# Galileo: The Added Value for Integrity in
## 1 Introduction  
Geolocation, _i.e._, the process of identifying the position of a given object, is present in everyday life; for example, mobile devices, be they smart phones or tablets, are generally equipped with a GPS receiver and cameras, so a user can geo-localize a picture taken with the device. This is only one of the applications classified as location-based service (LBS). Hence, there is a growing request for LBSs that can be carried out in different types of environments, from \"open-sky\" to indoors; hence, the navigation block has to be able to provide a seamless reliable navigation solution in different scenarios, such as open-sky, open-pit mines, urban canyons or even indoors. These scenarios are characterized by different signal availability and propagation conditions; in the first case, open-sky, a single GNSS is able to provide reliable, accurate and continuous navigation solutions with an accuracy of usually less than 10 m [1]. The good performance and the ease of operations of GNSS navigation in open-sky conditions have stimulated the need of extending GNSS-based navigation in signal-degraded environments, such as open-pit mines, urban canyons and indoors. These scenarios are characterized by a reduced availability of GNSS signals. Moreover, even when the minimum number of signals required to compute the navigation solution is available, gross errors due to multipath and fading effects can degrade GNSS observables. Signal availability and challenging propagation conditionsare just two of the major problems that have to be solved in such scenarios: an increased number of satellites and suitable processing techniques are required to enable seamless navigation. Availability, accuracy, continuity, coverage and integrity [2] are the parameters commonly used to assess the performance of navigation systems. Navigation is enabled only if minimum requirements in terms of these metrics are met. However, GNSS-based navigation may provide unreliable navigation solutions in harsh environments, due to the poor signal quality. Furthermore, in degraded scenarios, the performance requirements may not be satisfied using a single GNSS. For these reasons, several technologies have been proposed to extend LBSs in such environments. Among the proposed solutions, the multi-constellation approach, _i.e._, the joint use of measurements provided by several GNSSs, has the potential to enable GNSS usage in environments where navigation using a single GNSS is usually precluded [3; 4; 5; 6; 7]. Multi-constellation receivers are able to process an increased number of satellite signals and, thus, can provide a more continuous solution with respect to single constellation devices [3]. Hence, the solution availability (SA) can be significantly improved. Despite these advantages, signal availability is not sufficient to guarantee an accurate navigation solution. Measurements can be affected by gross errors induced by signal attenuation, multipath and fading effects. In this case, the redundancy provided by the multi-constellation approach has to be exploited to isolate and exclude distorted observations. Redundancy is essential for measurement consistency checks [8]; more measurements and stronger geometry conditions enhance the capabilities of quality monitoring procedures, for both detection and identification purposes [9]. Quality monitoring procedures are part of the integrity process, which refers to the ability of a system to provide timely warnings to users when the system should not be used [10]. In GNSS, integrity information can be obtained from different sources. First of all, GNSSs provide integrity information via the navigation message. This information usually refers to anomalies related to the system and satellite operations, such are satellite clock errors. The integrity information, provided by GNSSs themselves, cannot be used for real-time applications: because the ground control segments can take up to a few hours to identify and report the failure of a satellite [1].