China has bought more Russian Kh-31P (called by NATO; AS-17 Krypton) Anti-Radiation Missiles. This large missile, which reaches speeds of Mach 4.5 and has a range of 70 kilometers, is designed to take out radar systems like those used for the American Patriot anti-aircraft missiles, as well as the powerful U.S. Aegis radars found on many warships. The Kh-31P was built to be carried by Su-27 type aircraft, which China has many of. The Kh-31P is a variant of the Kh-31A, which is an anti-ship missile. The Kh-31P has a troubled history. The U.S. Navy bought some (for nearly a million dollars each) in the mid 1990s, for use as target drones. American engineers then suggested some design improvements, which overcame some technical problems the Russians had been having with the missiles sold to the U.S. Navy (as target drones). This assistance was criticized because it made the Kh-31P a more effective weapon. The Kh-31P is thought to still have several key components that still don't quite work right, or reliably. But if it does work, it threatens the two most vital American air defense radars (Patriot and Aegis).