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As a user of a particular Earth Science focused search interface (e.g., GEOSS broker, CWIC interface, etc.), I want all relevant Earth Science data to show up in my search results. The common two step standard for this is the two step OpenSearch Request.
A list of specifications and best practices for such an API follows:
http://www.opensearch.org/Specifications/OpenSearch/1.1
http://www.opensearch.org/Specifications/OpenSearch/Extensions/Geo/1.0/Draft_2
http://www.opensearch.org/Specifications/OpenSearch/Extensions/Time/1.0/Draft_1
http://www.opensearch.org/Specifications/OpenSearch/Extensions/Parameter/1.0
http://wiki.esipfed.org/images/9/97/Combined_Open_Search_Best_Practices_v0.4.pdf
http://www.opensearch.org/Specifications/OpenSearch/Extensions/Relevance/1.0/Draft_1
http://www.opengeospatial.org/standards/opensearchgeo
http://ceos.org/wp-content/uploads/2014/12/CEOSOpenSearchBestPracticeDocument-PublicComment.pdf
Fortunately there is a validator for this type of API available at http://testbed.echo.nasa.gov/cwic-smart/validations