# Relative Positioning in Remote Areas Using a GNSS Dual Frequency Smartphone
## 1 Introduction  
The use of Global Navigation Satellite Systems (GNSS) is nowadays an indispensable tool in areas such as terrestrial, maritime, or aerial transportation, agriculture and forestry, fisheries, financial services, emergency, leisure and recreational activities, mobile services, location, cartography, and many other in which the use of georeferenced information allows for efficiency gains. Providing Positioning, Navigation, and Timing (PNT) services, GNSS systems have been asserting themselves as enhancers of a wide range of applications with an impact on people's quality of life. These capabilities in mobile phones have evolved in a way that they allow today's precisions in positioning never achieved in this market segment and, until recently, only achieved using the expensive and high performance GNSS geodetic receivers. [PERSON] et al. [1] 2014, demonstrated for the first time that accurate positioning at the centimeter level, was possible based on raw code and phase data froma smartphone-quality GNSS antenna. This possibility can be explored in some areas as a lower-cost alternative to the expensive professional GNSS geodetic equipment. Three scenarios contributed to this evolution: the first was the introduction of the Android 7.0 Nougat, by the Google company in May 2016 [2], in the smartphones which allow access to the raw code and phase GNSS measurements; the second was the availability of Galileo signals in the smartphones, allowing access to the capabilities of this system characterized by the advanced atomic clocks, the new techniques of signal modulation as the AltBOC, the higher resistance to spoofing and jamming interferences; the third was the introduction, in May 2018, of the Broadcom BCM47755 chip with dual frequency L1/L5 [3] for smartphones, allowing the mitigation of the ionosphere effects, which is one of the biggest sources of error for the L1 only frequency receivers. Furthermore, the L5 frequency allows identifying the signal that makes the direct path (without reflections) easier, minimizing the multipath associated errors, and improving the positioning performance in urban areas. Several works have been done that show the potential of GNSS, including Galileo-enabled smartphones to reach sub-meter and centimeter accuracy in kinematic or static modes. The authors, [PERSON] et al. [4], used a PPP (Precise Point Positioning) approach to obtain results with a root mean square (RMS) in static mode of 0.20 m in the east, north, and up (ENU) components for the converged solutions and between 0.50 m and 1.09 m in the ENU components for the kinematic test using the dual frequency E1/E5a, L1/L5 from the Galileo, GPS systems and the G1, B1 single frequency from the GLONASS and BeiDou. [PERSON] et al. (2020) [5] also showed a PPP approach with accuracies of 0.33 m in the horizontal and 0.67 m in the vertical components achieved with a dual frequency smartphone, however, a long observation time is needed for the convergence of the solution, which can make the procedure inefficient. According to the authors, real-time PPP is yet to be realized. For many applications, the GNSS post-processing approach has some advantages fundamentally by allowing the edition of the processing parameters, choosing the best combination of the different GNSS systems, choosing atmospheric models and elevations masks, using precise ephemeris and clock information, discarding satellites that disturb the results, usually allowing to reach higher accuracies than the PPP. [PERSON] et al. (2021) [6] presented static mode results for GPS L1 solutions in relative positioning, using a very small baseline, with standard deviation values around 40 cm in the float solutions and below 10 cm in the fixed solutions. Static GNSS solutions with an accuracy of 0.10 m from a dual frequency smartphone using a small baseline were obtained by [PERSON] et al. [7]. The same author showed in a kinematic approach accuracy reaching 20 cm with dual frequency and 50 cm with a single frequency smartphone.