Several solutions exist to either improve the intersection determination for instance, based on spatial hierarchies such as bounding volume hierarchies or 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. 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 a GPU.
This issue is called aliasing. Aliasing artefacts appear, for instance, in parts of the scene with abrupt changes such as edges or in complex areas with lots of constituent objects. Solutions or anti-aliasing techniques , such as adaptive or stochastic samplings, exist that can deal with this phenomenon. Indeed, by means of a ray-tracing algorithm, software can simulate two phenomena of propagation see Figure As ray-tracing is a deterministic method, specular multipaths can be displayed in 3D and their attributes receiver power, phase, polarization, Doppler, geometry of the ray, and so forth are known.
GNSS Applications and Methods
It is therefore easy to see the contribution of some part of the environment on the propagation of the signal. The Synthetic Environment The main input of a ray-tracing algorithm is the synthetic environment, a virtual modeling of a real or realistic environment in terms of both geometry and physics. The geometries of the buildings or the objects define the shape of the rays.
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The definition of the intrinsic parameters of the materials of the objects permittivity, conductivity, and thicknesses of the walls, and so on enables computation of the interaction between the signal and the objects. Validation — Example of Simulated Data A measurement campaign was recently conducted to validate the performance of a simulator based on ray-tracing algorithms. Two parts comprised the simulation system screenshots shown in Figure 5 see inset photo, above right, for figures 5 — 8: The computation frequency has been set to 10 hertz.
We selected a U. These raw data, which model the geometry of the large structures buildings, large objects , have been created using photogrammetry techniques from satellite stereoscopic imagery. The resolution of the input images was 50 centimeters.
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Then, we post-processed these data to add geometrical details buildings windows, doors, street objects, and so on and to define the building materials. Figure 6 shows how the model evolves from low to high resolution.
Figure 7 compares measured and simulated data when only obscuration has been considered no multipath. The results of the measurement campaign indicates that general trends are globally reproduced: Regarding GPS, the drop of power is caused by the obscuration of the LOS signal as explained by the simulated data.
However, the simulated power does not correctly mimic the small variations of power, mainly due to multipath interactions and interferences. The graphs in Figure 8 show a better consistency between measured and simulated data. The variation of power is more erratic and the visibility of the satellites is similar to actual measurements of signals.
Nevertheless, some discrepancies can be observed in the power level. Several causes can explain such discrepancies, such as quality of the scene especially for low elevation satellites with only non-LOS reception or the surrounding traffic during the test vehicle or pedestrian and the vegetation for instance, isolated trees , which have not been modeled in the simulation.
Deterministic simulations never produce data that fits perfectly with real measurements. Indeed, they highly depend on the realism of the synthetic environment, and it is not possible with limited time and effort to perfectly mimic a real configuration.
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ARTECH HOUSE USA : GNSS Applications and Methods
Contains detailed theory and study cases to help the reader put the material into practice Contains a special chapter on next generation GNSS-RS, which is are seen as the future in GNSS Describes new potentials as well as advantages that complement existing techniques see more benefits.
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