NLOS-MULTIPATH EFFECTS ON PSEUDO-RANGE ESTIMATION IN URBAN CANYONS FOR GNSS APPLICATIONS. Rudy Ercek (1) , Philippe De Doncker (1) , Francis Grenez (1) (1) Université Libre de Bruxelles – Service Ondes & Signaux, Av. F. D. Roosevelt, 50 CP165/51 -1050 Brussel – Belgium Email: rercek,pdedonck,fgrenez@ulb.ac.be ABSTRACT Pseudo-Range (PR) errors due to NLOS-Multipath (Non-Line-Of-Sight-Multipath) are studied in an urban canyon model. In order to determine the different reflected and diffracted rays which compose the NLOS- Multipath, a dedicated ray tracing algorithm is applied. Two different methods are used in order to compute the PR error. The first one uses the error due to the maximum power ray and the second one uses an Early Minus Late (E-L) receiver model. Simulations in different urban canyon configurations are carried out in order to obtain PR error distributions and associated probabilities due to NLOS-Multipath rays above a given power threshold. 1. INTRODUCTION For a few years, research projects are carried out in order to use GNSS (Global Navigation Satellite System) in the safety framework. One of these recent projects is the Belgian PIST Project which aims to develop safety algorithms in order to calculate the position and speed of a mobile, with high integrity level, by using a GPS (Global Positionning System) receiver in combination with other sensors. Railway has been chosen as case study because train positioning and speed measurement require a high integrity level. The work described in this paper is part of the PIST project. Recent developments in GNSS consider safety applications by using satellite based augmentation systems (SBAS e.g. EGNOS) or GALILEO, offering, both, a safety service (SoL – Safety of Life) by monitoring GNSS signals. However, this service does not provide any protection against multipath since this effect is only local and depends on the environment. When a satellite is in direct view i.e. in Line-Of-Sight (LOS), the signal captured by a GNSS receiver is composed of a direct ray, and of diffracted and reflected rays called multipath [1]. These multipath components cause an error in Pseudo-Range (PR) measurement, but mitigation techniques exist in the receiver to decrease this error. The classical multipath model (Direct ray + one multipath ray) for the GPS C/A code shows that the maximum multipath errors of a narrow correlator receiver are proportional to the early-late chip spacing d (e.g. max. PR error ~15m for d=0.1 and Signal to Multipath Ratio (SMR) = 0dB)[6]. Moreover, the user equivalent range error (UERE) budget [1], where all errors are considered to have a normal distribution, takes only account of the error due to multipath with direct ray. In Non-LOS (NLOS) propagation conditions, when no direct ray is present, or if the power of the direct ray is too weak, the PR error can become very large. In this paper, a study of this error due to diffracted and reflected rays (NLOS-Multipath) is presented in an urban canyon model, with two different methods of PR error estimation. 2. URBAN CANYON MODEL This model has been chosen because it is a really worst case scenario for NLOS-Multipath and, so, a critical environment for safety aspects. The urban canyon configuration is also similar to the railway environment. Figure 1. Urban canyon Fig. 1 shows an urban canyon which is composed of two parallel brick walls (or two rows of buildings), each one at a given distance from the receiver Rx (w,w’). The receiver is placed at the beginning of the urban canyon (x=x’=0) and is equipped with an ideal omnidirectional RHCP (Right Hand Circular Polarization) antenna. The urban canyon follows the earth curvature, and the brick walls are modelled by thin lossy dielectric plates. The height profile of the walls (h(x) & h’(x’)) is generated from a Rayleigh distribution with a given mean height µ h [3] and the same constant width l, typically 10m. Two different height profiles have been mainly used: