###### Abstract
A very interesting comparison between different GNSS PPP processing software packages and related positioning accuracy is provided in [17], where a series of open-source software and online services are analysed with an extended dataset of 45 IGS stations, showing the capability of most of the PPP procedures to provide millimeter level accuracy in position determination. The positioning accuracy of the PPP approach is, however, strongly related to the accuracy of other parameters, including ZPD. The main goal of the present work is to set up an operational network of GNSS-based water vapour measurements from moving ships, over the north-west Mediterranean, an area of storm generation, to use in nowcasting and forecasting activities (through data assimilation procedures). The widespread observation system consists of Multi-GNSS receivers and weather stations installed on eight liners operating in the Mediterranean. For data processing, we adopted the PPP approach, which combines high accuracy with the possibility of separately processing data for each receiver. One of the main problems in the PPP approach is the time needed for the algorithm to converge in the ambiguity resolution, especially for float ambiguity choice. This results in a convergence time of approximately 15-30 min, from the first processed observation epoch, in the absence of external constraints for the ionospheric and tropospheric path delays [18]. This aspect is particularly relevant in challenging applications where moving systems, like those on board ships, can cause further increase in the convergence time and further fluctuations of the observations. Pioneering tests on GPS meteorology measurements from ships were undertaken in [19; 20], while some experiments over the Mediterranean area are described in [21]. In a research project conducted in the Baltic sea, the use of multi-constellations as possible means of improving measurement precision was examined [22]. A later experiment on the use of GPS receiverson board a ship for atmospheric IWV estimations was conducted during a two-month cruise in the equatorial Indian Ocean ([23]). The behaviour of hardware and software equipment was also evaluated during an Indian Ocean dipole event. As expected, errors from ship-based retrievals were significantly higher than those from fixed stations due to the unknown of the receiver position at any epoch. The results of the validation analysis, using ancillary data as reference and also including radiosonde launches, are given in [24]; the values of GPS-based IWV obtained were in agreement with the reference ones (rms errors less than 2.5 mm). Collection of ship-based GNSS meteorology observations was also recently conducted over the Atlantic Ocean, in the Barbados region, and in the Indian and Austral Oceans, as described in [25] and [26], respectively, highlighting the importance of this kind of measurement for climatological and meteorological applications. Finally, in [27], a very large dataset of shipborne GNSS-based ZPD measurements collected during six voyages all around the globe was discussed and validated. However, the test periods of the experiments were limited to a few weeks or a few months, so the potential application of the results obtained is limited with regard to longer time periods in different seasons and different types of meteorological situations. This paper provides for the first time a very large dataset (over more than three years) of automatic, continuous measurements, along different routes (so at different points) in the Mediterranean Sea, at the same time and in different weather conditions, spanning the seasons. The design and implementation of the infrastructure, including the hardware and software specific solutions, are described from both a technical and a scientific point of view in Section 2. A validation analysis, using as reference the ZPD values provided by a numerical weather reanalysis model, was conducted for the full 3-year dataset period.