GPS World, March 2018

SIMULATION MARCH 2018 WWW GPSWORLD COM GPS WORLD 29 for the simulation of realistic multipath in different environments and compares simulated and field test data The comparison is based on signal availability horizontal error carrier to noise C N 0 pseudorange and Doppler residuals RAY TRACING WITH 3D MODELING The model simulates the propagation of GNSS signals in constrained environments considering obscurations and multipath It uses a proprietary ray tracing kernel based on bounding volume hierarchy techniques using processing unit GPU resources coupled with geometrical optics and uniform theory of diffraction to compute the interaction between the signal and the local environment The computation uses as main input a synthetic environment that is geometrical and physical modeling of a real or realistic environment to assess the impact of obscurations related to signal availability issues and multipath the cause of fading effects and performance problems The objective of ray tracing is to find all the possible paths from the observer to the source of the signal considering a limited number of interactions per emitted rays A ray tracer or ray tracing algorithm uses a primary grid to cast primary rays Then it iteratively computes the possible interactions between these rays and the virtual scene often defined using triangles If those interactions exist if they comply with the law of physics and if the number of interactions to reach the emitter is below the maximum number of interactions set by the user then a ray or multipath is created This is a deterministic method that can be used to calculate the obscuration due to the local environment and therefore detect the signal availability and the geometrical characteristic of the computed path Combined with physics modeling path attributes such as received power delay Doppler and phase are also provided The main characteristics of ray tracing techniques to model GNSS propagation are All the signals arriving at the receiver can be modelbased on the virtual environment As it is a deterministic method the more realistic the environment modeling the more compliant with reality the results Moreover the simulation results are repeatable The specular multipath can be displayed in 3D and the attributes for example receiver power phase polarization Doppler geometry of the ray are known For example this is relevant when the effect and signature of the environment on the propagation signal need to be studied and understood Nonetheless ray tracing techniques must account for three major difficulties They are time consuming algorithms Indeed depending on the complexity of the scene defined in terms of the number of triangles a combinatorial problem to find the possible multipaths reaching the receiver makes the ray tracer very resource demanding That is the reason why the most difficult task to achieve during the coding of a real time ray tracing algorithm is to develop acceleration techniques to quicken the computation process Several solutions exist to either improve the intersection determination for instance based on spatial hierarchies such as bounding volume hierarchy BVH techniques or to decrease the number of cast rays often based on adaptive sampling techniques or even to replace rays with beams or cones Moreover it is possible today to use the resources of graphic boards to accelerate the computation Indeed as ray tracing can be coded by a large number of primary functions that can be treated simultaneously it can be easily ported into GPU Their accuracy depends on the resolution of the primary grid Details and therefore rays may be missed if the 3D scene is made of small details This issue is called aliasing Aliasing artefacts are raised for instance in parts of the scene with abrupt changes such as edges or in complex areas with lots of constituent objects Solutions or antialiasing techniques exist to overcome this issue such as adaptive or stochastic samplings When it is combined with geometrical optics these algorithms only compute the specular rays Even if some techniques exist to model the scattering signals only physical optics can render the global signal with high fidelity MULTIPATH SIMULATION SYSTEM The proposed system can model two of the main propagation issues encountered in urban environments such as obscuration which leads to limitations in signal availability and multipath which generates interference that causes fading of the signal and positioning errors To model realistically such a complex phenomenon the system uses a GPU ray tracing algorithm combined with geometrical optics and uniform theory of diffractions The ray tracing algorithm relies on 3D model reconstructions of the urban environment The computed obscuration and multipath effects are then used to generate signal corrections in terms of power delay and Doppler variation to be used in the GNSS simulator which generates the carrier code and navigation messages for different GNSS constellations into a single RF output Some of the advantages of this system is its ability to run in real time and to visually show all the reflections FIGURE 1 FACING PAGE Example of propagated signal simulation