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 - in the worst case when the direct signal is obscured - make signal tracking drift unboundedly. Recent research investigations carried out in the GEOLOC laboratory of the Ifsttar (the French institute of science and Technology for transport, development and network) with the SCM (Société de Calcul Mathématique) 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. These urban trenches can be computed from 3D-city-model using road and buildings layers. Initially, the rover is assumed to occupy the arc segment that represents the street. That way, only left and right distance and height attributes depict any urban trench section, which has the advantage of being easily compliant with the existing formats of navigable maps. Using the satellite positions in azimuth and elevation, our algorithm determinates whether their signal is in Line-Of- Sight or Not (NLOS), with respect to the modelled facades all around. In case of NLOS, a correction of the pseudorange measurement is made. The geometry of this model being simplified, and the initial position approximated, the first deterministic approach has been modified: a probabilistic approach is proposed. From this second approach, an urban trench protection level can be computed (UTPL), which provides an integrity indicator adapted to the urban environment. This article is divided into 2 main parts: the first part gives an overview of the deterministic approach, whilst the second part concerns the probabilistic approach. Afterward experimental results are presented, with improvement not only in terms of accuracy, but also in terms of accuracy.