## 2 Deployment Sites and Existing Infrastructure
### The W1-M3A Observing System  
The W1-M3A observing system [19] is part of the M3A network of permanent observatories established in the 90 s to collect data on long term physical and meteorological parameters in open sea [20] for the benefit of the Mediterranean ocean forecasting system [21]. It also belongs to OceanSITES and to the Italian network of fixed open ocean observatories known as IFON [22].  
There are two main installations that form the W1-M3A marine observing system: a spar surface buoy permanently moored on a 1200 m seabed, and a separate subsurface mooring, deployed about 4 km far from the main installation to avoid entaglement with the mooring-line of the surface buoy. The buoy is still one of the few examples of spar platforms operating in the Mediterranean Sea, designed to maintain stability even in rough seas and high waves and allowing scientists to make very precise measurements [23].  
The surface buoy is equipped with GPS, camera, a complete set of stand-alone meteorological sensors (barometer, 2D and 3D sonic anemometer, thermo-hygrometer, pyranometer, pyrgemometer, par, CO\({}_{2}\)/H\({}_{2}\)O analyzer) and a compact weather station providing measurements of wind speed and direction, air temperature, relative humidity, atmospheric pressure and rainfall. The latter parameter is measured using the RAINCAP sensor that infers the cumulate by the acoustic impact of drops on the sensing element [24]. This instrument was recently used for its reliability to monitor rainfall on both land and at sea [25; 26; 27].  
The submerged part of the W1-M3A observatory is equipped with conductivity-temperature-depth (CTD) devices, biochemical sensors (for monitoring dissolved oxygen, chlorophyll-a, turbidity, nutrient, pCO\({}_{2}\) and pH), and properly positioned echo-sounders to estimate wave characteristics (significant wave height, period and main direction of propagation). The subsurface mooring housed CTDs at different depths from the near surface to the ocean interior for monitoring the physics of the water column.  
The architecture of the W1-M3A observatory, consisting mainly of the scientific payloads, the data acquisition-transmission system and the receiving station ashorc, is depicted in Figure 2. The system devoted to the acquisition of signals is based on a real-time controller that collects the measurements provided by all instrumentation onboard the buoy and merges them in compressed data files, that are stored in the internal memory, on an hourly basis. The full time series of the meteorological parameters is processed on board to obtain statistical quantities such as average, standard deviation and range of variation, whereas the three-time series of the echo-sounders, properly corrected for buoy's motion [28], are processed to get estimates of significant wave height, period and main direction of propagation of the ocean waves.  
Statistical values with oceanographic measurements are merged, stored in the on board memory and transmitted in near-real time to the station ashorc by means of an IRIDIUM satellite link. The tasks of the receiving station are to: (a) automatically decode the received data files, (b) apply quality control procedures compliant with the Data Buoy Cooperation Panel (DBCP) Quality Control Guidelines, and (c) archive the data on a local MySQL relational database. Automatic routines have also been developed to format some subset of data in the OceanSITES NetCDF standard and in the Binary Universal Form for the Representation of meteorological data (BUFFR) of the World Meteorological Organization. These coded data are uploaded to international repositories such as the data center of the Mediterranean Operational Network for the Global Ocean Observing System (MonGOOS), the Marine Service of Copernicus, the European Marine Observation and Data network (EMODnet) physics portal and OceanSITES through the COROLIS Global Data Assembly Center (GDAC).