A demonstration prototype has been developed using the Open source Project for a Network Data Access Protocol (OPeNDAP) to illustrate the potential that distributed data access can provide to the HF radar backbone within the Integrated Ocean Observing System (IOOS). An individual HF radar site produces maps of radial surface velocity. Since two or more radar sites are needed to provide vector currents, most operators of these systems deploy radars at multiple sites and produce current vector maps in the region of overlapping coverage of their sites. The underlying premise of the prototype is that each HF radar site within the backbone is considered a unique and separate data source. In our approach the radials from each HF radar site are accessed directly to compute the surface current vectors in response to a user's request. This allows radials from different operators to be combined, with the potential to compute surface current vectors for regions outside the vector coverage region of either operator. Since each HF radar site is recognized as a separate data source, new HF radar sites can be readily incorporated to extend the backbone's spatial coverage. Additionally, by directly accessing the radials the prototype demonstrates the capability to specify the latitude/longitude grid that surface current vectors are computed for, which can be different from that of existing operators. The prototype can be configured with a default latitude/longitude grid representation on which vectors are calculated, or extended using the functional interface capabilities available within the OPeNDAP constraint syntax to allow requesting applications to specify a latitude/longitude grid, or list of grid cell locations. The demonstration prototype consists of two specialized OPeNDAP servers: a Radial Server that is installed at the HF radar sites to read and serve the CODAR radial files stored at the remote locations and the Combining Server, which is the primary application within the prototype. It operates to identify the HF radar sites within the backbone that have radials that can be combined to calculate the surface current vectors given the user's requested spatial and temporal extent. It communicates with the remote Radial servers, using the OPeNDAP protocol, to request only those radials necessary within the requested spatial and temporal extent. It then combines the radials from two or more HF radar sites to calculate the surface current vectors and the associated uncertainties at specific grid cell locations and returns these data to the requesting client.
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