Patriot Advanced Capability (PAC)

Patriot Advanced Capability, known today as the PAC-1 upgrade, was a software-only upgrade. The most significant aspects of this upgrade was changing the way the radar searched and the way the system defended its assets. Instead of searching low to the horizon, the top of the radar's search angle was lifted to near vertical (89 degrees) from the previous angle of 25. This was done as a counter to the steep parabolic trajectory of inbound ballistic missiles. The search beams of the radar were tightened, and while in "TBM search mode" the "flash", or the speed at which these beams were shot out, was increased significantly. While this increased the radar's detection capability against the ballistic missile threat set, it decreased the system's effectiveness against traditional atmospheric targets, as it reduced the detection range of the radar as well as the number of "flashes" at the horizon. Because of this, it was necessary to retain the search functions for traditional atmospheric threats in a separate search program, which could be easily toggled by the operator based on the expected threat. Additionally, the ballistic missile defense capability changed the way Patriot defended targets. Instead of being used as a system to defend a significant area against enemy air attack, it was now used to defend much smaller "point" targets, which needed to lie within the system's TBM "footprint". The footprint is the area on the ground that Patriot can defend against inbound ballistic missiles.

Through the 1980s, Patriot saw a number of smaller upgrades, mostly to its software. Most significant of these was a special upgrade to discriminate and intercept artillery rockets in the vein of the Multiple Launch Rocket System, which was seen as a significant threat to South Korea. This feature has not been used in combat and has since been deleted from United States Army Patriot systems, though it remains in South Korean systems. Another upgrade the system saw was the introduction of another missile type, designated MIM-104B and called "ASOJ" or "anti stand-off jammer" by the Army. This variant is designed to help Patriot engage and destroy ECM aircraft at standoff ranges. It works similar to an anti-radiation missile in that it flies a highly lofted trajectory and then locates, homes in on, and destroys the most significant emitter in an area designated by the operator.

PAC-2

In the late 1980s, tests began to indicate that, although Patriot was certainly capable of intercepting inbound ballistic missiles, it was questionable whether or not the MIM-104A/B missile was capable of reliably destroying them. This necessitated the introduction of the PAC-2 missile and system upgrade.

For the system, the PAC-2 upgrade was similar to the PAC-1 upgrade. Radar search algorithms were further optimized, and the beam protocol while in "TBM search" was further modified. PAC-2 also saw Patriot's first major missile upgrade, with the introduction of the MIM-104C, or PAC-2 missile. This missile was optimized for ballistic missile engagements. Major changes to the PAC-2 missile were the size of the projectiles in its blast-fragmentation warhead (changed from around 2 grams to around 45 grams), and the timing of the pulse-doppler fuse, which was optimized for high-speed engagements (though it retained its old algorithm for aircraft engagements if necessary). Engagement procedures were also optimized, changing the method of fire the system used to engage ballistic missiles. Instead of firing two missiles in an almost simultaneous salvo, a brief delay (between 3 and 4 second) was added in order to allow the second missile fired to discriminate a ballistic missile warhead in the aftermath of the explosion of the first.

PAC-2 was first tested in 1987 and reached Army units in 1990, just in time for deployment to the Middle East for the Persian Gulf War. Although no evidence of actual hits was ever recorded^[4], it was here that Patriot started to be regarded as a successful ABM system and a proof that ballistic missile defense was indeed possible. The complete study on its effectiveness remains classified.

There have been many more upgrades to PAC-2 systems throughout the 1990s and into the 21st century, again mostly centering on software. However, the PAC-2 missiles have been modified significantly, into four separate variants known collectively as "GEM" or "guidance enhanced" missiles.

The chief upgrade to the original GEM missile was a new, much faster proximity fused warhead. Tests had indicated that the fuse on the original PAC-2 missiles were detonating their warheads too late when engaging ballistic missiles with an extremely steep ingress, and as such it was necessary to shorten this fuse delay. The GEM missile was also given a new "low noise" seeker head designed to reduce interference in front of the missiles radar seeker, and a higher performance seeker designed to better detect low radar cross section targets, such as stealth aircraft. The GEM was used extensively in Operation Iraqi Freedom, with a perfect success rate.

Just prior to OIF, it was decided to further upgrade the GEM and PAC-2 missiles. This upgrade program produced missiles known as the GEM/T and the GEM/C, the "T" designator referring to "TBM", and the "C" designator referring to cruise missiles. These missiles were both given a totally new nose section, which was designed specifically to be more effective against low altitude, low RCS targets like cruise missiles. Additionally, the GEM/T was given a new fuse which was further optimized against ballistic missiles. The GEM/C is the upgraded version of the GEM, and the GEM/T is the upgraded version of the PAC-2. The GEM+ entered service in 2002, and the Army is currently having all of its PAC-2 and GEM missiles upgraded to the GEM/C or GEM/T standard.

PAC-3

PAC-3 missile launcher, note four missiles in each cannister

The PAC-3 upgrade is the most significant upgrade Patriot has received thus far, and is one of the most comprehensive upgrade programs ever undertaken on an American weapon system. Nearly every aspect of the system received a significant upgrade. The PAC-3 upgrade took place in three stages, and units were designated Configuration 1, 2, or 3 based on the stage of upgrade they were in.

The system itself saw another upgrade of its WCC and its software, and the communication setup was given a complete overhaul. Thanks to this upgrade, PAC-3 operators can now see tracks on the Joint Tactical Information Distribution System (JTIDS), which greatly increases the situational awareness of Patriot crews. The software can now conduct a "tailored TBM search", optimizing radar resources for search in a particular sector known to have ballistic missile activity, and can also support a "keepout altitude" to ensure ballistic missiles with chemical warheads or ERS ("early release submunitions") are destroyed at a certain altitude. For Configuration 3 units, the Patriot Radar was completely redesigned, adding an additional TWT (traveling wave tube) which increased the radar's search, detection, tracking, and discrimination abilities to previously unheard of levels. In fact, the PAC-3 radar is capable, among other things, of discriminating whether or not an aircraft is manned and which of multiple reentering ballistic objects are carrying ordnance.

The PAC-3 upgrade carried with it a new missile design, nominally known as MIM-104F^{[citation needed]} and called PAC-3 by the Army. The PAC-3 missile dedicated almost entirely to the anti-ballistic missile mission. Miniaturization has made the PAC-3 missile much smaller than the previous Patriot missiles; a single "can" (canister)now holds four missiles where one was once held. The PAC-3 missile is also much more maneuverable than previous variants, thanks to dozens of tiny rocket motors mounted in the forebody of the missile (called ACMs, or Attitude Control Motors). However, the most significant upgrade to the PAC-3 missile is the addition of a K_a band active radar seeker. This allows the missile to drop its uplink to the system and acquire its target itself in the terminal phase of its intercept, which improves the reaction time of the missile against a fast-moving ballistic missile target; the PAC-3 missile is, in fact, accurate enough to select, target, and home in on the warhead portion of an inbound ballistic missile. The active radar also gives the warhead a "hit-to-kill" capability that completely removes the need for a traditional proximity-fused warhead. This greatly increases the lethality against ballistic missiles of all types.

All told, the PAC-3 upgrade has effectively quintupled the "footprint" that a Patriot unit can defend against ballistic missiles of all types, and has considerably increased the system's lethality and effectiveness against ballistic missiles. It has also increased the scope of ballistic missiles that Patriot can engage, which now includes several intermediate range. However, despite its increases in ballistic missile defence capabilities, the PAC-3 missile is a less capable interceptor of atmospheric aircraft and air-to-surface missiles. It is slower, has a shorter range, and has a smaller explosive warhead compared to older Patriot missiles (although it generally relies on its kinetic "hit to kill" warhead).

Patriot's PAC-3 interceptor will be the primary interceptor for the new MEADS system, which is scheduled to enter service alongside Patriot in 2012.

Lockheed Martin Missiles and Fire Control, Dallas, Texas, is the prime contractor on the PAC-3 Missile Segment upgrade to the Patriot air defense system. The PAC-3 Missile Segment upgrade consists of the PAC-3 Missile, a highly agile hit-to-kill interceptor, the PAC-3 Missile canisters (in four packs), a fire solution computer and an Enhanced Launcher Electronics System (ELES). These elements are integrated into the Patriot system, a high to medium altitude, long-range air defense missile system providing air defense of ground combat forces and high-value assets.