# The Local Integrity Approach for Urban Contexts:
## 1 Introduction  
The concept of GNSS _integrity_ is conventionally defined as the measure of trust that can be placed in the correctness of the information supplied by a navigation system. This concept originates from the classic integrity framework, which was principally defined in the aviation context in the last decades. Since this framework was intended for _Safety-of-Life_ (SoL) applications, it also includes the ability of the system to provide timely warnings to users when the system should not be used for navigation. Extensive studies have been conducted on these aspects and a comprehensive literature is available (e.g., see [1] and references therein). In recent years the concept of GNSS integrity has started to become of interest not only in the aeronautical field, but also in other transportation sectors, where the availability of a \"reliable\" Position, Velocity, and Time (PVT) information has become a pressing need. Examples of such newcomers are _safety-critical_ or _liability-critical_ applications especially in vehicular contexts, where the availability of a Protection Level (PL) associated to PVT data from a GNSS receiver can offer a clear added value (e.g., autonomous driving, law enforcement, Pay as You Drive insurance, and electronic toll collection [2]). Nonetheless, it has been argued that the applicability of the aviation-born integrity to other transportation fields is far from being straightforward: a deep reconsideration of the approach is necessary in order to effectively exploit it in non-aviation operations [2; 3]. In fact, the classic GNSS integrity is known to be affected by conceptual and practical limitations when applied to urban contexts [4; 5]:  
* Conventional analytical models used to predict the pseudorange error variances (assuming an open-sky satellite visibility) may be inconsistent for mass-market automotive receivers. In fact themodels for the User Equivalent Range Error (UERE) are based on line-of-sight propagation with diffuse ground multipath only, and they often assume the availability of differential corrections. They do not account for possible time- and space-varying impairments (e.g., limited satellite visibility, non-line-of-sight signals, fast fading), whose probability of occurrence is high in urban scenarios [2];
* Typical requirements of integrity and continuity risks associated to aviation operations per phase of flight could be too conservative with respect to the requirements of vehicular applications, especially in non-SoL cases [3];
* In the case of _Satellite-Based Augmentation System_ (SBAS, for example EGNOS in Europe), the typical availability and applicability of GNSS integrity information can be limited in urban canyons. In detail, either the geostationary satellites can be blocked or the obtained PLs can result too conservative for a non-SoL vehicular application (e.g., as demonstrated by the measurement campaigns reported in [6]);
* The classic integrity approach does not take advantage of collaborative architectures, implying the potential availability of additional information from multiple GNSS receivers connected by Vehicle-to-Vehicle and Vehicle-to-Infrastructure communications (e.g., in a VANET) [5]. These theoretical problems and practical limitations are gaining special attention in both the GNSS and vehicular communities, where the investigation of possible solutions is an active research topic. Some standalone solutions have already been proposed, focusing on autonomous integrity approaches and/or fusion of a GNSS receiver with other sensors available on board of a vehicle (e.g., inertial measurement unit, odometer, vision sensors, _etc._) [4; 5; 7; 8; 9]. An innovative integrity monitoring solution has recently been introduced in [10]. This novel \"local integrity\" concept is based on a collaborative architecture tailored to the vehicular context and aims at overcoming the limitations of classic integrity.