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34 st / March 2015 www.sea-technology.com the HF radar coverage area and there- fore compared with HF radar data. These three drifters (named MLI001, MLI002, MLI004) were launched with an extra weight of 2 kilograms to re- duce the effect of wind on the ocean surface and without any drogue, so their measurements were superfcial. The G-ALTIKA experiment was part of the SOCIB and IMEDEA contribu- tion to the EU-funded MyOcean2 proj- ect. The objective was the validation and calibration of multisensor data sets (SARAL/Altika altimeter, gliders, HF ra- dar and surface drifter) in coastal areas. The drifter (ODI001) was launched in the HF radar coverage area with a 15-meter-depth drogue. In order to be consistent in the comparison with the time sampling for both instruments, drifter velocities have been averaged over 75-minute periods centered at each integer hour. Radial data, instead of total vectors, were used in the comparison, in order to avoid the smoothing and geometri- cal dilution of precision effect of total combined vectors. Drifter-derived ve- locities were decomposed and project- ed over the radial's components from each antenna (Ibiza and Formentera), making possible the radar-drifter com- parison. It is important to note, however, that the comparison between different measuring platforms is not straight- forward, since each system measures currents over different temporal and spatial scales. Drifter-derived mean velocities are compared with the closest radial data that fall within a 1-kilometer-diameter circle from the location of the radial data. Maps showing differences be- tween both measurements are useful for correct identifcation of spatial ar- eas where there is a higher agreement between the HF radar radial data and surface drifter velocities. The differ- ences are very low for the Ibiza radar station, where the values are generally below 10 centimeters per second RMS for the four drifters considered. For the Formentera Radar station, the differ- ence is still generally low; however, there are certain areas with larger val- ues, mainly due to the scarcity of radial data in the area. Correlation values between HF ra- dar radial data and drifter-derived ve- locities are high for drifters MLI001, MLI002, MLI004 and ODI001: 0.8, 0.9, 0.7 and 0.9, respectively, for the antenna in Ibiza Island, and 0.7, 0.7, 0.5 and 0.8, respectively, for the an- tenna in Formentera Island. Additional spectra analysis of the HF current data, outside the scope of this report, has shown that HF radars clearly capture the inertial, tidal and seasonal variations in the Ibiza Chan- nel currents, quantifying the range of magnitude of typical currents at each of these temporal scales of variability. All these validation results, togeth- er with the automated battery of tests performed over the data, give a good indication of the quality of the surface current measurements obtained by the SOCIB HF radar facility, and the strong potential of this coastal observing technology, which is the only observ- ing platform capable of measuring 2D surface current maps in real time. The HF radar monitoring of surface cur- rents is a promising tool to study the variability at different spatio-temporal scales of the surface circulation in the Ibiza Channel, an important hotspot for its biodiversity and transit as a stra- tegic point modulating the north-south transport exchange variability in the western Mediterranean Sea. Acknowledgments The authors are indebted to Dr. Alejando Orfla (TOSCA project) and Dr. Ananda Pascual (G-ALTIKA) for providing the drifter data from the oceanographic campaigns, and to ETD Division and Data Centre from SOCIB for their help in operating and main- taining the system and obtaining and archiving the HF radar data. References For a list of references, contact Arancha Lana at alana@imedea.uib- csic.es. n Dr. Arancha Lana is a physical oceanographer at the Marine Technologies Operational Oceanography and Sustainability department at the Mediterranean Institute for Advance Studies (IMEDEA-CSIC/UIB). Her research interests focus on the surface ocean circulation patterns at the Ibiza Channel from the SOCIB HF Radar Facility. Dr. Vicente Fernandez is a physical oceanographer experienced in ocean modeling and ocean observ- ing systems. He is now a freelance consultant for projects related to the marine environment. Dr. Joaquín Tintoré is a professor of physical ocean- ography at IMEDEA-CSIC and director of the Spanish large-scale marine infrastructure SOCIB, a multiplatform facility that responds to scientifc, technological and strategic society priorities related to the oceans and coasts in a global change frame- work. Iver3 Autonomous Underwater Vehicles Side Scan Bathy Water Quality Magnetometer www.ocean-server.com +1 508-878-0550 Rapid Data Collection For Coastal Applications