## 1 Introduction  
Global Navigation Satellite Systems (GNSS), like Global Positioning System (GPS) or Galileo, are getting more and more pervasive in many fields, including aviation, railway, maritime, and road. In order to satisfy the integrity requirements, becoming crucial for many applications, these systems do not only need to be precise and accurate, but they have to guarantee solutions with a certain level of reliability. In the last years, GNSS integrity increased its relevance in all the transportation sectors, especially for Safety of Life (SoL) services and liability critical applications, such as, for instance, road tolling applications, pay-as-you drive insurance, and other commercial services. However, the integrity algorithms for land applications are relatively new and not completely assessed, due to the fact that the number of new services continues to rapidly grow. Nowadays, the technology of the integrity systems in aviation is mature and many algorithms have been developed for this specific application. However, the absence of obstacles, the distance between the airborne and possible interference sources from the Earth, a reduced occurrence of multipath (MP) and the good visibility of the satellites make the techniques for aviation not directly applicable to other applications. In fact in sectors, such as road, railway, rural, and maritime, the environmental conditions around the receiver are in general worse than in aviation, and this implies that the integrity algorithms for aviation have to be modified. An example of integrity concepts applied to a urban environment can be found in [1]. A well assessed integrity technique is the so-called Receiver Autonomous Integrity Monitoring (RAIM) [2], which can be implemented by adopting different algorithms, such as range comparison [3], Pseudorange (PR) residual or Residual Based (RB) [4], and parity method [5]. They provide snapshot schemes and yield identical results under the assumption of equal alarm rate, as shown in [6], where a detailed comparison of these methods is given. Another well known RAIM scheme is the Solution Separation that works on the position domain [7]. A basic assumption of all these schemes is that only one fault at a time is possible. However, these schemes can be extended to the case of multiple faults, as described in the recent work [8]. The RAIM algorithms use PR and position data to implement the Fault Detection and Exclusion (FDE) module, which leads to the exclusion of faulty satellites in the position computation. However the identification of faulty satellites can be done also at the tracking stage of the receiver, by adopting adequate signal processing techniques, as shown, for example, in the recent paper [9]. Today, these methods cannot be implemented in a current commercial GNSS receiver, but a new generation of receivers can be devised with new blocks working in the correlation domain, and able to assist the FDE module of RAIM. The work presented in this paper analyses the feasibility of this concept of assisted-RAIM. In particular it is focused on the effects of environmental impairments, as MP, and how to assist RAIM to protect the receiver from this type of errors. MP occurs in the presence of objects, close to the receiver, which can reflect the signals coming from the satellites. The working principle of a GNSS receiver is based on the measurements of the delay between the time instant when a signal leaves the satellite and the time instant when the signal is received. Therefore, if reflected signals are received together with the useful signals, an error on the estimation of the delay of the GNSS signals may occur. The Delay Lock Loop (DLL) is the block devoted to the delay estimation. However a DLL error not always impairs the PR measurements. This fact has to be taken into account in the design of assistance methods based on correlation points estimated by a DLL. More details on this aspect are given in Section 3.