WASHINGTON (Reuters) – Ahead of Ukrainian President Volodymyr Zelenskyy’s visit to Washington on Wednesday, the Biden administration said it would provide another $1.85 billion in military aid to Ukraine, including the Patriot air defense system that the Ukrainian leader has said his country urgently needs to fend off Russian attacks.

Here’s what you need to know about the Patriot:

What Is the Patriot System?

The Patriot, which stands for Phased Array Tracking Radar for Intercept on Target, is a theater-wide surface-to-air missile defense system built by Raytheon Technologies Corp and is considered one of the most advanced air defense systems in the U.S. arsenal.

The system was first used in combat during the 1991 Gulf War, with batteries protecting Saudi Arabia, Kuwait and Israel, and later used during the U.S. invasion of Iraq in 2003.

It is a mobile system that usually includes powerful radar, a control station, a power generator, launch stations and other support vehicles.

The system has different capabilities depending on the type of interceptor used.

The PAC-2 interceptor uses a blast-fragmentation warhead, while the newer PAC-3 missile uses more advanced hit-to-kill technology.

The system’s radar has a range of over 93 miles, the North Atlantic Treaty Organization said in 2015.

What Does It Cost?

A newly produced single Patriot battery costs over $1 billion, with $400 million for the system and $690 million for the missiles in a battery, according to the Center for Strategic and International Studies think tank.

How Widely Is the System Used?

Raytheon has built more 240 Patriot systems and they are currently used by 18 countries, including the United States. The system has been in high demand in the Middle East because of the threat posed by Iran to the region.

According to Raytheon, the system has intercepted more than 150 ballistic missiles in combat since 2015.

How Would It Help Ukraine?

Ukraine has said it needs more air defense systems to protect against the barrage of missile and drone strikes from Russian forces.

So far, the United States has provided a pair of National Advanced Surface-to-Air Missile Systems to Ukraine.

While the Patriot system is designed to intercept threats like aircraft and ballistic missiles, it can also shoot down the “kamikaze” drones Russia has frequently sent to hit Ukrainian critical infrastructure. But it would be an extremely expensive way to destroy drones that only cost thousands of dollars.

Officials and experts have said that while the Patriot system is likely to save lives from incoming missiles, it probably won’t change the trajectory of the nearly 10-month conflict since it is a defensive system.

When Will It Arrive in Ukraine?

It will be several months before Ukraine can deploy the Patriot on the battlefield.

U.S. officials have told Reuters that the Patriot system will first arrive in Germany, where Ukrainian troops will learn how to use them.

The training can take months, with each system requiring dozens of troops to operate.

Ukraine will then have to decide how and where to deploy the system without it being destroyed by Russian forces. Russia has said the Patriot missile defense system would be a legitimate target for Russian strikes.