# On Mitigating the Effects of Multipath on GNSS Using Environmental Context Detection
## 1 Introduction  
The availability and accuracy of Positioning, Navigation and Timing (PNT) services can play a significant role in making transportation (road, rail, air and marine) more efficient, safer and sustainable by enabling route optimization and fuel economy planning [1, 2, 3, 4]. Real-world applications, such as precision agriculture, real-time tracking, asset management, emergency response services, disaster management and autonomous driving, rely heavily on accurate positioning and time estimations [5, 6, 7, 8, 9] for proper functioning. In addition, positioning and navigation technologies have recently played pivotal roles in the war against the COVID-19 pandemic by enabling the real-time tracking, tracing and locating of virus hot-spots by deploying geo-fencing [10, 11, 12] to monitor and control thespread of COVID-19. Among the various available solutions for precise positioning and navigation [13; 14; 15; 16], the Global Navigation Satellite System (GNSS) has established itself as a globally dominant and cost effective navigation technology [14; 17]. Positioning and navigation in urban canyons has always been a challenging task [18; 19; 20] as GNSS receivers encounter areas with an insufficient availability of satellites due to multipath (MP) and/or non-line-of-sight (NLOS) signal reception. This remains a potential vulnerability of satellite-based navigation systems due to biased range measurements and inaccurate positioning estimates [21; 22]. In [23], an initial study was performed on the characteristics of GNSS signals in highly obstructed environments by detecting and analyzing the possible deviations due to MP and NLOS reception. The quality of GNSS signals was analyzed and compared under different environments (i.e., LOS and NLOS) through field experiments. The results of these experiments showed that MP/NLOS could significantly affect the GNSS signal in terms of degraded tracking performance and frequent loss of lock. At present, there are four independent global satellite constellations (GPS, GLONASS, Galileo and BeiDou) with more than 100 units in orbit that are transmitting signals at several frequency bands to increase the satellite density [24]. This helps to improve the positioning availability and accuracy in constrained and highly degraded multipath environments. This was also demonstrated in [25] where it was shown that, when combining four constellations (GPS, GLONASS, Galileo and BeiDou), more than 35 satellites could be accessed and tracked at a given location. Similarly, researchers at Taoglas Inc. USA demonstrated a noticeable improvement in the positioning accuracy when using dual constellations (GPS and GLONASS) compared to single constellations [26]. The European GNSS Agency (ESA) and _Rx_ Networks Inc. performed several performance assessment tests in real-world environments and demonstrated that Galileo, when combined with GPS, provided more accurate positioning estimates compared to the GPS-only configuration [27]. In [28], researchers demonstrated that the performance of a tri-constellation navigation system in an obstructed environment (e.g., medium-height buildings) matched the performance of a GPS-only system in a clear open-sky environment when using only those satellites with elevation angles greater than 32 degrees. This implies that the impact of obstructions below 32 degrees can be mitigated by using an increased number of constellations. [PERSON] et al. [5] demonstrated that, in degraded environments (e.g., farms surrounded with dense trees), the availability and accuracy of positioning decreased significantly for the GPS-only system; however, limited impacts were observed for the multi-constellation GNSS case due to a greater number of visible satellites. This was experimentally validated in [25], where the impacts of using multiple constellations compared to a single constellation system in three distinct environments (i.e., open sky, partially obstructed and highly obstructed environments) were investigated.