# Accuracy of Code GNSS Receivers under Various Conditions
## 2 Background  
The rapid technological development and miniaturization of GNSS receivers has made their use in mobile phones possible. The first device of this type was the Benefon ESCI phone, released in 1999, equipped with a GPS positioning system operating on two GSM frequencies of 900 and 1800 MHz [7]. The appearance of a low-cost GNSS chipset after the 2010s has allowed for the development of mobile devices that are accessible almost to everyone [8; 9]. For example, the year 2013 started a new era: more smartphones were sold worldwide than any other type of phone [10]. In 2016, Google announced the Android Nougat (version 7) operating system, which allowed smartphones to receive raw GNSS measurements, i.e., pseudorange, carrier-phase, Doppler shift, and carrier-to-noise density ratio (C/N0) observations [11; 12]. According to the GNSS Market Report from the European GNSS Agency, in 2019, 1.8 billion GNSS receivers were sold; about 1.6 billion of these units were mass-market devices, costing less than EUR 5, and about 90% of these were mounted in smartphones or wearables (smart watches, fitness trackers, smart glasses) [13]. Smartphones, as navigation or positioning devices, are undergoing constant testing and improvement, and their usefulness and reliability in precision surveying are being tested [14]. Over the past few years, there has been a continuous and rapid development of both  
Figure 1: Receivers used in experiments: Garmin Etrex 30 and Xiaomi Mi 8 (Sources: [[https://www.garmin.com/pl-PL/p/87774/](https://www.garmin.com/pl-PL/p/87774/)]([https://www.garmin.com/pl-PL/p/87774/](https://www.garmin.com/pl-PL/p/87774/)), [[https://www.gsmarena.com/xiaomi_mi_8-pictures-9065](https://www.gsmarena.com/xiaomi_mi_8-pictures-9065)]([https://www.gsmarena.com/xiaomi_mi_8-pictures-9065](https://www.gsmarena.com/xiaomi_mi_8-pictures-9065));php: (accessed on 1 April 2022)).  
handsets and GNSS systems. Moreover, smartphones have been the object of continuous investigation by researchers in different fields of science and for applications, e.g., in terms of power consumption [15], usage of raw GNSS observations [16], or dual-frequency observations [17]. Nowadays, it is possible to achieve an accuracy of 3-5 m using GPS+GLONASS observations [7]. Such tests have shown that using satellite signals in addition to GPS provides more accurate measurements [18]. Tests have also been performed for various weather and environmental conditions [19] as well as regarding relative positioning [20]. Studies conducted using smartphones that record phase measurements offer the possibility of very high accuracy in static measurements: 1-4 cm [21] or even 2 cm for 60 min of measurement [22]. In addition, the use of external clock files and orbits allows the accuracy of the code solutions to be improved, and taking into account multi-tracking gives the possibility to achieve an accuracy of 2-3 m [23; 24]. The problem with this type of positioning is the fast TTFF (Time To First Fix), or time to full initialization of the receiver, which is currently as low as 0.5 s [15; 25]. Due to the nature and most common use of smartphones, they perform well in areas with high obscuration of the horizon, such as cities, mountainous areas, or forests [26]. In the case of such phones, the use of auxiliary information (IMU--Inertial Measurement Unit--sensors) even enables indoor position determination [27; 28]. Moreover, in recent years, raw (RINEX) measurements from Android smart devices have become more and more popular in accurate positioning, e.g., precise point positioning (PPP) [17; 29; 30; 31; 32; 33]. In addition to satellite techniques, other algorithms are becoming increasingly popular that refine positioning, such as SLAM (simultaneous localization and mapping) [34], stereo vision [35], or mixed algorithms [36].