Satellite navigation on Fehmarn: Tunnel elements assembled with millimetre precision
Tunnel workers will have to assemble the Fehmarn tunnel by lowering and assembling 89 tunnel elements at the bottom of the Fehmarn Belt. Each element is 217 meters long and weighs 73 500 tons. When navigating the elements on the ocean floor, they will be assisted by satellite signals that have travelled more than 20 000 km through space.
With a total of 89 tunnel elements, the construction will be an enormous undertaking, where even the smallest deviation from the first to the last tunnel element can have major consequences, says Henrik Schjøler Dahl, project manager at Femern A/S.
“We are talking about kilometre-long distances, so when we dig the tunnel trench and place the individual elements, even small deviations along the way can have great significance. Our work must be extremely precise, so when the last element is placed, we need an accuracy of very few millimetres,” he says.
Most of us have probably experienced our car’s GPS asking us to park the car in the neighbour's hedge. Satellite signals are weak and quickly run into many errors and disturbances en route from space down to Earth.
Therefore, ten stationary reference stations have been established, also known as GNSS stations (Global Navigation Satellite Systems) on both the Danish and the German side of the Fehmarn Belt. Femern A/S has built four primary ones, and Geoteam has added six more.
Their task is to correct for errors of the satellite signals so that construction machinery and ships can navigate with a very high accuracy. It is Danish company Geoteam from Ballerup that delivers the high accuracy with the ten GNSS stations around the Fehmarn construction area.
Within the next few years, we will gain access to corrections directly from the Galileo satellites, and then the accuracy will be up to 20 centimetres.
--Niels Rasmussen, COO, Geoteam
“It’s primarily about eliminating the errors that occur on signals from the satellite to Earth. It’s typically errors that occur in the ionosphere, but also in the troposphere, as well as errors in receiving equipment and clocks,” says Niels Rasmussen, COO of Geoteam.
Determining a position by measuring the difference in time is called trilateration.
GPS for tunnel elements
The system will provide an accuracy of 8 mm horizontally and 15 mm vertically. In comparison, satellite receivers in consumer electronics, such as telephones and cars, are typically limited to an accuracy of several meters.
“The contractor is towing the finished tunnel elements to their destination with tug boats. Our positioning system then delivers the exact coordinates, so that each element can be lowered into place at the right position,” Henrik Schjøler Dahl says.
It is expected that the first tunnel elements will be placed down using the positioning system in 2024.
However, the system has already been in operation for a little over a year and is extensively used by the many construction machines, which are currently in the process of building the 500 000 square meter work port on the south coast of Lolland, where the factory that will produce the large tunnel elements is also located.
When Ingeniøren talked to Niels Rasmussen on an ordinary Thursday in March, there were 69 systems connected to Geoteam’s reference stations on Fehmarn. It just requires an advanced version of a GPS receiver.
Geoteam’s reference network is called GPSnet.dk and covers the whole country. In addition to the construction industry, agricultural industry and surveyors also use the reference network.
“We are looking at a future with more autonomous devices, such as robots, drones, and maybe also cars in the long run. On Fehmarn, the most obvious uses are in vehicles and ships, but I think there are many other uses that we haven’t seen yet,” Niels Rasmussen says.
In addition to navigation, the system is also used by surveyors to determine locations and as documentation for which suppliers have been where and at what time.
In the Great Belt, Sund & Bælt uses GPS receivers on the bridge together with a reference station on land to monitor movements of the bridge, for example when heavy vehicles pass.
“Calculating the position between the three points, that is between the base station and the receiver, and then correcting for errors with the reference station on land is basic vector mathematics. This means that we can measure movements in millimetres,” Niels Rasmussen says.
If you are in the passenger seat the next time you drive over the Great Belt Bridge, it is quite easy to spot the small white mushroom-shaped devices with antennae that sit at eye level along the bridge.
More and more satellites
In fact, the first civilian use of satellite positioning was not navigation at all, but the timing of financial transactions, just as the satellites were also quickly put to use for synchronizing the power grid.
GPS signals are weak and prone to errors, and therefore the key to accurate positioning lies in spreading the receipt of signals.
A lot has happened here since Geoteam put GPSnet into operation 20 years ago. At that time, signals only came from American GPS satellites. Since then, the Russian GLONASS satellites, Chinese Beidou satellites, and most recently the European Galileo satellites have been connected to reference stations.
“The more satellites we have access to, and the more scattered they are, the more accurate positioning we can deliver,” Niels Rasmussen says.
Higher accuracy within a few years
While it today requires professional and costly reference stations, in a few more years, we will also have more accurate navigation systems for our mobile phones. While GPS, Glonass, and Beidou are primarily developed for military purposes, the European Galileo system is primarily civilian.
“Therefore, we will have services that deliver reference signals from the satellite. For years, we have received corrections from geostationary satellites in the EGNOS program, and over the next few years we will have access to corrections directly from the Galileo satellites, and then we can achieve an accuracy of 20 centimetres,” Niels Rasmussen says.
However, that service is not yet enabled on Galileo. The Fehmarn Belt tunnel is scheduled to be completed by 2029, and it will be the world’s longest immersed tunnel, spanning 18 kilometres.
