A number of upgraded Sparrow designs were developed to address these issues. In the early 1970s, the RAF developed a version with an inverse monopulse seeker and improved motor known as Skyflash, while the Italian Air Force introduced the similar Aspide. Both had the ability to be fired at targets below the launching fighter ("look-down, shoot down"), were more resistant to countermeasures, and much more accurate in the terminal phases. This basic concept was then made part of the US Sparrows in the M model (for monopulse) and some of these were later updated as the P model, the last to be produced in the US. Aspides sold to China resulted in the locally produced PL10. The Japan Self-Defense Forces also employ the Sparrow missile, though it is being phased out and replaced by the Mitsubishi AAM-4.