# Preliminary Results on Tropospheric ZTD Estimation by Smartphone
## 1 Introduction  
The applications exploiting Global Navigation Satellite System (GNSS) positioning have increased in recent years [1, 2]. The first GNSS-derived Zenith Total Delay (ZTD) estimates were carried out for scientific purposes using a post-processing strategy; the influence of ZTD was then considered in real-time applications to guarantee high-quality positioning, and it is now tackled in a Precise Point Positioning (PPP) strategy [3, 4]. The development of networks of Continuously Operating Reference Stations (CORSs) for Network Real-Time Kinematic (NRTK) positioning and the increasing diffusion of PPP strategies allow one to obtain a centimetric accuracy in a much shorter time with respect to relative post-processing. The first methodology (NRTK) has been investigated and considered for many purposes, where precision farming [5], autonomous navigation, maritime survey [6], and meteorological monitoring [7, 8, 9, 10, 11] are only a few examples. This positioning technique has allowed the attainment of impressive accuracy, even for single-frequency GNSS receivers [12], and it has permitted the increased performance of the low-cost ones [13, 14, 15, 16] since the implementation of studies into the potentialities of the GNSS system embedded in the smartphones [17]. Thus, 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 the interest in these systems and the managementof emergency scenarios. One primary advantages of using portable devices, such as those previously cited, is the possibility of completing a rapid survey and exploiting their embedded sensors, which results in them being useful for multidisciplinary teams cooperating in a coordinated manner on a common task, e.g., reconnaissance, inspection, and the survey of unstable structures. Starting in 2016, with the Android Nougat 7.0 Operating System (OS) development, Google has permitted direct access to the GNSS chipset raw measurements mounted on some Android-based smartphones. The possibility to manage pseudo-range and carrier-phase measurements from the GNSS chipset installed on smartphones and tablets with an Android OS has changed the concept of precise positioning with portable devices. Several studies have been conducted to verify the feasibility [18] and positioning accuracy [19; 20] of smartphones for different purposes, from urban [21; 22; 23; 24; 25] to pedestrian positioning applications [26; 27], always facing the problems related to the high-level Application Programming Interface (API) and the filtered measurements provided by the GNSS chipset. In [28], the authors demonstrated that it is possible to reach a decimeter level of accuracy in terms of positioning performances following the post-processing approach, made by double differencing raw smartphone observations. Meanwhile, the authors of [29] first focused their attention on single-base RTK positioning and then demonstrated the possibility of obtaining a centimeter-level accuracy through the use of NRTK corrections [30]. Recently, attention has been moved to PPP for obtaining the absolute position of a single receiver without the use of corrections or base stations [31]. However, to the best of our knowledge, nobody has focused their attention on the derivate products of GNSS positioning, such as the estimation of ionospheric or tropospheric delays. Recently, ZTD has been estimated using the PPP strategy [32; 33], but it was initially estimated within the GNSS station network adjustment to allow it to extend the local estimates to the entire area covered by the network through interpolation. In this context, this paper aims to investigate the possibility of retrieving ZTD estimates from smartphones and to evaluate their accuracy. To evaluate their quality, ZTD values obtained from smartphones have been compared to those estimated with a geodetic GNSS receiver settled a few meters away from the smartphone.