TOWARDS THE NEW CROATIAN TERRESTRIAL REFERENCE FRAME BASED ON CROPOS-PRELIMINARY RESULTS

Preliminary note Since the establishment of CROPOS, almost seven years have passed. Initial and also the last adjustment CROPOS network was carried out in 2008 using only seven 24h session data in GPS Week 1503 (ETRF2000 R05; e2008,83). In the recent period there have been significant changes Croatian State Geodetic Administration (CSGA) signed an agreement on the exchange of data of national positioning systems neighboring countries: Slovenia (SIGNAL), Hungary (GNSSnet.hu), Montenegro (MontePOS), Federation of Bosnia and Herzegovina (FBiHPOS) and Republika Srpska (SRPOS). This paper presents the results of long-term stability of coordinates of CROPOS stations through absolute comparison of ETRF2000 coordinates from initial adjustment with the combined networked solution of CROPOS and nearby positioning systems, calculated on the basis of 1463 daily solutions for the time span of four years.


Introduction
Nowadays, national positioning systems (POS), based on GPS/GNSS permanent stations are unavoidable part of everyday geodetic practice, providing 365/24/7 reliable services to its users.In 2008 Croatian State Geodetic Administration (CSGA) established CROatian POSitioning System (CROPOS) -network of 30 GNSS permanent stations approximately 70 km distanced apart over the territory of Republic of Croatia.CROPOS is the first positioning system in region with fully implemented 3D coordinate transformation model called T7D (Transformation 7-parameter Distortion) [1] thus providing modern approach to everyday geodetic tasks.
Possibilities of POSs are numerous, both in professional and in scientific field.The quality of POS is measurable by quality of providing services to its users.Precondition to this is primarily long-term stability of POS that is coordinates of its permanent GPS/GNSS stations.

An overview of CROPOS development
According to [5], Coordinate Reference System (CRS) is a coordinate system related to the real world by (geodetic) datum.Datum defines the position of the origin, scale and orientation of coordinate axes, including the definition of mathematical approximation of the Earth -ellipsoid.
Republic of Croatia has historically inherited the horizontal (2D) coordinate reference system as a former member of Austrian-Hungarian Monarchy (in 1901) colloquially called HDKS (hrv.Hrvatski državni koordinatni sustav), Hermannskögel or HR1901.HDKS was realized by astro-geodetic measurements of Military Geodetic Institute (MGI) in 1 st order triangulation network.It was inhomogenously adjusted in 7 separate blocks by conditional measurements [6].Fundamental point (origin) of HDKS was situated at Hermannskögel hill, near Vienna.For the mathematical approximation of Earth's shape, the Bessel 1841 rotation ellipsoid was selected.
Following modern European trends, as well as development of satellite positioning systems (primarily GPS and GLONASS), poor documentation of inherited horizontal reference system and its inhomogeneity (1÷2 m at state level, few dm at county level and ∼10 cm at city levels), were more than a good reason to introduce changes, that is to redefine a national horizontal (as well as height and gravimetric) system [4].On the proposal of the Working Group from University of Zagreb -Faculty of Geodesy (formed on request by CSGA), on 4 th August, 2008 the Government of Republic of Croatia brought the "Decree on establishing new official geodetic datums and map projections of Republic of Croatia" (hrv.Odluka o utvrđivanju novih službenih geodetskih datuma i ravninskih kartografskih projekcija Republike Hrvatske) [16].According to the Decree, Republic of Croatia has adopted a new horizontal reference system called "Croatian Terrestrial Reference System 1996" -CTRS96 (hrv.Hrvatski terestrički referentni sustav 1996 -HTRS96), based on European Terrestrial Reference System 1989 (ETRS89) with Geodetic Reference System 1980 (GRS80) level-ellipsoid as a mathematical and physical approximation of Earth's shape [6].
In October 2007 the Ministry of Finance (MFIN) of Republic of Croatia, representatives of European Union Delegacy in Zagreb and company Trimble Europe signed the contract on realization of Croatian national positioning system with funds ensured from PHARE-2005 (75 %) and Croatian state budget (25 %) [12].
On 9 th December 2008, CSGA introduced Croatian Positioning System -CROPOS to official public use.CROPOS represents a network of 30 GNSS (Global Navigation Satellite System; including: USA NAVSTAR GPS, European Galileo and Russian GLONASS) continuously operating reference stations (CORS) evenly distributed over the territory of Croatia, approximately 70 km apart.Each CROPOS CORS is equipped with Trimble Net R5 GNSS receiver and Zephyr GNSS Geodetic II antenna (TRM55971.00TZGD with elevation mask set to 5°).System is operative 365/24/7 providing real time positioning to its users through differential positioning service (DPS) and high-precise positioning service (hrv.visokoprecizni pozicijski servis -VPPS) with 0,3-0,5 m (for DPS), 0,02 m horizontal and 0,04 m vertical (for VPPS) declared accuracy.CROPOS also provides postprocessing through geodetic precise positioning service (GPPS) by downloading CROPOS CORS data or generating virtual reference station (VRS) data in receiver independent exchange (RINEX) format via CROPOS RINEX web-shop with possible below cm level of accuracy.

Long-term stability of CROPOS reference frame
As mentioned above, initial and also the last adjustment of CROPOS made, used only 24h session data in GPSW 1503 (e2008,83) [2].According to [9], a longterm solution of reference frame, defined using only GPS/GNSS measurements, can be given if there's a minimal period of 2,5 years of continuous measurements.
Processed data period of CROPOS and stations of nearby positioning systems in this work, was from 16 th November 2008 (GPSW 1506) to 31 st December 2012 (GPSW 1721), which is approximately 4 years of continuous data.CORS data processing as well as the adjusted (combined) networked solution (of CROPOS and nearby positioning systems) was done using AIUB Bernese GPS Software (version 5.0) [3].Total amount of 54 reference stations (in 15 seconds registration interval) were included in data processing and adjustment procedure: • 44 POS CORS (Fig. 1  Regarding large amounts of numerical indicators shown in Tab. 1, special attention was paid to input data sorting as well as frequent control of output data in order to avoid any coarse error in data processing and adjustment procedure.Adjustment is made by fixing 5 IGS stations (GRAZ, MATE, ZIMM, WTZR and PENC) using ITRF2008 [7] station coordinates, velocities as well as IGS final precise orbits in mean epoch of all 1463 daily solutions -2010,94.Ambiguity calculation was obtained using QIF (Quasi Ionosphere Free) strategy [3].For the a priori troposphere modeling, that is ZPD (Zenith Path Delay) model and mapping function, Dry Neill and Wet Neill models were used [3].Initial adjustment of CROPOS network included only original 30 CORS, without stations of nearby positioning systems.This combined networked solution includes above mentioned 44 POS CORS.In order to compare coordinates of 30 CROPOS CORS from initial adjustment (ETRF2000 R05; e2008,83) with ones from this combined networked solution, coordinates of original 30 CROPOS CORS were extracted and transformed to ETRF2000 according to [15].
The absolute comparison of original 30 CROPOS CORS coordinates is shown in Fig. 2. Corresponding statistics are presented in Tab 2. ETRF2000 absolute coordinate differences (∆E, ∆N, ∆U) shown in Fig. 2, indicate larger values for CROPOS stations DELN (∆N = 10,1 mm), IMOT (∆U = 7,4 mm) and GOSP (∆E = −5,9 mm) are probably caused by prominent multipath effect at these stations as shown in previous researches [13; 11].Mean standard deviation value of ∼2,5 mm for all components (E, N, U) indicates the existence of long-term stability of CROPOS coordinates for the time span of 4 years.Min.

Conclusion
The analysis of coordinate stability of CROPOS stations through the period of 4 years was obtained by comparing the results of the initial adjustment of CROPOS network, based on combination of 7 daily solutions in GPSW 1503 with combination of 1463 daily solutions for the period from 16 th November 2008 (GPSW 1506) to 31 st December 2012 (GPS week 1721).This comparison indicates to a high degree of stability and reliability of CROPOS reference frame.Preliminary results of the analysis in this work, draw the question whether we can consider CROPOS the "new" Croatian terrestrial reference frame.The answer to this question will be given through results of ongoing scientific research including longer operating time span of CROPOS and nearby positioning systems.
The potential that CROPOS offers is invaluable and therefore will certainly be a topic of future professional and scientific researches.
. Initial adjustment of CROPOS network was done using 24h session data for GPS Week 1503 (epoch 2008,83) [2; 12].In June 2009 at 1 st CROPOS Conference held in Zagreb, CSGA, Surveying and Mapping Authority of the Republic of Slovenia, Hungarian Institute of Geodesy, Cartography and Remote Sensing (FÖMI) and Real Estate Administration of Government of Montenegro signed an agreement on data exchange of border permanent stations of corresponding positioning systems: CROPOS, SIGNAL, GNSSnet.hu and MontePOS.With this agreement, CROPOS was supplemented with 13 reference stations (7 SIGNAL: KOPE, ILIB, TREB, CRNO, BREZ, PTUJ and VEPO; 4 GNSSnet.hu:NIZS, BARC, SIKL and BALE; 2 MontePOS: NIKS and TIVA) and represents total networked solution of 43 reference stations (Fig. 1).In December 2013, CSGA, Federal Geodetic Administration of Bosnia and Herzegovina (FGA) and Republic Administration for Geodetic and Property Affairs of Republic of Srpska signed an agreement on data exchange of border permanent stations of corresponding positioning systems: CROPOS, FBiHPOS and SRPOS.With this agreement, CROPOS was supplemented with additional 5 reference stations (3 FBiHPOS: SANM, BOSG and MOST; 2 SRPOS: NOVI and BIJE) (Fig. 1).On 13 th November 2011 (GPSW 1662, DOY 317) CSGA disconnected IGS station Dubrovnik (DUBI) and established a new one (DUB2), located just 22 m away (Fig.

Table 1
Numerical indicators of input and output data

Table 2
Statistics of absolute comparison of 30 CROPOS CORS coordinates