GNSS accurate positioning including satellite visibility check in a multiple hypotheses 3D mapping framework
Résumé
GNSS positioning in the urban environment remains challenging, because of the occurrence of many satellite signal reflection and diffraction; also called multipath, that, in the best case, combine with the direct signal, but – if the worst case when the direct signal is obscured – make signal tracking drift unboundedly. Recent research investigations carried out in the Geolocalization lab of the Ifsttar have shown that a simplified 3D modelling of buildings at both sides of streets, named “urban trench” for that reason, can benefit in the process of satellite visibility check and GNSS positioning. This model applies to street sections where the width between buildings and their height is homogenous. That way, two geometrical attributes only depict any urban trench section, which has the advantage of being easily compliant with the existing formats of navigable maps. Another parameter in the satellite visibility problem is obviously the lateral position of the rover. Our algorithm makes a hypothesis on the lateral position of the rover in a modelled street, and, using the satellite positions in azimuth and elevation, determinates whether their signal is in Line-Of-Sight or Not (NLOS), with respect to the modelled facades all around. The key problem resides in the initial hypothesis of lateral position. The research development presented in this communication consists in using a set of possible positions where one may drive on. Instead of checking a large set, which has a certain computation cost, the algorithm will just consider a set that corresponds here to the multiple lanes of the street. To deal with this set of lane candidates, an efficient solution consists in computing NLOS satellites detection and exclusion and the corresponding positioning solution for each lane separately. Finally, one will select among the possible positioning solutions the one that best matches the initial lane hypothesis. In addition to testing the coherence between hypotheses and solutions, the list of actually received satellites can also be examined and compared to that of theoretically received ones, similarly as “shadow masking” proceeds. Once NLOS satellites detection and exclusion have been made, the constellation of LOS satellites should optimally correspond to the occupied position of the roved if the best hypothesis is considered, whereas another hypothesis, less favourable, may exhibit mismatching in satellite direct visibility. This article gives also global results, not only on the part of the trajectory where the urban trench model applies, i.e. half in space in a city centre like Paris, but also anywhere else. That way, it has been checked that the most difficult areas have actually been modelled, whilst those un-modelled do not exhibit environmental singularities and let standard solutions relatively acceptable.