Security Terrorism

Improvised Mortars – Counter IED Challenges

Counter IED challenges faced from improvised mortars. An assessment of the lethality of the improvised mortar and its use in terrorism in Northern Ireland.


Throughout the Northern Ireland Troubles, the Provisional Irish Republican Army (PIRA) developed a range of technical capabilities including significant proficiency in the development and deployment of sophisticated improvised explosive devices (IEDs). PIRA experimented with a range of weapons and devices, for use in attacks against hardened targets such as military checkpoints, army bases and police stations. The improvised mortar bomb was first used by PIRA in the 1970s although initially it had a poor record of success (Oppenheimer, 2016, p. 227). PIRA continued to experiment with mortars (Birchall, 1997, p. 104), iterating through a series of different designs (Smith, 2006, pp. 131-149). PIRA launched several high-profile attacks using mortars. These included bombings of Royal Ulster Constabulary (RUC) bases in Northern Ireland, and attacks against Downing Street and Heathrow Airport (Oppenheimer, 2016, pp. 227-246). PIRA’s bomb making and engineering capabilities, bolstered by weapons and training supplied through Libya assisted in the development of complex threats (Coogan, 2000, pp. 589-590). In assessing the development of the improvised mortar in Northern Ireland a thematic discussion is presented considering the construction, reliability, lethality and threat profile of improvised mortars and various countermeasures used by the security forces.


PIRA mortars ranged in size from small hand fired horizontally launched devices, resembling rocket propelled grenades (RPGs) and drogue bombs, up to large Mk15 mortars which were made from domestic gas cylinders (Oppenheimer, 2016, pp. 239-239). Bottle gas cylinders from homes and pubs were readily available, providing good explosive containment and fragmentation as well as different calibre options. The use of mortars required significant co-ordination, from the construction of multiple devices (perhaps 4 to 8 mortars used in a single attack), along with preparation of attack platform (generally a stolen or hijacked van for large scale attacks) welded steel tubes and a portable stabilising platform for non-vehicle based attack.

The warheads of the mortars were generally homemade explosive (ANNIE HME) (ammonium nitrate and nitrobenzene) or high explosive Semtex detonated on impact using an initiator from a bullet or shotgun cartridge. The devices had arming mechanisms to prevent early detonation and required timed or remote-controlled firing mechanisms and arming circuits to allow the terrorist to set up the firing platform and make their escape prior to launch.

The constituent parts of the mortar system comprised, the firing platform, firing tubes and propulsion, aiming devices, flight stabilisation, timing circuits, power packs, warhead arming mechanisms, warhead explosive and impact detonation mechanism. Secondary devices including incendiaries could be rigged to destroy forensic evidence in the transportation / firing platform or to target or frustrate emergency responders.

There were many failed attempts by PIRA to use mortars, however there were notable multiple fatality attacks, including the death of 9 RUC officers in Newry in 1985 (Urban, 1996, pp. 206-210). PIRA also mortared Downing Street in 1991 (Whitney, 1991) and attacked Heathrow Airport in three linked incidents in 1994 using concealed firing points (Connett, McKittrick and Boggan, 1994). The ability to project multiple high explosive warheads against hardened or high-profile infrastructure was a significant capability for PIRA, and attacks on the Great Britain (GB) mainland garnered significant press and political attention (Moloney, 2007, p. 424).

Technical Description

There are a range of scenarios to consider in terms of countering the specific threat of mortars. In the immediate aftermath of an attack there could be uncertainty about the number of devices used, whether they had all deployed from their launch position, whether unexploded ordinance had landed on soft ground, or had been subject to technical failure, was stuck in launch tubes or was on a delayed timing circuit. There is a risk of secondary explosion, perhaps from a rigged or boobytrapped firing platform or as part of a multiple vector attack. The number of devices used in a co-ordinated mortar attack complicates response. The nature and purpose of the attack is also a factor and may be a complex ambush rather than an attack against a primary infrastructure target. In assessing and controlling firing points, remote camera equipment, aerial surveillance and jamming equipment could be deployed to assess and seek to reduce likelihood of ambush or secondary attack. Robots and disruptors would be useful tools to deploy to further assess firing platforms such as vans, with a range of cameras and explosive sensing capabilities to assess the likely presence of explosive devices. Disruptors might be used to separate detonators or firing circuitry from the mortar firing mechanisms or secondary devices such as traditional Vehicle Borne IED (VBIED).

Assessing and prioritising the risk of unexploded ordinance is a key challenge. Home made mortars present significant aiming challenges, and flight path and flight distance may be somewhat capricious. To maximise the possibility of an ‘on target’ hit, PIRA set launch tubes at different angles to give a wider spread of targeting. With warheads potentially made from Semtex, the potential blast radius with multiple mortars is significant. This potentially complicates the counter IED response as missiles may have significantly overshot target in multiple directions. Civilian homes were often close to police stations in Northern Ireland, adding additional safety challenges in the clearance of civilians and containment of risk from unexploded devices.

Large scale mortar attacks require significant coordination and present opportunities for interdiction by the intelligence community and security forces. Good security practices may detect terrorist surveillance of key sites, human intelligence (HUMINT) and signals intelligence (SIGINT) may provide important warnings. To prepare a large-scale attack, multiple bombs need to be constructed requiring time, skill and access to a sizable space such as a farm, garage, outbuildings or workshops. As attacks move from planning into execution, explosives will either need to be synthesised or (in the case of Semtex) moved from weapons hides. Watching premises and vehicle movements, along with covert source and neighbourhood policing intelligence may yield important clues about impending attack. Acquisition of explosives, precursor materials and detonators are critical steps for the terrorists, and PIRA had sophisticated supply chains through state sponsored terrorism in Libya (Taylor, 1998, p. 156).

Training security forces in detection of hostile surveillance, suspicious activity (such as a dry run) and vehicles and deploying security countermeasures to protect high profile infrastructure are important steps. Security force installations were hardened with grills and fencing to (at least in part) mitigate the impact of both improvised rocket and improvised mortar attack. Perimeter protection is however only one line of defence and as was painfully illustrated in Newry in 1985 (Hermon, 1997, pp. 168-169), the use of flimsy portacabins inside a base provides little or no protection against mortar fire successfully projected over the perimeter (Ryder, 2005, pp. 212-224).

Specialist training and inspection of devices by military and police technical operatives provides important weapons intelligence. Terrorists may make forensic mistakes and leave signature clues which may be used to build a picture of evolving capabilities, designs and even the nuances of a specific terrorist bomb maker. The use of specific electronic components, batteries, wiring, timers and detonators may provide further insight into the sourcing strategy of the terrorists and potentially usable forensic evidence in subsequent court cases.

Even at a very late stage in an attack, good surveillance and security hygiene might disrupt the assault. In their 1991 mortar attack on Downing Street, PIRA drove a van past Ministry of Defence HQ, positioning it at a firing angle just to the side of the Banqueting House in Whitehall. This is in direct line of sight of Horse Guards. Given the War Cabinet was meeting in Downing Street and security was heightened during the ongoing Gulf War, PIRA was able to construct and deploy the IEDs and make good their escape. Terrorist tradecraft and counterintelligence capability was perhaps a contributing factor, however there may have been opportunities for heightened security in the area to detect and subvert the attack. There seems to have been a significant intelligence blind spot throughout the planning and execution of the attack.


The primary countermeasures against improvised mortar attack are: a) intelligence, b) infrastructure and perimeter hardening, c) prevention of access to bomb making materials and precursors, d) general security hygiene and vigilance, e) forensics, f) training, g) use of robots and disruptors to disarm warheads and booby traps left on firing platforms or within areas of ambush and h) in limited circumstances, jammers. However, a key impediment to mounting successful mortar attack is their technical complexity. PIRA’s ingenuity and persistence overcame many difficulties, but this took years, if not decades and numerous technical iterations. Mortars provided options to attack fortified security installations or targets such as Downing Street which would be inaccessible to traditional VBIED attack (Holland and Phoenix, 1996, p. 128). Mortars were therefore an important development in PIRA’s overall threat profile and demanded a successful and holistic countermeasure response.

Intelligence was a key factor in the containment of PIRA. This ranged from intelligence about planned attacks, to technical intelligence about devices, key bombmakers, bomb construction, running covert agents and using SIGINT and other surveillance. After its reorganisation into cellular structure, PIRA became adept at counterintelligence and countersurveillance. The South Armagh brigade was particularly impervious and had a degree of operational freedom within South Armagh and the surrounding border. Bomb factories were concealable within farms where certain chemicals and engineering equipment would not seem out of place.

Fixed security force installations were fortified against mortar and rocket attack and these provided a degree of perimeter protection. Perimeter fencing is perhaps best suited to protect against horizontally fired rockets, and mortars could be landed within secure complexes. The death toll at Newry in 1985 illustrated the need for blast resistant buildings within the perimeter. PIRA’s experimentation with larger devices with high explosive (HE) warheads however illustrates that hardened perimeters can simply be attacked with higher force devices.

Frustrating terrorist access to bomb making materials is an important countermeasure however PIRA had access to several tonnes of Semtex acquired through Libya. There was also a ready supply of fertiliser that could be synthesised into ammonium nitrate based HME. PIRA had stocks of electronic detonators and a raft of experience in constructing IEDs with complex anti-handling mechanisms (Ivison, 2010, p. 30).

Security hygiene and surveillance around security force installations was important as a target would be scouted prior to attack. Watching for suspicious activity, vehicle thefts or vehicle movements might provide indicators of impending attack.

Large scale mortar attack launched form a platform such as a stolen van could provide rich forensics and intelligence, particularly if the attack was unsuccessful even in part. PIRA was adept at forensic hygiene, but mistakes could be made, and forensic evidence collected from vehicles and unexploded ordinance. Significant weapons intelligence could be collected from an attack and used to improve counter IED techniques such as warhead defusal. Understanding PIRA’s modus operandi and evolving capabilities was particularly susceptible to successful weapons intelligence collection.

Robots and disruptors developed in the fight against VBIEDs were useful in clearing attack scenes and protecting Ammunition Technical Officers (ATOs) involved in bomb disposal. Robots could be used to investigate and disarm unexploded mortars as well as inspect the firing point and any suspect vehicles within the vicinity. PIRA was proficient in mounting secondary attacks. Ambush and booby traps were ever present dangers and care required to ensure a safe clearance of a complex attack environment.

Jammers may in a very limited sense prove useful against mortars, but only if there is a remote-controlled launch mechanism. The use of command wire, or timed firing circuit would easily defeat jamming. As the mortars are ‘dumb bombs’ once armed and launched, electronic countermeasures are impractical. Such countermeasures however may be useful in detection of an evolving attack or to prevent terrorists communicating, or to protect ATOs and other responders from secondary attack triggered by remote control.

Arguably the use of mortars by PIRA was limited by a) the targets against which they were the best IED option, b) complexity in mounting a successful multi-device attack from planning to execution and c) the time and potential loss of material in a failed attack. Development of mortars as a capability seems to have been more frustrated by trial and error and technical complexity than the successful deployment of IED countermeasures. Intelligence, forensics, target hardening, and surveillance were perhaps most successful with disruptors, robots and jamming providing useful capabilities in bomb disposal.


In assessing the overall effect of PIRA’s use of improvised mortars it is helpful to consider impact on civilian and military targets, economic impact as well as an infrastructural impact on the security of military and police bases. The efficacy of government policy and countermeasures employed by the security forces are useful to consider and juxtapose with the tactical and strategic advancement of PIRA’s aims.

Arguably, primary disruption to civilian life in Northern Ireland was through terrorism against commerce, prosecuted using car bombs and blast incendiaries, often with some warning (although this could be confused and woefully inadequate). The proximity of civilian housing to security force bases in Northern Ireland did result in disruption, damage and injury to the civilian population. Mortar attack being somewhat capricious and indiscriminate, lacking any form of warning put civilian lives in greater peril. Attacks against targets on the GB mainland, notably in Whitehall and at Heathrow put civilians in the direct line of fire. Civilians at Heathrow had a very lucky escape when a mortar hit the roof on one of the airport’s terminals. Disruption to travel was also very significant at Heathrow, impacting a critical international transport hub. The ability of PIRA to mount a mortar attack against Downing Street had a broad terrorising effect on the populace.

Against military targets, particularly in fixed hardened positions the improvised mortar provided PIRA with options of attack that traditional VBIEDs did not afford. Attacks could be launched from low and high ground and from areas which might otherwise be inaccessible. The greatest security force loss was in Newry in 1985 where 9 police officers died in attack comprising 9 Mark 10 mortars. Other security force fatalities occurred in attacks at Newtonhamilton (Wharton, 2014, p. 161) and Carrickmore (Urban, 1996, p. 224). PIRA launched failed attacks on the RUC training centre in Enniskillen. An attack on a British Army base at Osnabrück in Germany in 1996 demonstrated PIRA’s capability to mount mortar attacks against security force targets outside Northern Ireland (Mortar attack rocks British army barracks in Germany, 1996) and the GB mainland. After approximately 11 years of experimentation with mortars, PIRA began to claim security force fatalities in the mid-1980s. A series of attacks around this period appears to show PIRA’s growing confidence in the technology and tradecraft necessary to mount successful mortar attack against military bases. In Northern Ireland, attacks against rural targets appears more prevalent, illustrating both the capabilities and operating freedoms enjoyed by South Armagh Brigade of PIRA along with the suitability of rural targets. The risk of scoring ‘an own goal’ in urban Belfast might have led terrorist planners to discount the use of heavy mortars in such environments.

The economic impact of the use of improvised mortars was perhaps most pronounced at Heathrow. Attacks over several days and the closure of runways had general disruption of air travel had a high economic cost. The fortification of army and police bases against a range of threats was also draw on security budgets (Geraghty, 2000, p.192) although traditional VBIEDs were much more likely to be used against commercial and economic targets in town and city centres.

PIRA shared their knowledge with other terrorist organisations, most notably with FARC guerrillas in Colombia (The IRA’s Foreign Links, 2003), (Hartnett, 2016, pp. 76-84). They are said to have made significant financial gain from the exchange, which continued after the signing of the Belfast Agreement in 1998.


PIRA’s development of improvised mortars took several years. The lethality of the devices is unquestionable, but it took almost a decade before the capability was successful in this regard against security force targets. PIRA used mortars in attacks against Downing Street and Heathrow, demonstrating their terrorising potential against civilian targets on GB mainland. Mortars were a significant boon to PIRA, providing attack options against fortified military bases, particularly around the border counties. Countermeasures such as denying access to explosives, intelligence gathering, surveillance and bomb disposal technologies and procedures were ultimately unsuccessful in preventing the development of the mortar capability or its use against certain targets. The complexity of clearing a scene of attack with multiple devices and a rigged firing platform was a significant challenge to ATOs. Attacks often took place in proximity to civilian housing and commercial premises, making area clearance complex. Secondary attack was a common PIRA tactic and ATOs needed to be exceptionally cautious of this threat. Experienced gained in countering PIRA IEDs and procedures was hard learned, but beneficial in other theatres.


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By Steve Nimmons

Steve is a Certified European Engineer, Chartered Engineer, Chartered Fellow of the British Computer Society, Fellow of the Institution of Engineering and Technology, Royal Society of Arts, Linnean Society and Society of Antiquaries of Scotland. He is an Electric Circle Patron of the Royal Institution of Great Britain, a Liveryman and Freeman of London and serves on numerous industry panels. He is a member of Chatham House, the Royal United Services Institute and the Chartered Institute of Journalists.