GPS - Galileo Interoperability Performance with the GPS - Galileo Time Offset
Vanschoenbeek, I.; Bonhoure, B.; Boschetti, M.; Legenne, J.
CNES

Satellite navigation offers an excellent mean to determine precisely one’s position everywhere on Earth. These days, GPS is the only global fully operational GNSS service, but within a few years, Galileo will also become available. It is expected that a combination of these two systems will provide better performances than those of the current GPS or future Galileo alone. However, there exists an offset, called GGTO (GPS Galileo Time Offset), between the time references of GPS and Galileo. Consequently, this GGTO will, when not accounted for, introduce a supplementary error on the navigation solution calculated by receivers using signals from both navigation systems. These receivers thus need to cope with this time offset if one does not want to degrade the final navigation solution.

By measuring the time delay between the transmission and reception of GNSS signals, receivers calculate the pseudo-distance to each satellite. However, the receiver does not only have to calculate the 3D user’s position, it also has to cope with a time bias between the receiver and the considered GNSS system. When combining measurements from GPS and Galileo, five unknowns need be calculated in order to accurately estimate a user’s position: 3D user position, GGTO and the time bias between receiver and GPS or Galileo.

Three different approaches to take the GGTO into account have been evaluated during this study: GGTO determination at user level, at system level and a combination of these two methods.
The GGTO will be calculated by the receiver as a fifth unknown from the equations of navigation, when a determination at user level is applied. This means that at least five pseudorange measurements from the receiver to the satellites are required. Thus, a minimum of five satellites is needed to accurately estimate a user’s position with this method. However, this prerequisite might affect service availability for users in constrained environments (e.g. urban canyons) due to signal masking.
On the other hand, Galileo and GPS will broadcast the GGTO within their navigation messages and receivers can than apply this GGTO to account for the time offset. The navigation solution can now be calculated from four pseudorange measurements, as if only one navigation system is used. The GGTO is determined at system level when this method is used.
The third approach is a combination of the two aforementioned methods and might be a solution for users in constrained environments who prefer to determine the GGTO as a fifth unknown, but have to cope with signal masking. Depending on the satellite geometry in the sky and the number of satellites available, the receiver will use the GGTO calculated on user level or the one broadcasted by the satellites. The choice between the different methods needs to be made for every measurement and is based upon user-defined thresholds.

New definitions for the equations of navigation are thus required in order to take the GGTO into account in the final navigation solution. Different equations have been defined, depending on the GGTO determination method chosen.

The Ergospace tool simulates GNSS performances in constrained environments and has been used to evaluate the proposed methods for taking the GPS Galileo Time Offset into account. This software tool determines all the signal paths between a ground receiver and the navigation satellites and simulates direct as well as reflected signals. The environment is simulated by applying 3D models representing mountains or urban canyons. This way, the GNSS performances are evaluated in a realistic environment. Simulations can be performed in a dynamic mode, where the receiver follows a predefined path, but also in a static mode, where the receiver stays at a fixed location. CNES has specified and financed the development of this specific module to assess the performances of the different GGTO determination methods which are explained above.

This paper will present a description of the obtained results, with a trade-off between the different GGTO determination methods. In addition, this study provides valuable conclusions on the increase in precision and service availability due to the use of Galileo in combination with GPS.