Multiconstellation satellite navigation is critical in signal-degraded environments where signals are strongly corrupted. In this case, the use of a single GNSS system does not guarantee an accurate and continuous positioning. A possible approach to solve this problem is the use of multiconstellation receivers that provide additional measurements and allows robust reliability testing; in this work, a GPS/GLONASS combination is considered. In urban scenario, a modification of the classical RAIM technique is necessary taking into account frequent multiple blunders. The FDE schemes analysed are the “Observation Subset Testing,” “Forward-Backward Method,” and “Danish Method”; they are obtained by combining different basic statistical tests. The considered FDE methods are modified to optimize their behaviour in urban scenario. Specifically a preliminary check is implemented to screen out bad geometries. Moreover, a large blunder could cause multiple test failures; hence, a separability index is implemented to avoid the incorrect exclusion of blunder-free measurements. Testing the RAIM algorithms of GPS/GLONASS combination to verify the benefits relative to GPS only case is a main target of this work too. The performance of these methods is compared in terms of RMS and maximum error for the horizontal and vertical components of position and velocity. 1. Introduction GNSS (Global Navigation Satellite Systems) are worldwide, all-weather navigation systems able to provide three-dimensional position, velocity, and time synchronization to UTC (Coordinated Universal Time) scale [1, 2]. The GPS system is the main GNSS and it is fully operational since almost two decades; in good visibility conditions (“open sky” scenario), GPS can provide a position accuracy of few meters for absolute positioning up to millimetre order for postprocessed relative positioning [2]. Satellite navigation in difficult scenarios (e.g., urban canyons, and mountainous areas) is more critical, because many GNSS signals are blocked or strongly degraded by natural and artificial obstacles; in these scenarios GPS only cannot guarantee an accurate and continuous positioning due to the lack of measurements and/or the presence of erroneous measurements. A possible way to fill this gap is the use of a GNSS multiconstellation receiver, considering the combined use of GPS with other GNSS such as Galileo, Beidou, and GLONASS. The performance of the integrated system is increased in terms of(i)continuity, directly related to satellite availability,(ii)accuracy, enhanced by observation geometry
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