The Fehmarnbelt Positioning System for a Mega Construction Site

Transcription

1 The Fehmarnbelt Positioning System for a Mega Construction Site Bastian Huck 1, Anna Jensen 2, Anders Almholt 3, Jürgen Rüffer 4 1 AXIO-NET GmbH, Hannover, Germany mail: bastian.huck@axio-net.eu 2 AJ Geomatics, Roskilde, Denmark mail: aj@ajgeomatics.com 3 Rambøll, Copenhagen, Denmark mail: adea@ramboll.dk 4 ALLSAT GmbH and AXIO-NET GmbH, Hannover, Germany mail: juergen.rueffer@allsat.de; juergen.rueffer@axio-net.eu Abstract The paper describes the intention for and the establishment and operation of an RTK positioning system, including the geodetic basis, four new permanent GNSS stations, and a service called the Fermarnbelt Positioning System (FBPS) for the Fehmarnbelt Fixed Link, a rail and road causeway between the islands of Fehmarn (Germany) and Lolland (Denmark). The fixed link crosses the Fehmarnbelt, a 20 km stretch of water of the Baltic Sea, connecting the northern European countries with Central Europe (see Figure 1). BIOGRAPHIES Bastian Huck is head of operations and quality management with AXIO-NET. He is a university-level geodesist and certificated project management practitioner with more than 10 years of experience in RTK projects. Anna Jensen is owner and CEO of AJ Geomatics in Denmark. She holds a Ph.D. in geodesy and has worked with research and development within GNSS and geodesy for 20 years. Anders Almholt is a senior analyst at Ramboll Denmark. He holds a M.Sc. degree in geophysics from the University of Copenhagen and has worked with ground engineering for the past 5 years.jürgen Rüffer is co-owner and CEO of ALLSAT and former MD of AXIO-NET. He is a university-level geodesist, a publicly certified expert for GNSS positioning at the chamber of engineers in Germany, working with GPS and GNSS since Figure 1: Main Northern European traffic routes. Source: Femern A/S A fixed road-and-rail link across the Fehmarnbelt in the Baltic Sea will connect the German island of Fehmarn and the Danish island of Lolland by It will provide a cost-efficient trade and traffic link between central Europe and Scandinavia. Initially proposed as either a bridge or a tunnel, an immersed tunnel is now the solution (Figure 2). It will be placed in a trench excavated on the sea floor, and covered with a layer of stones. It will be the longest immersed tunnel in the world at 17.6 kilometers. The immersed depth is up to 40 meters. During planning and construction of the fixed link, it is very important to be able to permanently (24/7) perform reliable positioning with high accuracy and with constantly high reliability. I. INTRODUCTION Large infrastructure projects require increasing efforts in logistics, machineries and technology in order to finish such projects in time within the budgets allocated. The estimated cost for this project is 5.5 B (for comparison: Galileo was planned initially with 3.5 B, the new main station in Stuttgart with 4.2 B ).

2 Figure 2: Conceptual designs of the tunnel portal (top) and interior (bottom), Source: Femern A/S This requires a well defined geodetic basis, the Fehmarnbelt Coordinate System (FCS) including a 3D reference system and a reference frame for GNSS positioning, a height system and a geoid model for working with heights, a map projection for plane maps and drawings, and coordinate transformation parameters in order to connect to the different reference frames of the two countries. II. THE FEHMARNBELT POSITIONING SYSTEM (FBPS) From experience in comparable large projects in the past the project management team of Femern A/S knew that an inhomogenous geodetic basis may complicate and thus delay the realization of such a large project, resulting in significantly increasing project cost. Furthermore the stopping of expensive machinery due to missing or inaccurate positioning may exceed the multiple of the cost required for a sophisticated real-time positioning system that has not been designed in a similar way before. 1) The Fehmarnbelt Coordinate Systems (FCS) The reference system for the Fehmarnbelt Coordinate System is the International Terrestrial Reference System, realized by the ITRF2005, at the time of planning the newest and most accurate realization of the ITRS. Four permanent GNSS stations were established around Fehmarnbelt during the autumn and winter of 2009/2010: two on Fehmarn and two on Lolland (Figure 3). After establishment of the GNSS stations, seven days of GNSS data were collected in February Coordinates for the stations were determined by the Danish Geodata Agency (GST), using the Bernese GPS software. Data from six GNSS stations of the network of the International GNSS Service (IGS) was included in the data processing, and these stations with coordinates in the ITRF2005 were used as reference stations. Hereby, the ITRF2005 was introduced in the Fehmarnbelt area, and a reference frame for positioning in three dimensions has been established. Figure 3: Situation of the FBPS 2) The Fehmarnbelt Height System (FCSVR10) To determine heights relative to MSL with GNSS it is necessary to utilize a geoid model. The Danish National Space Institute (DTU-Space) performed precise leveling and new gravity surveys for development of a local geoid model for the Fehmarnbelt. The geoid model is fitted to a new height system for this project and to the ITRF2005 by the four new permanent GNSS stations, the model can be used for conversion between MSL heights and ellipsoidal heights. 3) The Fehmarnbelt Map Projection The last item of the geodetic basis is the definition of a map projection, using a transverse Mercator projection. The projection is fitted to the area to obtain a scale factor as small as possible within the construction area. Also, a false Easting value was chosen to provide FCS Easting values within the construction area which are different from Easting values of the ITM, UTM, or Gauss-Krüger projections used in Germany and Denmark. 4) The Fehmarnbelt GNSS Stations The four permanent GNSS stations for the Fehmarnbelt Positioning System are established as geodetic-grade stations, as shown in picture 1. Individually calibrated GNSS choke ring antennae are mounted on concrete pillars, with foundations 3-9 meters into the ground. The concrete antenna monuments are planned for reducing multipath effects and constructed for thermal insulation.

3 communication between the GNSS stations and the control centers, and RTK correction data is distributed to users in two different ways, via ultra-high frequency (UHF) radio and mobile Internet. The four primary GNSS stations broadcast the standardized RTK correction data on four separate radio frequencies. By switching their radio modem to one of the frequencies, users receive the correction signal from the control center via the respective station. RTK corrections via UHF radio can be used where radio signals from one of the four primary GNSS stations can be received. Users who wish to receive RTK corrections via mobile Internet may connect to the Service via NTRIP protocol. Figure 4 shows areas of signal coverage. Areas 1 and 2 are covered by UHF radio and mobile Internet. Area 3 is covered by mobile Internet. Picture 1: GNSS Station of the FBPS The GNSS reference station receivers are capable of processing GPS L1, L2 and L5, GLONASS L1 and L2, Galileo E1, E5a, E5b, and Alt-BOC signals. In view of the long-term demand for the FBPS, its compatibility with Galileo signals makes the system more robust and future-proof. GNSS reference station receivers, access points to power grids, and uninterruptible power supply are mounted adjacent to the antenna pillars. Additional equipment in each cabinet comprises an industrial PC, Internet router, GSM/GPRS/UMTS router, satellite communication equipment, transmitting and receiving radio modems, and a heat exchanger to cool the incabin room if required. At each station, a radio mast of about 10 meters height carries a satellite dish for wireless Internet access, and an antenna to broadcast GNSS correction data into the construction area in the Fehmarnbelt. III. THE SERVICE FOR THE CONSTRUCTION AREA To ensure accurate GNSS positioning, an RTK service has been defined and established, based on GNSS data from the four new permanent GNSS stations (primary stations) as well as four already existing GNSS stations located in the hinterland in Germany and Denmark (secondary stations), which strengthens the network with respect to temporary station failures and for better modeling of errors from GNSS observations. All eight GNSS stations transmit their data via redundant communication lines to the control centers, which derive and transmit RTK correction data to FBPS RTK users in the project area. The RTK service has been developed with focus on robustness, with two control centers at different locations in Germany under the control of AXIO-NET. Three different communication carriers provide data Figure 4: Coverage Area of the FBPS The FBPS RTK service can be used with any commercially available geodetic GNSS receiver that is capable of using and processing RTK data based on RTK corrections in RTCM format version 3.1. The service generates and broadcasts RTK corrections in two different modes: master-auxiliary corrections (MAC) mode, and virtual reference station (VRS) mode. MAC and VRS are two different calculation methods to generate RTK corrections in a standard format defined by the Radio Technical Commission for Maritime Services (the RTCM format). The GNSMART reference network software (by Geo++) generates both types of corrections and the user can choose according to his preferences and the capability of his rover. RTK data in MAC mode can be received by users of RTK rovers via both possible types of communication, UHF radio and GPRS. With the VRS concept, the user s RTK rover requires a two-way communication and thus in VRS mode the corrections need to be transmitted via GPRS/UMTS using NTRIP protocol.

4 Multiple RTK rovers even more than 100 simultaneously - can receive RTK corrections from the FBPS with any of the connections described above. IV. THE SYSTEM SETUP AND OPERATION The contract for the installation and operation of the Fehmarnbelt Positioning System was awarded in 2009 to AXIO-NET GmbH with a sub-contract to ALLSAT GmbH, both based in Hannover, Germany by Femern A/S in Denmark. During 2009 until mid 2010 the whole system was installed and delivered to Femern A/S, comprising of system design obtaining permissions for site construction and for transmission of radio signals geotechnical investigation of proper station locations construction, delivery and installation of four reference stations implementation of redundant communication implementation of the GNSS reference network and the service performance tests offshore and onshore The RTK service was established during the spring of 2010 and was run in test mode until 31 July Those tests analysed system accuracy, signal coverage area, signal availability and robustness of the service. Since then the system has been operated by AXIO-NET without any significant outages. in the entire coverage area for more than 99 percent of the time. Availability is defined as the time where all elements of the positioning system are available for end users and where the described accuracy can be obtained for all users within the coverage area. Availability is evaluated in percent of time per day: The system must be available for at least 23 hours and 45 minutes per day. A control segment has been established to monitor RTK service accuracy and the availability of the system. The results are available through the system s website. Evaluation of availability is carried out automatically by the control segment, and an overall evaluation of availability is performed every month. Recent results from evaluation of availability are listed in Figure 5 below. Figure 5: Availability of FBPS service during 2011 and 2012, percent of time per day VI. CONCLUSION AND OUTLOOK V. SYSTEM PERFORMANCE RESULTS FBPS system inspection is carried out monthly. 1) Service Accuracy The accuracy obtainable by end users is better than 1.0 centimeters in the horizontal and better than 1.8 centimeters in the vertical. Values are provided as one sigma, and are valid during normal ionospheric activity. Applying an RTK rover and RTK corrections received from the FBPS RTK service, users inside the coverage area can determine the coordinates of distinct points repeatedly with these accuracies. 2) Service Coverage The RTK service coverage area shown in Figure 4 is defined as the geographic area where the described accuracy can be obtained for end users at any time. Test measurements of UHF radio signal strengths from the four primary GNSS stations have been carried out onshore Lolland and Fehmarn, as well as offshore across the Fehmarnbelt. 3) Service Availability The Fehmarnbelt Positioning System is designed using up-to-date technology, redundancy, and back-up to ensure that the system is operational and available System test results regarding accuracy, coverage area, and availability show that the positioning system and the RTK service fulfill all specification requirements since mid The first RTK user was registered in July 2010, and the complete system was already used for environmental, geotechnical, and geophysical investigations. User benefits of the FBPS include: Ensured consistent and uniform geodetic reference throughout the planning, construction and operation phases of the Fehmarnbelt Fixed Link, available to all stakeholders at any time; Seamless, real-time data flow from the point measurement at the construction site into computer-aided design (CAD) or geographic information systems (GIS); Simplified geodata transfer across interfaces between project stakeholders and project phases; Cost efficiency, reducing costs in both surveying and data management, particularly in precise operation of large, expensive offshore equipment, especially during critical procedures in the construction phase.

5 The positioning system for the Fehmarnbelt Fixed Link is an example of a homogeneous, consistent, coherent, and highly accurate GNSS-based positioning system. Comparable systems and services can be established and used for any major construction site or infrastructure project. The wise and careful planning of the clients project management team provided a positioning system that will save more cost and time for the project of construction of the Fehmarnbelt Fixed Link than its own cost. Disclaimer: The opinions and conclusions presented in this paper do not necessarily cover the opinions and conclusions of Femern A/S. ACKNOWLEDGMENT This work is funded by Femern A/S. The authors acknowledge contributions from the Danish Geodata Agency, Danish National Space Institute (DTU-Space), Land Survey Office of Schleswig-Holstein in Germany, German Federal Agency for Cartography and Geodesy. Establishment, operation and maintenance of the GNSS stations and RTK service was entrusted by Femern A/S to AXIO-NET GmbH, with ALLSAT GmbH as subcontractor for implementation of the four GNSS stations (both companies in Hannover, Germany). Ramboll Arup JV was entrusted by Femern A/S with project coordination and geodetic consultancy, using AJ Geomatics as subcontractor. More information about the fixed link is available at and more on the RTK service at femern.axio-net.eu. REFERENCES [1] A. Jensen, D. Hermsmeyer, B. Huck, J. Rüffer, P. Skjellerup, 20 Kilometers, Heavy Construction - World s Longest Immersed Tunnel, 40 Meters Underwater, GPS World May 2011 [2] D. Hermsmeyer, A. Jensen, B. Huck, P. Skjellerup, J. Rüffer, Positionierungssystem für die feste Fehmarnbeltquerung VDVmagazin June 2011 [3] J. Rüffer, Die Fehmarnbelt-Querung Anforderungen an ein präzises Positionierungssystem in der Ingenieurvermessung Lectures at the Sachverständigentag der Ingenieurkammer Niedersachsen September 2010 and at the Karlsruhe Institute for Technology February 2012