# Preliminary Results on Tropospheric ZTD Estimation by Smartphone
## 5 Conclusions  
The spread of low-cost technologies such as smartphones and tablets, with their rapid evolution in terms of the quality of their installed sensors, has increased interest in these systems and their employment for positioning purposes. Of course, because they are not primarily developed for these applications, their performances are not comparable with those obtainable with geodetic receivers. However, starting from 2016 with the advent of the Android Nougat 7.0 OS, their use as positioning tools has been investigated, and attempts have been made to reach a sub-meter accuracy by exploiting different techniques. In this work, one of these devices was employed for the ZTD estimation, comparing the obtained values with those obtainable with geodetic receivers, commonly used for these applications. Two different types of software (one online and one open-source) and two different case studies have been considered to obtain solutions independently from the used processing software and the test site.  
The obtained results are encouraging: the ZTD estimations obtained with the smartphone receiver are comparable with those obtained with the geodetic one, especially if RTKLIB is used. Indeed, the online tool (CSRS-PPP) incapable of producing ZTD estimates for smartphone-derived RINEX files, whereas it produces consistent ZTD estimates for geodetic GNSS CORSs, without losing the quality of the positioning solutions. The measurement campaigns considered in this work do not cover an entire day due to the smartphone battery capacity. In the future, we plan to find solutions to extend the session lengths to more than 24 h, mainly by exploiting an external power supply. It is quite challenging to perform measurement campaigns considering different consecutive days because we have not had the possibility to provide a power supply to the smartphones up to now. Furthermore, another improvement will be considering different smartphones to verify the results obtained with the model considered in this research activity. Because there are few GNSS receivers installed inside smartphones, we believe that interesting results could also be obtained with other devices, and our goal is to prove that in the future.  
This study represents initial analysis regarding the capability of online software to positively process GNSS measurements extracted from portable devices, exploiting the PPP strategy. Moreover, it has also shown the possibility to exploit raw GNSS measurements obtained from smartphones for ZTD estimations. This aspect opens new frontiers, not only in the positioning solutions with portable devices, but also for the contribution that smartphones could provide for atmospheric monitoring; if used correctly, they could become an interesting and widespread tool that is useful for creating dense sensor networks, even ones that are deployable for atmospheric monitoring. This aspect will be investigated in the future, and it is one of the next steps that the authors want to pursue.  
Conceptualization, data interpretation, and analysis, writing--original draft preparation, all authors; GENU/GESM GNSS tests, L.B. and D.S.; TORI/TOSM GNSS tests, P.D.; GNSS processing, L.B. and P.D.; writing--review and editing, I.F. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data that support the findings of this study are available upon reasonable request.  
The authors wish to thank the two anonymous reviewers whose precise and valuable comments helped to significantly improve the manuscript.  
The authors declare no conflict of interest.