Active Protection Systems (APSs) are subsystems integrated into or installed on a combat vehicle to automatically acquire, track, and respond with hard or soft kill capabilities to a variety of threats, including rocket-propelled grenades (RPGs) and anti-tank guided missiles (ATGMs). APS technologies are not new, and a number of nations have already employed APS on the battlefield. The U.S. military is now beginning to include APS as part of its formal combat vehicle modernization plans and, if the initial deployment of APS proves successful, could expand the use of APS to potentially thousands of tactical military vehicles—a complex and potentially costly undertaking. Congress, in its oversight and authorization and appropriations roles, has expressed interest in past and current U.S. military APS efforts and will likely continue to be actively involved as these technologies mature and possibly are integrated onto U.S. combat vehicles. A combination of evolving threats, advances in technology, experiences in combat, and the possibility of future conflicts has served to heighten the perceived need to pursue APS for U.S. military use more aggressively. Some of the factors affecting current U.S. APS efforts are discussed in the following sections. The proliferation of advanced RPGs and ATGMs is of concern to some defense officials and policymakers alike. Recently, the conflict in Syria has brought this concern into greater focus. These weapons—RPGs in particular—have been especially popular with insurgents because they are readily available, relatively inexpensive, and require little training. RPGs are basically single man-portable, shoulder-fired, unguided rockets. RPGs have been widely proliferated but can be mitigated to a degree by installing passive nets and bar armor on vehicles and by tactics, techniques, and procedures. The Lexington Institute notes: The threats aren't standing still. Both Hezbollah and Hamas have acquired advanced Russian rocket-propelled grenades (RPGs) and anti-tank guided missiles (ATGMs), some with tandem warheads designed to defeat the reactive armor on most Western tanks. In its 2006 operation against Hezbollah, the Israeli Defense Forces lost a number of its top-of-the-line Merkava tanks and the lives of soldiers to these ATGMs. These same weapons have shown up in the hands of Ukrainian separatists. Figure 1 depicts a Russian RPG-30, which reportedly has a 105 mm tandem warhead supposedly capable of penetrating over 650 mm of steel armor, even if it is located behind reactive armor —a central armor protection feature on a number of modern tanks. The RPG-30's special feature is a second tube that fires a smaller-caliber decoy rocket a fraction of a second before the main rocket fires. This design is intended to cause an APS to engage the decoy rocket instead of the main rocket. A wide variety of both traditional, less sophisticated RPGs (such as the RPG-7) and advanced RPGs are being employed on battlefields throughout the world by both armed forces and non-state actors. Figure 2 depicts a Russian Konkurs ATGM. Open-source U.S. military analysis reports there are eight types of ATGM systems currently in use by non-state groups operating in Syria—including the U.S-made Tube-Launched, Optically Wire-Guide (TOW) ATGM. It is estimated by U.S. military sources that 130 countries and various non-state groups—including Jabhat al-Nusrah and the Islamic State in Iraq and Syria—have ATGMs. ATGMs have some tactical advantages over weapons such as Improvised Explosive Devices (IEDs) and RPGs, as ATGM gunners can attack targets from several thousands of meters outside the range of direct fire. They also have some disadvantages. For example, the Konkurs ATGM in Figure 2 has a 4 kilometer range, which, while providing a great deal of standoff protection, can also reduce the accuracy of the ATGM as the operator could lose track of the intended target at such an extended range. Most ATGMs in use require little operator training—due to the effectiveness of the link between the missile and its launcher—and operators tend to build proficiency with their systems relatively quickly. Some experts cite Israeli combat experiences in 2006 and 2014 as demonstrative of the value of APS. In July 2006, after Hezbollah rocket and missile attacks killed and wounded a number of Israeli soldiers, Israel launched a series of air and ground attacks against a number of Lebanese towns and villages. Some analysts believe the war ended inconclusively and later characterized Israeli ground operations as "ineffective." Hezbollah's use of RPGs and ATGMs was considered a major factor in the conflict. One study notes Israel's "armored forces [were] not prepared for swarming of Hezbollah ATGMs and other anti-tank weapons [RPGs]." Specifically, about 50 Israeli Merkava tanks were reportedly hit by ATGMs and about 20 of these tanks were penetrated, resulting in 23 crew casualties. ATGMs also reportedly produced major casualties among dismounted infantry forces who were occupying buildings. Anti-tank weapons were credited with causing the majority of Israeli casualties during the war. These lessons and others were apparently not lost on the Israeli Defense Forces (IDF). In late 2010, Israel began to install the Trophy APS on Merkava tanks and Namer armored personnel carriers (APCs). In July 2014, after rocket attacks against Israeli cities and infrastructure targets, Israel undertook military operations in the Gaza Strip designed to stop the rocket attacks. The operation lasted until the end of August 2014. Hamas anti-armor operations were deemed less effective than in 2006. One report suggests: Hamas was less effective with these tactics. Not a single IDF tank was confirmed destroyed, nor were any Namer heavy APCs lost in combat. Other armored vehicles appeared more vulnerable, including the aging M113 APC, in which seven Israeli troops were killed in an RPG blast. Armored corps personnel were killed and wounded by sniping and mortar fire, but by and large Hamas anti-tank weapons and tactics were not of great effect. This was due to the Trophy anti-ATGM system employed on Merkava Mk 4 tanks, the protection provided by Merkava tanks and Namer APCs, and probably Israeli tactics that employed heavy firepower against ATGM threats. Given the contrast between Israel's combat experiences in 2006 and 2014, U.S. defense officials, who might have originally been skeptical about the effectiveness of APS, may have taken a renewed interest in adapting APS for use on U.S. combat vehicles. According to one defense expert: In the aftermath of the Ukraine invasion, Western military planners no longer think they can predict how Russian leader Vladimir Putin might react to perceived provocations or opportunities. So the possibility of war in Europe is back on the table as a priority concern, and that means land warfare in which the U.S. Army would have to carry most of the burden. Some senior U.S. military leaders have suggested a "resurgent" Russia is the "nation's top national security threat," with some claiming the U.S. Army is "outranged and outgunned by many potential adversaries." Others contend, however, that the heavy emphasis placed on the Russian threat is merely a ploy by the Army to gain "a bigger chunk of the budget." Aside from these competing views, the Army, Marines, and the Department of Defense (DOD) have expressed concerns about the vulnerability of U.S. armored vehicles to evolving anti-tank threats. It has been suggested that an APS should meet both technical and operational criteria to be effective, including be able to work under extremely demanding circumstances and compressed timelines; be robust against countermeasures; minimize the threat to friendly forces and civilians; fit in the space and power allocated to it on the vehicle; and be affordable. An effective APS requires a capability to detect and classify incoming projectiles. The means by which this is accomplished is small on-board radars and/or sensors such as optical sensors. Once detected and classified in very short order, the system calculates an intercept point away from the vehicle and, if the projectile poses a threat to the vehicle, countermeasures (soft kill and/or hard kill) are employed to defeat the incoming threat. This entire transaction is accomplished almost simultaneously; the closer the point of attack, the more critical this becomes. This sequence of events is depicted in the following figure: Ideally, an APS should provide "hemispheric" coverage to combat vehicles. This is particularly important, as some advanced countries have designed weapons that can use advanced infrared or radar sensors and guidance to attack the tops or engine compartments of vehicles, which are usually less armored than the front, sides, and underside of armored fighting vehicles (AFVs). However, absent hemispheric coverage, APS coverage of certain aspects of a vehicle might still be sufficiently effective to justify its adoption for use. Another important aspect of coverage is the APS's ability to address multiple incoming attacks. Common enemy tactics involve using multiple RPGs and/or ATGMs against a single vehicle, as was seen in Gaza in 2006. The aim of these "swarm" attacks is to overwhelm a vehicle's defenses and improve the chances of disabling or destroying the vehicle with multiple hits. APSs that cannot respond to multiple, incoming threats from different directions are considered by some experts as of limited utility. Also, it is considered an important capability that, after being attacked, an APS can quickly be reloaded or "reconstituted" to respond to further attacks.