for various research programs; one such initiative was the definition of new Federal Aviation Administration communication standards for unmanned aerial vehicles operating in US airspace. However, a lack of spare parts and increasing difficulty supporting the type meant their use could not continue in the long term. The last of the NASA's S-3Bs, which were the final working members of the type in existence with any operator at that point, were retired on 13 July 2021. Naval analysts have suggested that the U.S. Navy return to service an unspecified quantity of the stored S-3s in order to fill gaps that were left in the carrier air wing when it was retired. This move was promoted as a response to the realization that the Chinese navy is producing increasingly capable weapons that can threaten carriers beyond the range their aircraft can strike them. Against the DF-21D anti-ship ballistic missile, carrier-based F/A-18 Super Hornets and F-35C Lightning IIs have about half the unrefueled strike range, so bringing the S-3 back to aerial tanking duties would extend their range against it, as well as free up Super Hornets forced into tanking. Against submarines armed with anti-ship cruise missiles like the Klub and YJ-18, the S-3 would restore area coverage for ASW duties. Bringing the S-3 out of retirement could at least be a stop-gap measure to increase the survivability and capabilities of aircraft carriers until new aircraft can be developed for such purposes. Potential revival and proposals In October 2013, the Republic of Korea Navy expressed its interest in acquiring up to 18 ex-USN S-3s to augment their fleet of 16 Lockheed P-3 Orion aircraft. In August 2015, a military program review group approved a proposal to incorporate 12 mothballed S-3s to perform ASW duties; the Viking plan was sent onto the Defense Acquisition Program Administration for further assessment before final approval decision by the national defense system committee. Although the planes are relatively old, being in storage has supposedly kept them serviceable, and using them is an affordable means of fulfilling short-range airborne ASW capabilities that were vacated by the retirement of the S-2 Tracker. Refurbished S-3s could be returned to use by 2019. In 2017, the Republic of Korea Navy canceled plans to purchase refurbished and upgraded Lockheed S-3 Viking aircraft for maritime patrol and anti-submarine duties, leaving offers by Airbus, Boeing, Lockheed Martin, and Saab on the table. During April