Houthi ballistic missile and drone attacks targeting Saudi Arabia have surged in recent weeks, an alarming escalation in the six-year Yemen conflict.
On Monday, Royal Saudi Air Force strikes continued targeting the origin of these strikes, hitting ballistic missile launchers and weapons depots.
Iran-backed Houthi militias have been targeting what the Saudi Arabian government has said is the "backbone of the global economy" – oil infrastructure.
The kingdom produced on average 9.2 million barrels of oil per through 2020 and has almost 200 million barrels in storage.
But it's not just oil infrastructure that has been attacked: civilian residential areas and airports have also been targeted.
Despite this threat, record numbers of drones and missiles are being shot down as the Saudi Arabian military builds what is referred to as "layered" air defence.
Attacks are being foiled by a combination of cutting-edge weapons systems like Patriot PAC-3 missiles – which can travel at four times the speed of sound – and novel approaches using new versions of long-serving missiles, such as AIM-9 Sidewinder air-to-air missiles.
Saudi Arabian F-15 jets have been hunting Houthi drones over the kingdom’s skies, a new front in a distinctly 21st-century form of warfare.
What weapons are the Houthis launching at Saudi Arabia?
Houthis are using mostly old weapons systems, but modern innovations which exploit the proliferation of modern satellite navigation, have made them more dangerous.
The most headline-grabbing, and most frequently deployed, have been explosive drones, sometimes called "kamikaze drones".
The Qasef-2K, a propeller-driven aircraft, is slow moving but has a long range. It is based on the Iranian Ababil drone and built from foreign parts used on commercial drones, plus parts that have been reserve engineered.
It is originally thought to have been developed as a target drone for training air defence crews, at some point in the late 1980s or early 1990s.
More dangerous, arguably, are Iran-supplied ballistic missiles, as they carry large explosive warheads.
Burkan-2 missiles, which a UN panel of experts concluded in 2018 were made by Iran, have been smuggled into Yemen in pieces and then re-assembled for use, according to analysis by US-based defence expert Michael Knights.
The Burkan-2 shares its origins with the Iranian Qiam short-range ballistic missile.
"Some are based on foreign designs with Iranian modifications, like the Qiam/Burkan-2," Fabian Hinz, a specialist in Iranian drones and missile systems, told The National.
"Others are Iranian designs but use foreign components."
The Burkan traces its history back to Iran's efforts to reverse engineer Soviet-era Scud missiles, which threatened much of the region during Saddam Hussein's invasion of Kuwait in 1991.
It is these weapons – fast moving missiles with a high-altitude trajectory – which have been the focus of US missile defence efforts for years.
Washington has allocated tens of billions of dollars to missile defence to combat these threats.
So why are small drones and copies of Iranian cruise missiles, such as the Houthi Quds-2, still a threat?
The simple reason is that research and development of countermeasures, like the Patriot missile, were not designed to destroy them.
This is because when the Patriot was developed in the 1980s, the US assumed that small rival states or militias would never have access to cruise missiles or drones.
Focus was kept on intercepting high-altitude, fast-moving missiles like the Scud, 80 of which were fired by Iraq at Kuwait and Saudi Arabia during the First Gulf War.
As Scuds could travel at two kilometres a second, Patriots struggled to intercept them, though it was not impossible.
Since then, a lot has changed.
Enter the PAC-3 Patriot
To deal with smaller threats, the Americans designed a smaller and cheaper upgrade to the Patriot, a more flexible system known as the Patriot Advanced Capability (PAC-3).
Capable of targeting ballistic missiles or smaller drones, they have the capability to fly at 5,000 kilometres an hour. But more importantly, their computer targeting systems have been trained to recognise much slower and smaller targets.
Advances in computing power and radar technology since the First Gulf War have had a huge impact.
US geopolitical risk consultancy Gulf State Analytics estimates Saudi Arabia has shot down more than 300 ballistic missiles and more than 340 Houthi drones using Patriot PAC-3 missiles.
Evolving air defences
"The Patriot system was originally designed for ballistic missiles, but it's having to adapt to a target it wasn't originally designed for," said David, an analyst at open-source analysis company Aurora Intel, who only gave his first name.
"Why are they improving? Because of the increase of usage. You can't be expecting it to just work. These things evolve, computer algorithms change, radars improve, equipment is updated and upgraded," he said.
This evolution, combined with cheaper air defence options, like the US Hawk system which Saudi Arabia has also deployed, is bad news for the Houthis, especially as efforts progress to reduce the cost of air defence.
Next generation drone defence
David says this evolution process will continue and in future, it is possible that drones like the Qasef-2K will be easily downed by defence systems that fill the gap left by a focus on intercepting ballistic missiles.
"We are seeing that adaption happening. If you look at Israel, with Iron Dome [Israel's mobile air defence system], they have taken what is ultimately a C-RAM, made it 'smarter' and now they have a newer version being deployed which can now go against cruise issiles and UAVs [drones], simultaneously" he said.
C-RAM is an acronym for the US Counter-Rocket, Artillery and Mortar system, which fires 4,500 rounds a minute to destroy incoming weapons, once they are detected by infrared sensors.
Newer systems currently in development, such as the US Lower-Tier Air and Missile Defence Sensor (LTAMDS) radar-based system, are being prioritised to counter a range of threats, from small drones to missiles.
Until they enter service, Saudi Arabia will remain on the front line of a new type of proxy warfare – and observers will be watching and learning.