Grand Admiral a naval rank in one of the highest in the several European navies that use this system, what system are they using?
Grand admiral Grand admiral is a historic naval rank, the highest rank in the several European navies that used it. It is best known for its use in Germany as "Großadmiral". A comparable rank in other navies is that of fleet admiral.
Admiral (United States) Admiral (abbreviated as ADM) is a four-star commissioned naval flag officer rank in the United States Navy, the United States Coast Guard, and the United States Public Health Service Commissioned Corps, with the pay grade of O-10. Admiral ranks above vice admiral and below fleet admiral in the Navy; the Coast Guard and the Public Health Service do not have an established grade above admiral. Admiral is equivalent to the rank of general in the other uniformed services. The National Oceanic and Atmospheric Administration Commissioned Officer Corps has never had an officer hold the grade of admiral. However,  /201 § 201  of the U.S. Code established the grade for the NOAA Corps, in case a position is created that merits the four-star grade.