# A Controlled-Environment Quality Assessment of Android GNSS Raw Measurements
## 2 GNSS Positioning and Basic Performance of Android Smartphones
### Basic PVT with Raw Measurements  
The raw GNSS data used in this section were collected during a dedicated campaign at Politecnico di Torino premises, on 19 October 2018. Ten minutes of raw GNSS data were stored using the GNSSLogger App. L1 C/A signals from GPS constellation were considered. Three different devices have been used:  
* Xiaomi MI 8 (single-frequency mode) running Android 8, denoted as **MI 8**. * Samsung Galaxy S8 (Exynos 8895) running Android 8, denoted as S8. * Huawei P10 running Android 8, denoted as **P10**. The position of the phones and of the georeferenced point are shown in Figure 1. The georeferenced point is used as a reference position for error computation. It has to be noted that, although it is a roof-top location, an incomplete open-sky view is available due to some higher buildings in the surroundings. Some known sky obstructions, as well as some multipath reflections are expected to affect the quality of the collected data. GNSS signal strength is commonly evaluated through \(C/N_{0}\), defined as the ratio between carrier power \(C\) and noise power density \(N_{0}\)[12]. \(C/N_{0}\) is a good indicator of the quality of the signal and of the hardware components of the receiver, such as the antenna [16]. The \(C/N_{0}\) of three GPS satellites, common to all three devices, was observed: Pseudo Random Noise (PRN) number 18 with the highest elevation (67\({}^{\circ}\)), PRN number 20 with the lowest elevation (4\({}^{\circ}\)), and PRN number 14 at medium elevation (31\({}^{\circ}\)). Figure 2 displays the comparison between the \(C/N_{0}\) as measured from the three different devices. The quality of the S8 and MI 8 devices is comparable, at least for the closer satellites, while the P10 device performed the worst in all cases. In the case of PRN 18, the three measurements differ by almost 5 dB. Furthermore, the S8 showed a more stable \(C/N_{0}\) estimate. The MI 8, on the contrary, had the highest sensitivity, being able to gather measurements from a weaker signal, i.e., the farthest satellite, with more continuity and stability. Figure 1: Setup of the data campaign carried out using smartphones. The screw indicates the georeferenced known point. Figure 2: Comparison of the \(C/N_{0}\) of three different satellites (low, medium, and high-elevated Pseudo Random Noises (PRNs)), as estimated by the three different devices. The comparison can also be made in terms of positioning solution errors. The position solutions obtained by running the Google Toolbox [17] were considered after the addition of atmospheric corrections to the toolbox by the authors. For this, the Klobuchar ionospheric model using broadcast ephemeris and the Hopfield troposphere model were implemented. For a fair comparison, a common subset of satellites were selected for all the smartphones, including GPS signals with PRN 1, 10, 11, 14, 18, 22, 27, and 32. Even though data collection was performed simultaneously and at the same location, it was not possible to ensure that all satellites were always in a tracking state for all the smartphones and, thus, to guarantee perfectly equal conditions. However, the appearance and disappearance of some of them is something to be expected considering the different hardware, and is indeed very representative of a realistic situation. For the sake of fairness, all signals that were in a tracking state for at least one epoch in all smartphones were considered. Furthermore, PRN 20 was excluded from analysis given its very low quality, also shown in Figure 2. Excluding a few outliers, Horizontal Dilution Of Precision (HDOP) always ranges between the excellent values of 1.1 and 1.3 for all smartphones, with a common slight increasing trend. The impact of the HDOP could then be considered negligible. Figure 3 compares the horizontal positioning solutions of the three devices and of the reference control point obtained by means of Equation (4).