A Note on the Threat and Recent Attacks
Yemen has become a key theater for unmanned aerial and maritime vehicles (quadcopter drones and radio-controlled water-borne improvised explosive devices [RC-WBIEDs]) used for intelligence gathering missions and for close and long-range attacks. The ongoing crisis in Yemen has turned it into the ultimate test ground for the latest developments in this field.
Recent attacks of this type took place on May 14, 2019, when two pumping stations along the Aramco East-West pipeline west of Riyadh were apparently attacked by Houthi GPS-guided Qasef-1 Unmanned Aerial Vehicle (UAV), with a 30 kg warhead.
A UN report submitted to the UN Security Council in January 2018 found compelling evidence that Houthi-made drones had an almost identical build and capability as the Iranian Qasef-1 UAV. (photo: Houthi controlled media outlets hailed the operation)
The attack highlights the persisting threat of Houthi unmanned weaponized systems targeting vital onshore and offshore Saudi and Saudi affiliated sites. The Houthis have also regularly attacked Saudi targets with missiles in the past. If accurate, this is the first time the Houthis have managed to hit a target 800 km inside Saudi territory with UAVs. According to Saudi sources, the drone was guided using satellite technology, as beyond a certain range drones need a satellite data link for information to be sent back to the pilot. Despite the minor damage caused by the attack, there is now increasing concern that, given the current tension, any further attack could spark an escalated regional conflict.
Weaponized and intelligence gathering UAVs have been used increasingly by the Houthis in operations against the Saudi-led coalition in Yemen. In July 2018 a drone exploded at Abu Dhabi airport causing minor damage and sending a strong message to the UAE and their dominant role in the southern parts of Yemen.
The targeted pipeline was built during the Iran-Iraq war as an alternative to Saudi Arabia should the Straits of Hormuz be blocked, as in the case of the oil terminal at the Emirati port of Fujairah where Saudi tankers were sabotaged. The message sent to the UAE and/or Saudi Arabia is that “we know where your vulnerabilities are” should you choose “to circumvent the Straits of Hormuz”.
Critical Infrastructure targeted
In January 2019, a senior intelligence chief and senior officials were killed at the al-Anad air force base just outside Aden by a weaponized drone that exploded above them as they attended a military parade. 
Last March, the Houthis released video footage of a drone flying past the Saudi al-Shuqaiq water treatment and power plant, 130 km from the Yemeni border. The facility was not attacked but the message was clear, with water infrastructure a critical resource in the area and in many countries around the world that heavily rely on desalination plants.
Al-Qaeda Attack on the Elite Force
On June 24, 2017, Al-Qaeda in the Arabian Peninsula (AQAP) disseminated a full video of the June 12, 2017, attack on the Hadrami Elite Force checkpoint in the Budha area of Doan, Hadramout (Yemen) that apparently killed 15 soldiers. The video features both ground and aerial reconnaissance footage taken prior to the coordinated attack. The aerial imagery intelligence (IMINT) footage was likely taken using a drone fitted with a camera. It was the first time an aerial footage from a drone is featured in one of AQAP’s videos. The use of UAVs for intelligence, surveillance, target acquisition, and reconnaissance (ISTAR) objectives provides terrorist groups with a tactical advantage previously unavailable.
Radio-controlled water-borne improvised explosive devices (RC-WBIEDs)
The documented boat with the two Yamaha outboard engines bears a strong resemblance to another Houthi RC-WBIED that was intercepted in April 2017 and to the RC-WBIED documented in Armament Research (CAR) report from December 2017. The vessels had been retrofitted with a hydraulic autopilot and an armature attached to the throttle which, in conjunction with an artisanal manufacture control box, permitted the vessel to be remotely piloted. The explosive consisted of a Soviet-era anti-shipping missile warhead fitted to improvised pressure switches and a detonator.
RC-WBIEDs remain a threat to commercial shipping due to mounting tensions between Iran and the United States, and the regional rivalry between Iran and the Sunni Gulf States.
UAVs, RC-EBIEDs and quadcopter drones enable terrorists or other extreme groups to carry out target identification and selection in specific locations with sufficient detail to effectively employ weapons. UAVs can be used to support artillery fire or guide suicide vehicle-borne improvised explosive devices (SVBIEDs) by acting as an aerial artillery observer, providing real-time information to allow artillery crews to adjust their fire or by enabling the control room to guide the suicide driver.
UAVs allow terrorists to film and document operations for strategic communication purposes. The videos are used to demonstrate various capabilities as well as being used as professional-grade recruitment tools for attracting new recruits.
Commercial UAVs can also be weaponized to serve as either aerial-borne IEDs (ABIEDs) that detonate on impact (e.g., the military parade in Yemen), or as a delivery platform for dropping munitions from above. The Islamic State (IS) has successfully demonstrated its ability to weaponize commercial UAVs to lethal effect in Syria and Iraq.
Although commercial off-the-shelf (COTS) RC drones have limited operational range, endurance and payload capacity, they are nevertheless increasingly being used in attacks and intelligence collection. This is a global trend that is growing rapidly. The variety of benefits commercial UAVs provide terrorist actors represents an evolving threat in both the domestic setting and for military forces deployed overseas.