###### Abstract  
This work presents the design and implementation of an operational infrastructure for the monitoring of atmospheric parameters at sea through GNSS meteorology sensors installed on liners operating in the north-west Mediterranean Sea. A measurement system, capable of operationally and continuously providing the values of surface parameters, is implemented together with software procedures based on a float-PPP approach for estimating zenith path delay (ZPD) values. The values continuously registered over a three year period (2020-2022) from this infrastructure are compared with the data from a numerical meteorological reanalysis model (MERRA-2). The results clearly prove the ability of the system to estimate the ZPD from ship-based GNSS-meteo equipment, with the accuracy evaluated in terms of correlation and root mean square error reaching values between 0.94 and 0.65 and between 18.4 and 42.9 mm, these extreme values being from the best and worst performing installations, respectively. This offers a new perspective on the operational exploitation of GNSS signals over sea areas in climate and operational meteorological applications. GNSS meteorology; atmospheric water vapour; ship meteorology; ZPD estimation; ZPD validation; reanalysis weather prediction models 2024 24 3177 1224 2 and its space-time dynamics over the sea is very important in territories largely surrounded by the sea, such as the Italian peninsula--the concentration of humidity over the sea is one key driver of intense or extreme precipitation events in coastal and inland areas. This work is directed towards the design, implementation and testing of an operational ship-based infrastructure, as a pilot experiment for the continuous monitoring of atmospheric parameters over the sea. Atmospheric humidity can be expressed as a quantity integrated along the observing direction and is defined as integrated water vapour (IWV). IWV can be estimated using Global Navigation Satellite System (GNSS) signals. Measurements from GNSS receivers are combined with surface meteorological observations, whose data are processed to obtain the zenith path delay (ZPD), i.e., the delay of satellite navigation signals refracted in the troposphere and projected to the zenith, which mainly depends on atmospheric pressure, temperature and humidity ([4]). The atmospheric water vapour estimation problem from GNSS measurements can be addressed with different techniques, such as geometric projection of slant delays ([5]), probabilistic inference ([6]), and tomographic approaches [7; 8]. A very comprehensive review of GNSS meteorology theory and methods is provided in [9], where several related applications are described. Through extensive investigation, it has been found that the precision of ZPD estimations using GNSS equipments is strongly dependent on the possibility of modelling certain ancillary information, such as the precise satellite ephemeris, clock parameter estimations [10], and ocean tide loading [11]. ZPD values are systematically provided by the International GNSS Service (IGS) center for reference stations around the world, together with other products needed for their computation, such as precise combined final orbits and clocks of the different constellations [12]. Such ancillary features are mandatory to process GNSS data using the most recently introduced processing technique called precise point positioning (PPP) based on a single receiver and the observations of as many constellations as possible (Multi-GNSS) to increase the precision of the estimates of the various parameters [13; 14]. An exhaustive introduction to PPP is provided in [15; 16], with a detailed mathematical formulation of the GNSS data processing, including the critical issue of ambiguity resolution of the signal phase observations.