## 5 Conclusions  
This work describes the implementation and operation of an infrastructure of GNSS meteorology sensors, consisting of multi-constellation receivers and weather stations, installed on board a series of ferries operating in the north-west Mediterranean Sea (mainly in the Ligurian and Tyrrhenian Seas). The observations were processed by the MG-APP software system that was used in the best possible conditions regarding knowledge of the satellite orbits, i.e., using the final ephemeris, which are the most precise, but available with a delay of about two weeks. This was to allow the assessment of the complete potential of the system by considering the best ZPD estimates usable for scientific (e.g., climatological) studies, but not reachable at the moment in real-time application. This means that for real-time applications, the resulting considerations should be adapted to real operating conditions, i.e., considering the real-time available orbit and clocks and updating the software configuration, as suggested in some reports (e.g., [35]). However, apart from the use of the best ephemeris values, the remainder were as available as in an operational context. A comparison of the measurements with a reliable global reanalysis model, namely the MERRA-2, supported the use of this infrastructure, as the values were in very significant agreement with the reference values, and very similar to those obtained for the two fixed stations that were included in the analysis. This infrastructure is a promising tool for monitoring water vapour patterns, from climatological trends to fast dynamics in meteorological events, using measurements over the sea that are normally very sparse. This network can, in fact, using a high sampling rate, record the trend in the ZPD, a parameter strongly dependent on the vapour content in the atmosphere, which is the most abundant and variable greenhouse gas and a critical climatic feedback variable. At the same time, it is also a crucial input in numerical weather models because it is the main player in the mass-energy exchange between the Earth's surface and the troposphere, including precipitation phenomena and their extreme manifestations. The performances of such a measurement system (integrated water vapour between 2.3 kg m\({}^{-2}\) and 4.6 kg m\({}^{-2}\)) are also compatible with the requirements of [50] both for nowcasting, numerical meteorological prediction and climatological applications. One of the main limitations of the proposed system is the convergence time for achieving reliable ZPD values. In some receivers (such as METH), we experienced some outliers and anomalous oscillations in the very initial period of processing. This was probably due to the time needed for convergence to ambiguity resolution. Future work should investigate the concatenation of successive days of processing. This should reduce, by a smoothing approach, the occasionally anomalous oscillations in ZPD values due to the transition from one daily processing step to the next. There is also the possibility that these issues will be reduced by technological innovation and improvement in the performance of processing software, as well as by the growing number of satellites simultaneously tracked by each receiver. We are planning further analyses to compare these and new data from the network with other reanalysis products and with radiosounding data from ships. At the same time, we will perform some tests to understand if the outlier occurrence in some equipment is due to the solutions adopted for the installation of the apparatus, very often resulting from ship-specific constraints, for example, due to the proximity to other operating instruments that can cause interactions, such as the presence of reflective surfaces near the antenna. Conceptualisation, A.A. and A.O. ; methodology, A.A. and L.F.; technical implementation, A.S., M.V., S.M. and A.A.; software, L.F., M.V., A.S. and S.M. ; validation, A.A. and L.F.; formal analysis, A.A., L.F. and A.O.