A deep knowledge of all propagation effects has become an essential issue to design future communication and navigation systems and optimise their performances. Here, we will target Land Mobile Satellite (LMS) systems, with focus on Global Navigation Satellite Systems (GNSS). The urban environment is one of the most critical for LMS systems, since shadowing, multipath fading and time spreading are often present. This study aims at developing more efficient propagation channel models using physical-statistical approaches. In order to build these models, numerical asymptotic tools are to be used to avoid costly extensive measurements. These tools are theoretically valid for large objects. So, it is necessary to know which level of simplification of the environment is acceptable. Thus, this article performs a rigorous analysis of the influence of small scatterers at different levels of the transmission channel (up to the GNSS receiver), using the exact Method of Moments technique as a reference.

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Multipaths represent a common predominant and uncontrolled component on channel impairments for all terrestrial and Land Mobile Satellite systems. Without restrictions w.r.t mobile terrestrial applications, the addressed multipath problematic in this paper is focused on Land Mobile Satellite applications where delayed signal replicas are highly impacting performances on communication systems while they induce strong positioning errors for navigation systems. The actual trend in propagation channel modelling is to improve the multipath characterisation and representation by using semideterministic and hybrid physical-statistical models into channel simulators instead of narrow-band empirical approaches. In this context, this paper presents a new simplified model, called 3CM (3-Component Model) to reproduce building scattering in an efficient way which strongly improves computation performances. This model is based on asymptotic methods, namely, PO (Physical Optics) which allows the 3CM to be frequency scalable, polarimetric, and dielectric materials oriented. Note that the proposed model and the retained approach can be integrated into more complex tools such as existing ray tracers. However, this issue is not discussed in this paper.

When crossing the ionosphere layer, GNSS signals may be impacted by scintillation, a dynamic effect which causes very fast variations of the amplitude and the phase. The GNSS receivers may lose lock, which lead to unavailability of the satellite link as well as the reduced reliability of the available information. In the present state of the art, the understanding of scintillation phenomena is still insufficient, and the available models are still not adequate for GNSS system design. Thus, many studies are carried out in order to model scintillation [1] [2] [3] and evaluate the effect on the GNSS receivers and augmentation system [4] [5]. These studies monitor the scintillation thanks to specific scintillation index. However, the estimation of these scintillation indices may be affected by the receiver itself and by the estimator. The goal of this paper is to understand how receiver or estimator tuning may impact the estimation of scintillation index. To conclude, this paper gives recommendation on how to tune GNSS receiver or estimator in order to minimize their impact. I.