World’s Longest Immersed Tunnel, 40 Meters Underwater
By Anna Jensen, Dirk Hermsmeyer, Bastian Huck, Jürgen Rüffer, and Peter Skjellerup
The Fehmarnbelt Positioning System between Denmark and Germany includes a geodetic basis, four permanent GNSS stations, and a real-time kinematic (RTK) service for construction of a road and rail causeway between the islands of Fehmarn, Germany, and Lolland, Denmark, across the Fehmarnbelt, a 20-kilometer stretch of open water in the Baltic Sea. This homogeneous, consistent, coherent, highly accurate GNSS-based positioning system exemplifies comparable systems and services that can be established for any major construction site or infrastructure project. Now in use for environmental, geotechnical, and geophysical investigations, it provides cost-efficient operations and facilitates the precise navigation of large, costly offshore equipment.
A fixed road-and-rail link across the Fehmarnbelt body of water in the Baltic Sea will by 2020 connect the German island of Fehmarn and the Danish island of Lolland. It will provide a critical time- and cost-efficient trade and traffic link between north-central Europe and Scandinavia.
Geophysical and geotechnical pre-investigations have been completed as well as an environmental assessment of the fixed link. Initially proposed as either a bridge or a tunnel (Figure 1), an immersed tunnel is now the preferred solution. 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, excluding peninsulas on both sides to be constructed for easier entrance to the tunnel. The strait is 20 kilometers wide at the site. The immersed depth is up to 40 meters.
During planning and construction of the fixed link, it is very important to be able to perform reliable positioning with high accuracy. This requires a well defined geodetic basis — a 3D reference system and a reference frame for GNSS positioning, a height system and a geoid model for working with heights, and a map projection for plane maps and drawings. The ability to determine positions with high accuracy in real time within the project area is also very important. Therefore a carrier phase-based GNSS positioning service, a real-time kinematic (RTK) service, has been established.
Altogether, we refer to the geodetic basis and the RTK service as the Fehmarnbelt Positioning System (FBPS), and the geodetic basis as the Fehmarnbelt Coordinate System (FCS). In this article we describe the geodetic basis and the RTK service, including four new permanent GNSS stations established for the purpose.
Geodetic Reference Frame
The reference system for the FCS is the International Terrestrial Reference System, realized by the ITRF2005, the newest and to date 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 2).
After establishment of the GNSS stations, seven days of GNSS data were collected in February 2010. Coordinates for the stations were determined by the National Survey and Cadastre-Denmark, 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.
Height System and Map Projection
The height difference between Germany and Denmark is known from a 1987 hydrostatic levelling between Puttgarden and Rødbyhavn. For the Fehmarnbelt Fixed Link, precise levelling has been carried out between the connecting points of the hydrostatic levelling and stable point groups further inland. Levelling points with a large displacement since 1987 were eliminated, and the hydrostatic levelling was then used for transfer of the height difference between Germany and Denmark.
The next step was determination of present mean sea level (MSL) in the Fehmarnbelt and establishment of a project-specific height system with the zero-level as close as possible to the actual MSL of Fehmarnbelt. In this area of the Baltic Sea, a slow rise of MSL relative to the neighboring land is taking place, and therefore water-level data from Heiligenhafen on the German mainland, and from Puttgarden and Rødbyhavn, was analyzed in cooperation with the Danish National Survey and Cadastre and the Danish National Space Institute.
Analyses of the last 20 years of water-level data show an increase in the water level of approximately 2 millimeters per year at Rødbyhavn. Data from Heiligenhafen was also analyzed; as Heiligenhafen is not directly adjacent to the site, the time series was not used directly for establishing the MSL datum but instead used as an independent control.
Water-level data was used for estimation of the present MSL in Fehmarnbelt, and the zero level for the FCS Vertical Reference 2010 (FCSVR10) coincides with MSL at Rødbyhavn in 2010. The zero level of FCSVR10 thus deviates from both the German and the Danish height systems.
The Danish National Survey and Cadastre conducted precise levelling to determine FCSVR10 heights to the four new permanent GNSS stations, and determined FCSVR10 heights to a number of existing height benchmarks on Fehmarn and Lolland. Local land uplift on Fehmarn and Lolland causes differences between the FCSVR10, the national German DHHN92 height system, and the national Danish Vertical Reference 1990 height system. Differences between the height systems are not constant values but vary within the area, so it is very important to use the geoid models when converting heights for high-accuracy applications.
To determine heights relative to MSL with GNSS it is necessary to utilize a geoid model. The Danish National Space Institute performed new gravity readings to supplement the existing gravity database. Then all existing gravity data from the area was used for development of a local geoid model for the Fehmarnbelt. The geoid model is fitted to the height system FCSVR10 and to the ITRF2005 by the four new permanent GNSS stations, and the model can be used for conversion between MSL heights and ellipsoidal heights.
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. Table 1 gives the defining parameters for the map projection.
Permanent GNSS Stations
The four permanent GNSS stations are established as geodetic-grade stations, as shown in the photo. Individually calibrated GNSS choke ring antennae are mounted on 3-meter tall concrete pillars, with foundations 3 meters into the ground at stations 1, 2, and 4, with predominantly silty glacial till of stiff consistency at about 0.70 (stations 1 and 2) and 1.70 meters (station 4) below soil surface. At station 3, foundations for the antenna monument are built 9 meters into the ground. Soil conditions are sandy at this location to about 7 meters below soil surface, where stiff glacial till is met. In geotechnical investigations and analyses carried out before establishment of the GNSS stations, the glacial till at the station locations was rated as a good to very good foundation ground, with little tendency to settlement.
The concrete antenna monuments are surrounded with about 0.30 meters of styrofoam for thermal insulation. The monument head is bevelled with an angle of 30° from vertical, reflecting GNSS satellite signals striking the monument head underneath the antenna away from it, to further minimize signal multipath effects.
The GNSS reference station receivers are capable of processing GPS and GLONASS L1 and L2, GPS L5, and Galileo E1, E5a, E5b, and Alt-BOC frequency band signals. Galileo signals can be processed when Galileo satellites are available; a firmware update on the receivers will be required. In view of the long-term demand for the FBPS (until 2020 or longer), its compatibility with Galileo signals in particular makes the system future-proof.
GNSS reference station receivers, access points to power grids, and uninterruptible power supply are mounted in cabinets adjacent to the antenna pillars. Additional equipment in each cabinet comprises an industrial PC, Internet router, GSM/UMTS router, satellite communication equipment, transmitting and receiving radio modems, and a heat exchanger to cool the in-cabin room if required.
At each station, a radio mast of about 10 meters height carries a satellite dish for wireless Internet access, and a Yagi antenna to broadcast GNSS correction data into the proposed construction area in the Fehmarnbelt. Radio masts are located directly north of the GNSS antennae.
To ensure accurate GNSS positioning, an RTK GNSS service has been established, based on GNSS data from the four new permanent GNSS stations (primary stations) as well as four GNSS stations located further away in Germany and Denmark (secondary stations), which existed previous to our work. Figure 3 shows the locations of the eight stations used for the RTK service. The stations relay GNSS data to the control center, which derives and transmits RTK correction data to surveyors in the project area with RTK rovers.
The RTK service has been developed with focus on robustness, with two control centers at different addresses in Germany. Three different communication carriers provide data 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. Figure 4 shows the communication lines of the RTK service.
FBPS RTK users who wish to receive RTK corrections via UHF radio require a UHF radio modem and antenna, in addition to an RTK rover. The four primary GNSS stations broadcast 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.
From the users’ point of view an advantage of using UHF radio over using a mobile Internet connection is that the UHF connection is free-of-charge and can be collected from four different sources.
Users who wish to receive RTK corrections via mobile Internet must connect via General Packet Radio Service (GPRS) and require a GPRS modem, antenna, and a subscriber identity module (SIM-card) in addition to their RTK rover. GPRS connections will be charged according to tariffs of the respective mobile phone network provider.
Figure 5 shows areas of signal coverage. Areas 1 and 2 are covered by UHF radio and mobile Internet. Area 3 is covered by mobile Internet.
The FBPS RTK service generates and broadcasts RTK corrections in two different modes: master-auxiliary corrections (MAX) mode, and virtual reference station (VRS) mode. MAX 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 version used for the FBPS RTK service is the RTCM version 3.1.
With MAX corrections, the RTK rover does not send its position to the reference network software. The GNSMART reference network software calculates and sends MAX corrections to the rover. These contain the measurements from a master station and correction data from the auxiliary reference stations. The rover individualizes the corrections for its position, which means it determines the best suitable RTK corrections. RTK data in MAX mode can be received by users of RTK rovers via both possible types of connection, UHF radio and GPRS.
With the VRS concept, the user’s RTK rover transmits its approximate position to the control centre, which returns to the rover observations or corrections of an individual VRS near the user’s position. Data is transmitted back and forth between the RTK rover and the control center. Therefore a two-way communication link must be established with VRS. Because the UHF radio connection is one-way, GNSS correction data in VRS mode can be received via digital cellular phone (GPRS) only. For data transmission via GPRS, the FBPS RTK service uses the networked transport of RTCM via Internet protocol (NTRIP).
Multiple RTK rovers (that is, multiple users) can receive RTK corrections from the FBPS simultaneously with any of the connections described above, while every user may select his or her favourite connection type. The RTK service can be used with any commercially available geodetic GNSS receiver that is capable of processing RTK data.
System Test and Results
The RTK service was established during the spring of 2010 and was run in test mode May 12–July 31 to test system accuracy, signal coverage area, and signal availability.
Accuracy. An error budget of the RTK service is provided including all known error sources and latencies in the system, and a description of how these errors are handled. 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 a marked survey point repeatedly with these accuracies.
System inspection is carried out monthly. Part of monthly inspection is the visit of marked control points with an RTK rover. ISO 17123-8:2007 (ANSI, 2007) standard procedures are applied to determine control point coordinates.
Coverage Area. The RTK service coverage area shown in Figure 5 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 (see photo). Modelled UHF radio signal broadcasting areas are closely verified during these tests.
Availability. The positioning system and the RTK service are designed using necessary technology, redundancy, and back-up to ensure that the system is operational and available 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. During the first year of operation it is accepted that RTK correction data from the system are available to end users for 97 percent of the time or more per day.
A control segment has been established to constantly monitor RTK service accuracy and the availability of the system. The control segment is installed in such a way that all relevant output and data streams from the GNSS stations 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. Results from evaluation of availability during the test operation are listed in Table 2. During test operation, the required availability of 97 percent per day during the first year of operation was reached on all days. Availability only fell below 99 percent, as is the required availability during following years, for 5 out of 81 days (5.6 percent) of the test period.
Conclusions and Outlook
System tests results regarding accuracy, coverage area, and availability show that the positioning system and the RTK service fulfil all specifiecation requirements.The first RTK user was registered in July 2010, and the complete system is now being 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, including during critical procedures in the construction phase.
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.
This work is funded by Femern A/S. The authors acknowledge contributions from the National Survey and Cadastre, Denmark, Danish National Space Institute, Land Survey Office of Schleswig-Holstein in Germany, German Federal Agency for Cartography and Geodesy, Richter Deformationsmesstechnik GmbH, Günther Steimann, and Ohms Nachtigall Engineering GbR. Also Mr. and Ms. Thomsen, Stadt Fehmarn, Mr. Henriksen, and Mr. Boserup for permitting establishment of FBPS GNSS stations on their property.
Establishment, operation and maintenance of the GNSS stations and RTK service was entrusted by Femern A/S to AXIO-NET GmbH, with ALLSAT 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, and more on the RTK service.
The RTK service is based on GNSMART software (GEO++ GmbH). The permanent GNSS stations are equipped with Leica Geosystems AR25 antennas and GRX1200+ receivers.
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 more than 15 years.
Dirk Hermsmeyer holds a Ph.D. from the University of Hannover, and is a project management professional. He previously worked at ALLSAT and is now with the Chamber of Commerce in Lübeck, Germany.
Bastian Huck is head of operations and quality management with AXIO-NET. He is a university-level geodesist and certificated project management practitioner with 10 years of experience in RTK projects.
Jürgen Rüffer is co-owner and CEO of ALLSAT and 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 1977.
Peter Skjellerup is chief advisor on geotechnology with Ramboll Denmark. He has worked with ground engineering for many years, and holds a M.Sc. in physics-geophysics from the University of Copenhagen.
Note from author Anna Jensen (2/27/13):
“Since publication of the article, the opening year for the Fehmarnbelt tunnel has been changed to 2021.”