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SPRING - Overview

Thales Services together with French Space Agency is developing a GNSS simulator named SPRING. One of the aims of this joint activity is to create an innovative and reliable tool that will simulate the propagation of GNSS signals. In SPRING, two simulation levels are previewed: a global service volume simulation and a local simulation with accurate 3D environment modeling. The global level of the simulator provides the assessment of the performances of navigation systems over a global or a regional area by computing the performances parameters, such as Dilution of Precision (DOP). It allows the analysis of the evolution of navigation performances obtained with the current GNSS constellations and the deployment of new ones. This type of simulation does not need the implementation of 3D models. The following figure shows an example of service volume performances.

VDOP map produced by the SPRING simulator

VDOP map produced by the SPRING simulator

The local level will often represent simulations conducted in urban canyons. Therefore, the environment representation has to be more realistic and must take into account as precisely as possible interactions with local surrounding area, especially multipath. To correctly assess positioning performances, a 3D model of local environment and a realistic propagation modeling are needed.

Studies over the light ray propagation have already been conducted by specialists of the computer graphics. Several techniques focused on obtaining the best possible 3D rendering have been developed. The most common techniques are rasterisation, ray casting and ray tracing. Among these, ray tracing provides the best realism but on the other hand has the highest computation needs.

Due to the fact that light rays and GNSS signals are both electromagnetic waves, they are subject to the same effects. Therefore, ray tracing can be applied to simulate the propagation of the satellites signals. To successfully accomplish this task, it is assumed that the GNSS satellites are considered as light sources, and that the GNSS receiver antenna is the camera that is watching the 3D scene.



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