Photo: DTU Space

DTU tests super precise positioning system

Tuesday 30 Oct 18


Kristian Pedersen
DTU Space
+45 20 11 74 77


Lars Stenseng
DTU Space
The Testbed in Aarhus for Precision Positioning and Autonomous Systems (TAPAS) is being made available to businesses and public authorities wanting to develop new concepts, for example autonomous vehicles.

The degree of accuracy will be down to within one centimetre if a car uses the new TAPAS platform from DTU Space to navigate. On Friday 26 October, the TAPAS platform was inaugurated in Aarhus by Lars Christian Lilleholt, Denmark’s Minister for Energy, Utilities and Climate.

In the coming years, TAPAS will be the starting point for developing and testing new systems and technologies involving super precise positioning systems, for example for use in urban environments.

“With the TAPAS project, we are ensuring that Denmark is at the forefront of developments within super precise satellite-based positioning systems. I see enormous potential, which will ultimately benefit the individual citizen,” said the minister at the inauguration.

Eleven reference stations in and around Aarhus

TAPAS comprises a network of 11 reference stations covering Aarhus and the port.

"The TAPAS project is an outstanding example of universities and the public authorities working together to develop technologies that can specifically benefit Danish companies and society as a whole."
Professor Kristian Pedersen, DTU Space

Via the reference stations, corrections are calculated which significantly improve satellite navigation signals from, among others, the American GPS system and the corresponding European Galileo system which is currently being built up. In this way, it will be possible to achieve unprecedented precise positioning down to one centimetre in real time in the test area.

TAPAS was designed and established as part of a cooperation between DTU Space, the City of Aarhus, and the Agency for Data Supply and Efficiency which runs and owns the finished TAPAS platform.

“The TAPAS project is an outstanding example of universities and the public authorities working together to develop technologies that can specifically benefit Danish companies and society as a whole,” says Professor Kristian Pedersen, Director of DTU Space.

Huge scope for businesses and the public authorities

 Businesses, research institutions, and public authorities will have access to the system. As a result, they will have the opportunity to test how they can use the precise positioning of units in motion to develop innovative and efficient solutions for deployment in cities. In addition to systems for autonomous vehicles, the equipment can in principle also be used to develop aids for the visually impaired, so that they can be guided safely through the urban environment.

“The project brings Denmark right to the very forefront within the field of precise positioning, and this will be a decisive element in the development of tomorrow’s autonomous systems. Also, we have a big advantage here, as we can draw on our extensive experience within accurate and reliable positioning systems from space. And we’re more than happy to make this know-how available with this project,” says Kristian Pedersen.

How does TAPAS interact with GNSS satellite navigation systems such as GPS and Galileo?

Positioning starts with the satellites

Precise positioning via satellite navigation systems requires reference stations on the ground, such as the TAPAS system, which can be used to correct for the various factors that affect the satellite signals.

An object with a receiver which can receive satellite signals from Global Navigation Satellite System(GNSS)—for example the American GPS system, the European Galileo system or the Russian Glonass system—can be positioned accurately using four satellites: three satellites to determine the coordinates (x, y, and z) and one satellite that registers the time differences between the satellite and the receiver on Earth.

The measurements are based on the time it takes for the signal to move from the satellites to the receiver. The positions of the satellites are sent together with the time signals from the satellites.

Signals need correcting for several factors

Corrections have to be made in respect of a number of factors that affect the precision of the signals which are received by the receiver on the ground. The signal from the satellite is slightly deflected and delayed on its path towards Earth by conditions in the atmosphere and ionosphere, and there are also errors in the satellites’ clocks and their positions.

It is dynamic conditions such as temperature, air pressure, and moisture in the troposphere as well as the density of electrons and variations in electron density in the ionosphere that play a role. The dynamic conditions in the ionosphere are closely linked to the Sun’s activity and the Earth’s magnetic field.

Also, the satellite orbits vary slightly. In addition, it is necessary to correct for the precision differences between the two clocks that measure the time at which the signal is sent and received, i.e. the clock on board the satellite and the clock in the receiver unit.

Corrections that take these factors into account are calculated on the basis of fixed reference points on Earth where a GNSS receiver is located with a known position. The better the mathematical models which can be made to correct for these factors, the more precise the positioning. For this to be possible with a high degree of precision for objects in constant movement, it is necessary to have the fixed reference points on Earth, as they can minimize and virtually eliminate the error indication providing the distance between them is not too large.

In the civilian version of the American GPS system which is available, the level of precision is approx. 10 metres, but by using corrections sent from other satellites (geostationary satellites such as USA’s Wide Area Augmentation System (WAAS) and the European Geostationary Navigation Overlay Service (EGNOS), it is possible to achieve a degree of accuracy down to approx. 1–2 metres. With the Galileo system, the level of accuracy will be approx. 30 cm. Partly because the atomic clocks in the Galileo satellites are more accurate, and partly because the signals (‘packets with codes’) are transmitted from Galileo with a higher degree of resolution while being more immune to noise.

TAPAS and Galileo closely interlinked

The TAPAS system comprises 11 permanently fixed reference stations in and around Aarhus.

The TAPAS stations use data from GPS and Galileo as well as other GNSS systems. The exact position of each station is calculated on the basis of satellite data, and corrections are calculated which are sent to moving units with positioning equipment, after which their exact position can also be determined in real time.

Positioning with corrections takes place in two ways

Method 1:

The satellites transmit signals which are received by both the GNSS reference station and the moving unit. Both units pick up the same ‘packs of codes’ with signals.

By subtracting the two sets of measurements/data from each other (the measurement from the mobile unit is deducted from the measurement from the fixed reference point), the factors or sources of error affecting both sets of measurements/data are eliminated.

This leaves a baseline where the sources of error are eliminated, and which is thus the actual distance vector between the two units.

Based on this, the position of the mobile unit can now be calculated and, if necessary, displayed on a navigation device. The calculation is updated every second, and provides a level of precision down to one centimetre on a map.

Method 2:

The satellites’ clocks and orbit eccentricities are determined in real time, just as the troposphere and ionosphere are also mapped in real time. These and other deviations are determined and sent to the GNSS receivers on Earth. The GNSS receiver on the ground can then correct for the sources of error, and thus also determine a precise position.

TAPAS-platformen består af 11 fast placerede referencestationer i og omkring Aarhus (Illustration: DTU Space)

The 11 reference stations in the TAPAS platform are located in and around Aarhus. There are only 4–22 km between them. In other words, it is a very close and precise reference system compared to other existing systems. Combined with data from the Galileo satellites, it is therefore possible to achieve extremely precise positioning. (Illustration: DTU Space)