According to the manufacturer the American Patriot anti-aircraft missile system has had a success rate of 100 percent against ballistic missiles fired from inside Yemen during the last year. The Shia rebels in Yemen captured over twenty SCUD and SS-21 ballistic missiles when they moved south in early 2015. Many army units joined the rebels, including troops who knew how to operate these missiles. The SS-21 has a half ton high explosive warhead and a range of about 70 kilometers and can land within 75 meters of its aiming point. In 2011 Yemen had about four SS-21 transporter/launcher vehicles and over a dozen SS-21 (also called OTR-21) missiles. There were also at least a dozen longer-range (300 kilometers) SCUD missiles.
The Patriot has been in service since 1984 but did not shoot down its first manned aircraft until 2014 (when an Israeli Patriot shot down a Russian made Su-24). Patriot didn’t get its first combat use until 1990, when it was used against Iraqi ballistic missiles. There the success rate was 70 percent against missiles fired at Saudi Arabia and 40 percent against those fired at Israel. The relatively low success rate here had to do with the crude modifications Iraq made to its SCUD missiles to give them more range. This caused them to come apart when intercepted and, over an urban area, the large bits of missile debris caused casualties. Do you count that as a successful intercept? Even against aircraft, if you destroy it the debris are going to come down in fatal velocities in dangerously large fragments. This is an issue that does not get much media attention.
Israeli Patriot batteries have had more combat experience than anyone else but have shot down more ballistic missiles, cruise missiles and UAVs than manned aircraft. Israel has six Patriot batteries, enough to cover all 1,280 kilometers of land and coastal borders. Patriot is not infallible. Recently Israel admitted that two PAC-2 anti-aircraft missiles missed when fired at a UAV coming out of Syria. An F-16 firing an air-to-air missile missed as well. The Israelis are still trying to figure that out.
Each Patriot battery is manned by about a hundred troops, and each contains a radar and four launchers. A battery can fire two types of Patriot missile. The $4 million PAC 3 missile is smaller than the anti-aircraft version (PAC 2), thus a Patriot launcher can hold sixteen PAC 3 missiles, versus four PAC 2s. A less expensive PAC 2 missile weighs about a ton, about three times what a PAC 3 does. The PAC 3 has a shorter range (about 20 kilometers) versus 160 kilometers for the PAC 2 anti-aircraft version used against low flying UAVs. Given this long range and the small size of Israel the six Israeli Patriot can also take down cruise missiles, which is basically what a UAV is if equipped with a warhead and sent on a one-way bombing mission.
The Patriot system was in development since the early 1960s and is expected to remain in service until the 2040s. Over 10,000 Patriot missiles and over 1,500 launchers have been produced so far. Most of those missiles served for decades and then were retired and scraped unused.