Starting in 1987, some S-3As were upgraded to S-3B standard with the addition of a number of new sensors, avionics, and weapons systems, including the capability to launch the AGM-84 Harpoon anti-ship missile. The S-3B could also be fitted with "buddy stores", external fuel tanks that allowed the Viking to refuel other aircraft. In July 1988, VS-30 became the first fleet squadron to receive the enhanced capability Harpoon/ISAR equipped S-3B, based at NAS Cecil Field in Jacksonville, Florida. 16 S-3As were converted to ES-3A Shadows for carrier-based electronic intelligence (ELINT) duties. Six aircraft, designated US-3A, were converted for a specialized utility and limited cargo Carrier onboard delivery (COD) requirement. Plans were also made to develop the KS-3A carrier-based tanker aircraft, but this program was ultimately cancelled after the conversion of just one early development S-3A.