# Galileo: The Added Value for Integrity in
## 1 Introduction
Therefore, additional sources have to be used to obtain the integrity information. Examples of such sources are aircraft autonomous integrity monitoring (AAIM), ground-based augmentation system (GBAS), satellite-based augmentation system (SBAS) and RAIM [1]. AAIM systems use information provided by sensors mounted on-board an aircraft to obtain integrity information [8]; whereas GBAS and SBAS systems compute integrity information exploiting a network of ground stations and satellite constellations, respectively [10]. In the GBAS and SBAS cases, additional infrastructures are required, increasing the complexity and the overall cost of the navigation system. However, errors due to local effects, such as multipath and fading, are not identified using the above-mentioned techniques. These limitations promoted the development of RAIM, which is considered here: this approach can be the key enabler for error isolation and exclusion. RAIM is a technique that exploits only the information available using a GNSS receiver; RAIM algorithms may detect user-level errors [1]. Such techniques are used for quality monitoring to identify, and eventually exclude, observables affected by gross errors [11]. RAIM algorithms were originally conceived of as part of air navigation systems where they are adopted for safety-critical applications. The classical algorithms, considering only GPS, assume only one measurement failure at a time. In harsh environments, such an assumption may not be sufficient, since often, two or more outliers can be present in the measurement set. In order to overcome the limitations of traditional RAIM algorithms, different strategies have been proposed using multi-constellation solutions. Moreover, in harsh environments, the most stringent assumptions is related to the presence of a single blunder in the measurements set. Hence, modified algorithmshave to be developed to take into account the presence of multiple blunders and to overcome the limitations of the classical RAIM algorithms. The RAIM algorithm proposed in this research is a modified version of the classical forward backward (FB) [12] approach and considers a GPS/Galileo multi-constellation scenario. The proposed algorithm provides improved performance with respect to the classical FB algorithm exploiting the introduction of a preliminary check to verify the geometry conditions. Such a test has been adopted since the capabilities of detecting outliers is highly dependent on the measurement redundancy. Moreover, the presence of an outlier can cause the rejection of good measurements; hence, in order to avoid the rejection of blunder-free observables, a separability check is introduced. The fault detection and exclusion (FDE) algorithm implemented is made up of two phases: the forward and backward phases. The first one uses four tests, specifically a preliminary check based on geometry considerations, a global test (GT) to check the consistency of the measurement set [12; 13], a local test (LT) to identify the outlier [14] and, finally, a separability test to check the correlation between the observables [15]. These tests will be described in Section 3. The backward phase, where the erroneously-rejected measurements are re-introduced, is based only on a GT test. A complete description of the algorithm is provided in Section 3. In order to validate the proposed algorithm, several hours of GPS and Galileo data were collected in a signal degraded environment where a multi-constellation receiver was placed between high reflecting buildings. The performance of the GPS/Galileo multi-constellation solution is assessed and compared with respect to that obtained using only GPS. The improvements achieved using the proposed RAIM algorithm are assessed. The navigation system performance is evaluated using three parameters, specifically standard deviation (SD), mean and maximum errors. These parameters are computed for the horizontal and vertical components of the position and velocity solutions.