protected], [email protected]
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)