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.
As early as 1950 Douglas examined equipping the Sparrow with an
active radar seeker, initially known as
XAAM-N-2a Sparrow II,
...
The most common version of the Sparrow today, the
AIM-7M, entered service in 1982 and featured a new inverse monopulse seeker (matching the capabilities of Skyflash), active radar
proximity fuse, digital controls, improved
ECM resistance, and better low-altitude performance. It was used to good advantage in the 1991
Gulf War, where it scored many USAF air-to-air kills. Of 44 missiles fired, 30 (68.2%) hit their intended targets resulting in 24/26 (54.5%/59.1%) kills. 19 kills were obtained beyond visual range.
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https://en.wikipedia.org/wiki/AIM-7_Sparrow