GNSS Time Offset

Effects on GPS – Galileo Interoperability Performance

Time is a crucial factor in satellite-based radionavigation. The elapsed time between the transmission of a GNSS signal and its reception by a receiver, multiplied by the speed of light, provides the basis for calculating ranges to the satellites. However, GPS and Galileo will use different reference time systems creating a time offset, which can complicate the positioning computations in user equipment employing signals from both satellites. A team of researchers at the French space agency CNES describe the effect of this time offset on positioning accuracy and examine three methods proposed to eliminate the offset.

Inge Vanschoenbeek, Bernard Bonhoure, Marco Boschetti, and Jérôme Legenne
(CNES, Le Centre National d’Études Spatiales)

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By measuring the elapsed time between the transmission and reception of GNSS signals, navigation receivers calculate the pseudorange to each satellite and use this information to calculate their position on Earth. However, receivers not only have to calculate the user’s position in a 3D environment; they also have to cope with an unknown time bias between the receiver and the GNSS system.

Therefore, to precisely estimate a user’s position, receivers need to solve for four unknowns, which requires at least four pseudorange measurements between the receiver and the satellites. This means that a minimum of four satellites needs to be available to accurately estimate a user’s position.

Because the positioning accuracy will improve as more satellites become available (better geometry measured with the well-known dilution of precision or DOP parameters), we should expect that a combination of Galileo and GPS will provide better performance than those of both systems separately. In the future, therefore, most GNSS receivers will calculate the navigation solution using measurements from GPS and Galileo together.

However, the European navigation system Galileo will not use the same time reference as GPS and, thus, a time difference arises — the GPS-Galileo Time Offset (GGTO). The navigation solution calculated by receivers using signals from both navigation systems will consequently contain a supplementary error if the GGTO is not accounted for. Users wanting to use measurements from both systems need to cope with this time offset if they do not want to degrade the final navigation solution.

This article discusses three different approaches to solve the problems faced when using both navigation systems together, proposed in the work by A. Moudrak et alia cited in the Additional Resources section near the end of this article. These three approaches are the following: GGTO determination at user level, at the system level, and a combination of these two methods.

We have extended the previous research into the methods of GGTO determination by examining the differences among the three proposed solutions with the aid of a software simulation tool in representative environments, including a 3D model of the city of Toulouse, France. Our research also took into account not merely the directly received signals from the satellites, but also signals reflected from the surrounding structures.

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Manufacturers

The simulation tool used in the research described in this article is the software ergospace, developed by the company Ergospace, Toulouse, France.

Author Profiles

Inge Vanschoenbeek graduated from Delft University of Technology with a M.Sc. degree in aerospace engineering. Currently she is working as a simulations and systems engineer in the field of satellite navigation at CNES.

Bernard Bonhoure graduated as an aeronautic engineer from ENSICA. He joined CNES in 1989 and has worked on several successful space programmes such as TOPEX/POSEIDON, SPOT, ENVISAT, and JASON. As a navigation system expert, he is now involved in many activities linked to satellite positioning and performances.

Marco Boschetti graduated as telecommunications engineer from UPC (Universidad Politécnica de Cataluña) in Barcelona and aerospace engineer from Sup’Aéro in Toulouse. He entered CNES in 2003 and he is responsible for GNSS simulation activities in the Location and Navigation System Engineering Department.

Jérôme Legenne was graduated as an aerospace engineer from Sup’Aéro in Toulouse. He occupied several positions at CNES Toulouse and French Guiana space centers, including four years integrated within the European Space Agency’s EGNOS project team. Since 2004, he has been the head of CNES’s location and navigation system engineering department.