18.4. The PERD Data Record - Regular Wave Period

      2   5  7   11         21        
    - --- -- ---- ---------- --------- 
   |X|   |  |PERD|XXXXXXXXXX|         |
    - --- -- ---- ---------- --------- 
       |  |   |                 |
       |  |   |                 |
       |  |   |                 |_Wave Period (F10.0)
       |  |   |
       |  |   |_Compulsory Data Record Keyword (A4)
       |  |
       |  |_Optional User Identifier (A2)
       |
       |_Compulsory END on Last Data Record in Data Category (A3)