## 4 Discussion  
The beginning of this work dates back to 2010, when a small number of initial experiments testing the feasibility of GPS meteorology applications from ships was available (e.g., [20; 24]). We approached the concept of measuring water vapour over marine areas using GNSS signals from ship-based receivers as the main content of a successful FP7 proposal named COSMEMOS [49]. There was an inherent contradiction in seeking to use a mobile station to retrieve atmospheric parameters that are essentially retrieved from the errors in the positioning produced by the atmosphere, measured from receiver stations of well-known positions (i.e., geographic coordinates, including height), such as for fixed stations, installed on stable grounds. Thanks to the simultaneous availability of GNSS measurements from many constellations, in addition to the upgraded accuracy of present systems, we can say that, nowadays, GNSS meteorology instruments can be used operationally on board ships to accurately measure ZPD in whatever conditions, as testified by the results of three years of operational data acquisition, as we have shown. As explained, measurements were compared with independent data that we considered as reference, i.e., ZPD values derived from the atmospheric parameters provided by the MERRA-2 reanalysis products. It is known that reanalysis data are very accurate as they are reconstructed ex post assimilating as much information as possible from different observations, from in situ, airborne and satellite platforms. Therefore, we can be confident in the reliability of the reference data, while keeping in mind that they provide data on grid points that refer to an area (or better to a flattened volume) and not to a dimensionless point (we note that a MERRA-2 pixel has a horizontal dimension greater than 65 \(\times\) 50 km\({}^{2}\)). The results of the comparison highlight a very strict agreement between the quantities, with skill scores comparable with the two other fixed GNSS stations. Moreover the quantitative assessment of the ZPD estimation showed a difference of about 15-30 mm that is consistent with the differences reported in [21; 27]. A relevant part of the remaining difference was still due to the presence of outliers that could be further removed considering seasonal acceptance ranges instead of a constant one. The present results and the identified room for improvement enable us to say that mobile equipment provides performances so close to fixed ones that their variability with respect to the MERRA-2 derived values is intrinsic to the differences in the data we are comparing, that we stress again reflect in situ high sampling rate measurements vs. gridded model data. In other words we infer that such differences are in large part real (and correct), coming from the intrinsic (somehow artificial) smoothness of the model parameters, mainly because of their limited space and time effective resolution, and from some errors in the model values and their synchronisation with the real atmospheric state and its dynamics, that even the reanalysis process can never fully cancel. Therefore, the system showed high precision, accompanied by robustness and reliability, demonstrated during the long period of operation that was analysed. In addition, the use of these systems at sea for several years, subject to critical atmospheric agents, was a real stress-test that we can say was fully passed, as over the time, the performances of the systems were absolutely stable. As demonstrated, the stations installed on board ships showed relevant fluctuations in the ZPD values and also a greater occurrence of outliers, though particularly on some ships (CRRM, METH, MEAN). The opposite occurred for fixed stations where the dispersion of values was very limited, with very high correlations around 0.95. On the other hand, the MBE was greater, showing a systematic small underestimation error.