SJ radar was the name of a type of S-band (10-cm) radar set used on United States submarines during the Second World War. The widespread use of the SJ radar, combined with the very low use of radar in the Imperial Japanese Navy gave great operational flexibility to the U.S. Navy's submarine campaign in the Pacific Ocean. The SJ provided directional information, provided highly accurate distance information, and provided information about surface contacts as well as low-flying aircraft.