## 3 Signal Quality Assessment  
The performance assessment of a standalone positioning with code pseudoranges is preceded by a quality evaluation of GNSS signals acquired by the employed smart devices. As previous studies showed, GNSS observations tracked by smartphones are subject to several unwanted effects and suffer from substantial observation noise and low suppression to multipath effect [9; 15; 27]. Such outcomes are said to be related to low strength of the GNSS observations acquired by smartphones. This section provides selected characteristics of GNSS observations and verifies, with regard to the most recent smartphones, the common assumption that smartphone GNSS signals are of poorer quality with respect to these collected by geodetic grade receivers. Our investigations are focused on the carrier-to-noise density ratio (\(C/N_{0}\)) that characterizes the GNSS signal power and noise of code pseudoranges [10]. We assess the quality of code observations since their noise directly propagates into the position estimates in single point positioning. We begin with the presentation of the time series of \(C/N_{0}\) of GNSS signals that were tracked by analyzed smart devices in Figure 2. In the plots, the values corresponding to different satellites are distinguished by different colors. As we may read from the figure, the maximal power of acquired signals does not exceed 50 dB-Hz. We find that this value is significantly lower when compared to high grade receivers since the latter acquire signals with power reaching even over 50 dB-Hz [6; 28; 29]. What also transpires from the plots in Figure 2 is that the \(C/N_{0}\) records are subject to frequent outliers characterized with ultra-low values. This effect holds true for all analyzed smartphones. We attribute this effect to higher susceptibility to multipath and worse elimination of outlying observations by smart devices with respect to high grade receivers and antennas. In the consecutive skyplots in Figure 3, we present the \(C/N_{0}\) of GNSS observations as a function of azimuth and elevation of the satellite signal. The figure gives us the first impression of the \(C/N_{0}\) dependence on signal elevation and verifies the assumption of azimuthal symmetry that holds true for high-grade receivers and antennas. Looking at the skyplots in Figure 3, we may conclude that this is not the case for GNSS observations collected by the smartphones. We do not just discover a low \(C/N_{0}\) dependence on satellite elevation but also a clear azimuthal asymmetry of a signal gain. We also noticed a number of drops of \(C/N_{0}\) that are unexpected for high elevations e.g., in the results for Xiaomi Mi 8 which are given in the left panel of Figure 3. Such results provide us with the first hint that, commonly employed, an elevation-dependent function may not be optimal for smartphone GNSS observation weighting and suggests an application to be the \(C/N_{0}\)-dependent one. To complete the quality analysis of smartphone GNSS observations, now we focus on the stochastic properties of code pseudoranges. We take advantage of the satellite to receiver (absolute) observations that were double differenced in a time domain (DDTD). Such combination isolates the observation noise as the impact of low-frequency effects such as geometric distance, satellite orbits and clocks, receiver clock, atmospheric propagation errors as well as other time correlated effects are eliminated or significantly reduced [9; 30]. Figure 2: \(C/N_{0}\) time series of GNSS signals collected by Xiaomi Mi 8, Xiaomi Mi 9, and Huawei P30 in left, middle, and right panels, respectively. The values corresponding to different satellites are distinguished by different colors. Therefore, the variations of such combination residuals should be predominately attributed to the noise of observations. In Table 2, we report the standard deviations (STD) of code pseudoranges obtained from the DDTD observations, whereas, in Figure 4, we show the distribution of DDTD observations for the consecutive smartphones and signals.