The ESEAS-RI Sea Level Test Station: Reliability and Accuracy of Different Tide Gauges By Belén Martin Mfguez, Begona Pérez Gomez and Enrique Alvarez Fanjul, Puertos del Estado, Madrid, Spain ) 0 2 A b stract In December 2002 a test station of six tide gauges using four differ­ ent technologies (acoustic, pressure, pulse radar and FMCW radar) was established by Puertos del Estado at the port of Vilagarcfa de Arousa (NW Spain), as part of the ESEAS-RI (European Sea Level Service- Research & Infra­ structure) project. The aim was to compare the performance of the tide gauges in order to support the future decisions concerning the improving of the sea level observing systems. Although the comparison of the sea level time series showed that all the tide gauges met GLOSS (Global Sea Level Observing System) quality standards, the experiment also revealed some differences in the quality of the data for certain ranges of frequency. I B m m Résum é En décembre 2002, dans le cadre du projet européen ESEAS-RI (Euro- pean Sea Level Service Research&lnfrastructure), une station pilote de six marégraphes utilisant quatre technologies différentes (acoustique, à pression, radar par impulsions et radar à onde entretenue modulée en fréquence) a été installée par Puertos del Estado au port de Vilagarcîa de Arousa (NO de l’Espagne). L’objectif était de comparer le fonctionnement des marégraphes afin de soutenir les futures décisions relatives à l’amélioration des systèmes d’observation du niveau de la mer. Bien que la comparaison des séries chronologiques du niveau de la mer ait montré que tous les r> * V * marégraphes satisfont aux normes de qualité GLOSS (Système mondial d’observation du niveau de la mer), l ’expérience a également mis en lumière certaines différences dans la qualité des données pour certaines gammes de fréquence. Resum en En diciembre de 2002, y como parte del proyecto europeo ESEAS-RI (European Sea Level Service Research&lnfrastructure), Puertos del Esta­ do instalô una estaciôn piloto de seis mareôgrafos de tecnologîas diferentes (acusti- ca, de presiôn, radar de pulso y radar de barrido de frecuencias) en el puerto de Vila- garcia de Arousa (NO Espaha). El objetivo era comparar el funcionamiento de los mareôgrafos con el fin de apoyar decisiones futuras concernientes a la mejora de las redes de medida del nivel del mar. A pesar de que las comparaciones realizadas con las series temporales de nivel del mar mostraron que todos los equipos satisfacen los requerimientos exigidos por GLOSS (Global Sea Level Observing System), el experi- mento también revelô algunas diferencias en la calidad de los datos para ciertos ran- gos de frecuencia. Introduction ments for a GLOSS-quality tide gauge are the main reference. These requirements are described in Apart from the most practical and immediate appli­ the Implementation plan for GLOSS (IOC, 1997) cations such as harbour operations or navigation, and the IOC manuals (IOC, 2002), and in brief, they the monitoring of the sea level is crucial for under­ state that the equipment must measure to cen­ standing processes related with Global Climate timetre accuracy in all weather conditions for the Change. As recently stated in the Galway Declara­ temporal averaging indicated (typically hourly). tion (EurOCEAN, 2004), one of the challenges of the European Union is "responding to the implica­ In this report, after describing the test site and tions of global climate change and its impacts on based on our experience during the two years of marine and coastal environments and communi­ operation of the test station, some considerations ties". Sea level monitoring requires a network of are made regarding the functioning of the tide tide gauges, adequately located and managed, and gauges. In a second step we will compare the data this is one of the aims addressed in the ESEAS-RI sets over a 6-month period (the longest period project (ESEAS-RI, 2002). At present, there are when all the tide gauges were working simultane­ several technologies available for measuring the ously) in order to assess their accuracy. sea level and it is not an obvious issue to deter­ mine which is the most reliable. In this study, and within the aforementioned project (EVR1-CT-2002- Description of the Experiment 40025), some of the most relevant ones were examined: acoustic, pressure, pulse radar and Fre­ In Table 1 are listed the tide gauges that we have quency Modulated Continuous Wave (FMCW) radar. evaluated in this study. Some of the tide gauges With this aim, Puertos del Estado has maintained were loaned from public institutions such as the a test station of tide gauges in the port of Vilagar- United States National Oceanic and Atmospheric cîa de Arousa (NW of Spain) for almost 2 years. It Administration (NOAA) or the Proudman Oceano­ was the first time that so many tide gauges were graphic Laboratory (POL). In other cases, they were tested simultaneously over such a long period and loaned by private companies (ENRAF). Finally, Puer­ this provided an excellent opportunity to compare tos del Estado (PE) owned three of the sensors. their advantages and disadvantages. For tide gauges employing acoustic or pressure sensors, The test station was installed in the port of Vila- there is already an important amount of experience garcîa de Arousa (Figure 1). The port of Vilagarcta accumulated (IOC, 2002). Radar systems, howev­ is situated on the Northwest coast of Spain, in the er, are a relatively new type of tide gauges, that is sheltered waters of the inner RTa of Arousa (Longi­ becoming popular due to its economical pricing and tude: 8° 46' W Latitude: 42° 36'N), a partially low maintenance (Barjenbruch et al., 2000). mixed estuary (Âlvarez-Salgado et al., 1993). This location has several advantages: it has an ade­ The examination of the performance and adequacy quate tidal range (mesotidal, up to 4.2m), it is of the equipment can be approached from different perspectives which, for our purposes, we will divide in two. The first perspective, comprises all that has to do with the operation of the equipment and is eventually related with the total cost of the data. The installation and maintenance expenses vary greatly and must be taken into account when pur­ chasing a tide gauge. In addition to this, some tide gauges can be more robust than others and turn out to be more suitable for certain environments. Secondly, once the data are obtained, it is neces­ sary to assess their quality. The quality of the data, •10'00' 10' -8' 20' -T 30' -6140' considered in a broad sense, includes their accu­ racy, lack of spikes and gaps, stability of the meas­ Figure 1: Location of the port of Vilagarcfa de Arousa in urements, etc. In this respect, GLOSS require­ the inner Ria of Arousa (NW of Spain), the test site. Tide gauge Type of Sensor Provider the greatest number of tide gauges were working (SHORT NAME) simultaneously and without interruptions. Aquatrak (AQU) Acoustic NOAA Geonica (GEO) Pulse Radar PE Miros (MIR) FMCW Radar MIROS Operational Aspects of the Tide Gauges Paroscientific (POL) Bubbler Pressure POL Radac (RAD) FMCW Radar ENRAF As seen in Table 1, the sensors involved in the exper­ Seba (SEB) Pulse Radar PE iment basically belong to four types: acoustic, pres­ Sonar (SRD) Acoustic PE sure, pulse radar and FMCW radar. In short, the first Table 1: Tide gauges evaluated during the test: type of sensors measure the travel time of acoustic commercial name and short name used in the paper, pulses reflected vertically from the air/sea interface. type of sensor and provider of the equipment. Pressure sensors use the changes in the pressure exerted by the water column as the tide progresses. affected by varying meteorological conditions, and Finally, radar sensors detect microwave pulses that it has 24 hour surveillance. At this port, Puertos are reflected by the air/sea interface, either by del Estado (PE) has operated an acoustic Sonar measuring the transit time of the signal (pulse radar) Research and Development (SRD) tide gauge sta­ or the phase shift between the reflected and the emit­ tion since 1997, which forms part of the REDMAR, ted wave (optical phase ranging, Mai and Zimmer­ the PE tide gauge network (Âlvarez Fanjul et ai., man, 2000). These different measuring techniques 2001; Pérez and Lôpez Maldonado, 2004). The involve changes in the way the tide gauges were sea level data obtained from this station undergo installed -and operated. Acoustic sensors such as near-real time quality control (automatic detection Aquatrak (AQU) or Sonar (SRD) must estimate the of spikes, interpolation of short gaps and adjust­ speed of sound, which depends on the air conditions. ment of the time of measurement) and are This requires their installation in a calibration tube processed and analysed in more detail annually. where that process is performed continuously. In Throughout the experiment the SRD permanent order to reduce the appearance of temperature gradi­ REDMAR station was used as a reference for the ents that might influence the estimation, the acoustic analysis of the data. The tide gauges that were gauges had protective tubes painted white. Radar part of the test station were placed on a different pulses, on the contrary, are not affected by air condi­ dock, approximately half a kilometre from the SRD tions, which implies that the radar sensors, i.e. Geon- permanent station. Computers and other electron­ ica (GEO), Miros (MIR), Radac (RAD) and Seba (SEB) ic devices necessary for the operation of the tide could be placed directly above the surface of the sea gauge equipment: data loggers, the power supplies without any further protective structure needed (see etc.