For decades, experts have deemed advanced low-flying cruise missiles to be one of the more challenging and numerous threats air defense systems around the world were set to face. Especially as these systems are easier and cheaper to acquire or develop than traditional ballistic missiles. Nonetheless, these systems have received less attention than the proliferation of ballistic missiles around the world.
In this piece, I will provide a short overview of the PATRIOT’s anti-cruise missile capabilities, its development over the years, and status today. For a more thorough and in-depth read on the PATRIOT, I direct the reader to my digest on this system found here.
The situation in the early 1990s
As it stood when the PATRIOT Advanced Capability (PAC) program was being rolled out, the Tactical Ballistic Missile (TBM) interception was of prime importance (especially due to the events leading up to and during the Gulf War). However, during the same period, the low-altitude cruise missile threat was of major concern, not to the US Army, but to the US Navy. The issue was stressed enough to Congress, and to that effect, funding was provided for PATRIOT testing as part of the Navy’s attempts at overcoming this problem under the Mountain Top Program.
The issue that faced both the US Navy and US Army at the time was the compression of detection-to-decision time imposed by extremely low-flying cruise missiles (now using radio altimeters) with low-radar cross section (RCS).
The issue was complicated by the fact that both the US Navy and US Army’s main missiles (PAC-1/2 and Standard Missile 2) used semi-active radar homing (SARH), meaning the missiles could only intercept targets within the guiding radar’s view. This is not problematic on its own, but it is extremely problematic when the low-flying cruise missiles compress your detection horizons from hundreds of kilometers to tens of kilometers.
So, to engage these targets at range, the defenders not only needed to detect targets beyond the radar’s horizon but also have missiles capable of engaging these threats at those distances; the Navy and Army’s approaches to these problems were entirely different.
The problem, on the missile end, was one of guidance schemes. The US Navy’s solution was keeping the missile guidance the same (semi-active). Thus, if you aren’t going to change the missile, you need a new illuminating source that could illuminate a target beyond the horizon. It was deemed that its fighters (F-14 Tomcat) and support aircraft (E-2 Hawkeye, which did detection of low-flying threats) would carry a small radar pod that would illuminate targets beyond the radar horizon of vessels that would launch the Standard Missiles
During this period, the US Army was working on the PAC-3 program; the missile segment of this program was aimed at finding a successor to the PAC-2 missile capable of handling TBM threats better. There were two competing missiles, a modified PAC-2 called the Multi-Mode Seeker Demonstration (MMSD) proposed by Raytheon and co-developed with Germany (Memorandum Of Understanding was signed on 17 May 1989).
The other missile was called the Extended Range Interceptor Technology (ERINT); it was proposed by Loral Vought and would eventually become what we know as the PAC-3 today. What both missiles had in common was the incorporation of active radar seekers, meaning they didn’t need any external sources to provide target illumination for them.
During the early testing to determine the winner of the PAC-3 program, the ERINT missile was supposedly favored for the overall mission and the Anti-TBM mission in particular. On the other hand, the PAC-2 MMSD missile proved to be better against fast aerial threats and low-flying cruise missile. However, the US Army was not particularly after these latter target sets, especially as the new PAC-3 missile also had some anti-cruise missile capabilities, albeit at reduced ranges.
Nonetheless, during the testing phases of the Mountain Top Program, a captive PAC-3 seeker was flown aboard an aircraft and underwent “simulated” firings. These were cued by external sensors (these would be E-3 Sentry or E-2 Hawkeye Airborne Early-Warning aircraft during combat), and the simulated firings took place against low-flying cruise missile targets The PAC-3 missile seeker proved capable of detecting and tracking these low-flying targets against both land and sea clutter using external information.
In the late 1990s, the US Army received $60M in funding to further test out the PAC-2 MMSD seeker as part of the Patriot Anti-Cruise Missile (PACM) Research and Development (R&D) effort. After a few successful flight tests, the goals of the program were met, and testing was discontinued. Following budget uncertainties in the mid-1990s to early 2000s, the Army opted to focus on fielding the new PAC-3 missile and upgrading older PAC-2 missiles to GEM (Guidance Enhancement Missile) variants. The PAC-2 PACM variant modifications would have added the MMSD active seeker, and the cost per missile in 1999 dollars was 600k or $1.1M today.
The situation in the early 2000s
At the conclusion of the above segment, the reader might come to believe that the US Army was on the right track while the US Navy was pursuing a more complicated and yet less elegant solution. This conclusion is only partially true.
The US Navy ended up abandoning this over-the-horizon variant of the Standard Missile cued by external sources (it would have been designated SM-5). Instead, the Navy opted to start fresh in the early 2000s with a new missile variant designated the Standard Missile 6 (SM-6). For this variant, the US Navy incorporated an active seeker from the AIM-120 Advanced Medium-Range Air-to-Air Missile (AMRAAM). The seeker’s diameter was increased to take advantage of the 34 centimeter (13.5 inch) diameter of the SM-6, resulting in a final solution similar to the Army’s missiles.
During this period, the US Navy further developed what it calls the Cooperative Engagement Capability (CEC). This is a capability that allows for formation-wide real-time sensor and tracking sharing and data fusion.
In layman’s terms, CEC allows all of the sensor information within a group of vessels and their supporting assets to be synchronized and combined into information that is usable for engagement by any vessel. During the development of CEC, it was deemed that this information was accurate enough to be used for weapons engagement, and, as such, algorithms were developed to facilitate these functionalities.
These new engagement functionalities included the capability for one vessel to fire a missile at a target it doesn’t see on its own sensors, but is visible to another vessel beyond the horizon. Once the missile is fired, it flies based on this information towards the area where the target is. When it passes from the sensor horizon of the launching vessel, it enters the sensor horizon of the forward vessel that was providing the information. This vessel then takes control of this missile and guides it to the target.
It is also possible to use aircraft such as the E-2 Hawkeye to provide detection of a target obscured by terrain from a firing ship, fire a missile towards the target based on the E-2 Hawkeye information, and have the missile turn on its active seeker and seek out its target independently.
By the early 2000s, through the Cooperative Engagement Capability (CEC) and the Naval Integrated Fire Control (NIFC) efforts, the Navy was able to completely overcome the low-flying threat while significantly improving fleet integration and sensor fusion.
On the other hand, the army made little progress on its end in the early 2000s. The new PAC-3 missile was capable of engaging low-flying cruise missiles, but the Army possessed no external cuing assets. The US Air Force’s E-3 Sentry and the US Navy’s E-2 Hawkeye were both integrated with PATRIOT, but not to the extent that they could be used for what the Navy had just demonstrated under CEC and NIFC. Moreover, the PAC-3 variant in use at the time was a missile purely optimized for mass engagement of TBM threats at close ranges of about 30 kilometers (18 miles).
It wasn’t until the US Army’s Integrated Battle Command System (IBCS) program truly kicked off that the problem was even seriously looked at. Since the late 1980s the Army was aware of the one big drawback all of its air defense platforms suffered from: they were meant to fight as a part of an integrated and layered network, but none of its systems could truly communicate in a fashion necessary to fulfill this mission. (This was the case in the late 1980s to early 1990s, and it remains partially the case as of today.)
The US Army’s Avenger units, which are responsible for short-range air defense, were not fully integrated with PATRIOT, which did medium- to long-range air and missile defense, which was itself not fully integrated with THAAD, which did medium- to long-range ballistic missile defense.
IBCS was intended to solve this problem and facilitate sensor fusion and integrated fire control capabilities similar to those of the US Navy’s CEC and NIFC, but on land. One particular focus for this system was the detection, tracking, and engagement of low-flying cruise missiles hidden from the main PATRIOT radar. The forward sensors that would be used for this were the Army’s AN/MPQ-64 Sentinel radars.
In a test that took place on the 29th of August, the US Army demonstrated the ability to detect, track, and fire at a low-flying cruise missile target using two Sentinel radars. At the time of firing, the target was obscured from the main PATRIOT radar by terrain. However, the IBCS was able to fuse the tracks from multiple radars and provide a track accurate enough for a PAC-3 missile to be fired and successfully engage the target.
Subsequent IBCS test events have further expanded the range and scale of testing to include assets such as the F-35 Lightning II and the US Army’s TPY-2 radar for the THAAD system.
The situation today
Today, the PATRIOT’s newest missile, the PAC-3 Missile Segment Enhancement (MSE) has a robust anti-cruise missile capability. The older PAC-2 had a anti-cruise missile optimized version proposed and tested but never fielded.
The PATRIOT as a system started off with a rather lackluster sensor setup against cruise missiles, and nations that still field PATRIOT as a standalone system will face a more daunting task if they try to employ the system against low-flying cruise missiles.
However, the US Army and International operators that field IBCS-capable PATRIOT batteries will have far more robust capabilities against cruise missiles. The best way to improve the PATRIOT’s anti-cruise missile capability is to use it with both better and more sensors that are fully integrated with IBCS.