## 1 Introduction  
In Global Navigation Satellite System (GNSS), the user computes its position by means of measured distances between the receiver and a set of in-view satellites. These distances are calculated estimating the propagation time of signals transmitted by each satellite. For such a reason GNSS receivers are only interested in estimating the delays of signals in line-of-sight (LOS) carrying the information on the relative distance. In some environments, reflected versions of the LOS (multipath) may exist due to obstacles surrounding the receiver [1]. When such reflected rays have a delay smaller than one chip duration, they are not mitigated by the shift-orthogonality properties of the codes used for the ranging, and they may cause a bias in the delay and carrier-phase estimations. As a result, the positioning accuracy of GNSS receiver is badly affected. The problem of multipath mitigation has been extensively tackled in literature [2], since the early years of GNSS. Several techniques have been studied and developed along the years, in order to mitigate the effects in the pseudorange computation [3]. To further improve the robustness to multipath, the recent trend in multipath mitigation is the use of more complex receiver architectures able to estimate the multipath features, thus enabling a mitigation based on a wipe-off of the spurious components. A different approach in order to increase the robustness of the receiver is the coupling of the GNSS receiver with sensors of different nature, with complementary features with respect to the GNSS receiver. Such techniques include the integration of GNSS signal with other information sources and sensors, e.g., inertial navigation system (INS) [4]. In this paper, we present a preliminary work performed, targeting as a final objective the assessment of the integration of the GNSS receiver with a Light Detection and Ranging (LiDAR) system. LiDAR is an optical remote sensing technology that can measure the distance to a target, or other properties of the target, by illuminating the target with light, often using pulses from a laser. LiDAR can be used with a wide range of targets including metallic and non-metallic objects like buildings in urban canyons. A narrow laser beam can be used to map physical features with very high resolution, which is a key point in modeling close reflections. A LiDAR can be used to obtain digital surface models of the environment, thus using it as an external source of information on the expected reflections impinging the receiver antenna [5]. The objective of this work is (1) to investigate the feasibility of using the LiDAR measurements for the estimation of the GNSS multipath reaching the receiver, thus to cross-check in an urban environment and (2) to validate the possibility to have a run-time estimation of the environment and get aiding information by it. Modeling of the environment surrounding the receiver, taking into account the specific features of GNSS measurements, is becoming of growing importance in order to define the reliability of the position obtained. Classical integrity determination procedure are in fact useless in urban environment where the impact of the multipath ask for different definitions of the protection levels [6,7]. The work presented in this paper is performed by means of real GPS signals and LiDAR measurements recorded during the data collection campaign in an urban environment. The raw GPS data were processed in order to characterize the multipath error using a GNSS fully-software receiver implementing the Coupled Amplitude Delay Lock Loop (CADLL) architecture [8, 9, 10], able to estimatethe multipath features. Results obtained from this test campaign show the feasibility of the developed approach for multipath error characterization. The paper is organized as follows: Section 2 briefly summarizes the CADLL architecture, while Section 3 describes the methodology for the data collection and the environment characterization.