# A Controlled-Environment Quality Assessment of Android GNSS Raw Measurements
## 1 Introduction  
Nowadays, a variety of applications and services supporting our daily activities and needs, in a ubiquitous fashion, are based on mobile-communication technologies. Indeed, Location-Based Services (LBSs) are included in many mobile applications, widespread in our smartphones, and they constitute to an essential part of everyone's daily life [1]. Global Navigation Satellites Systems (GNSSs), such as the Global Positioning System (GPS), Glonass, and Galileo, play a crucial role in enabling and improving positioning performance in smartphones. To support and encourage this rapid innovation trend, in 2016 Google made available raw GNSS measurements, retrieved from the enabled GNSS chipset for mobile devices. The measurements could be retrieved from the on-board GNSS chipset through Android's Application Programming Interface (API) 24 on devices running Android 7+ equipped with enabled chips, thus improving their positioning and navigation performance. Starting from the availability of raw-code pseudoranges and Doppler measurements, developers implemented a number of precise-positioning algorithms based on such consumer-grade GNSS receivers [2; 3]. Among these implementations, for instance, Doppler filtering and augmentation were investigated in Reference [4] to reach submeter accuracy. By exploiting carrier-phase observations on the enabled models, it is also possible to smooth code-based pseudorange measurements and reach decimeter-level accuracy without phase-ambiguity resolution, as successfully proposed in Reference [5]. The recent push to achieve precise positioning from Android smartphones thanks to the availability of their raw measurements has also been boosted by the release of a white paper from the European GNSSAgency (GSA) [6]. This allows for a clearer reference of the hardware performance of the devices, helping in more effective integration with other sensors and analysis of multiple GNSS constellations among many other applications. The aforementioned effective examples of improved positioning and navigation capabilities suggest valuable implementations of affordable smartphone GNSS hardware in different contexts. Signal impairments, such as multipath, have been seen as the major deterrent in achieving accurate positioning affecting the Carrier to Noise Power Density Ratio (\(C/N_{0}\)) up to 10 dB in certain Android devices, largely extending ambiguity-resolution time periods and causing cycle slips [7]. The problem of cycle slips, however, is mostly attributed to the presence of a power-saving function known as _duty-cycling_, a mode in which the GNSS chip is active only for a fraction of each second [8]. While code measurements are unaffected by interrupted signal tracking, the continuity of phase measurements is not achieved, resulting in cycle slips, which make any carrier-phase-based processing unusable [9]. Duty-cycle implementation is different in each Android phone and not explicitly declared by the manufacturer, which makes it difficult to distinguish from bad measurements. The option of switching duty cycle off has been implemented in the latest Android release (P), but not yet available at the time of writing. To quantify the effects caused by signal impairments in a real environment, it is important to first determine the results of an ideal condition. In this work, differently from previous performance assessments [10], the performances of positioning algorithms based on raw GNSS measurements were investigated in a controlled environment: an anechoic chamber. The study allowed to determine the quality of the measurements without the presence of multipath and external spurious signals. The tests were performed by means of the record-and-replay technique [11] applied to simulated GNSS signals by means of an IFEN(tm)NavX hardware signal generator and constellation simulator. Positioning solutions were evaluated with different carrier-smoothing algorithms to reach high precision and verify the performance of the obtained navigation solutions.