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109-665: The AN/APG-81 is an active electronically scanned array (AESA) fire-control radar system designed by Northrop Grumman Electronic Systems (formerly Westinghouse Electronic Systems) for the Lockheed Martin F-35 Lightning II . The Joint Strike Fighter AN/APG-81 AESA radar is a result of the US government's competition for the world's largest AESA acquisition contract. Westinghouse Electronic Systems (acquired by Northrop Grumman in 1996) and Hughes Aircraft (acquired by Raytheon in 1997) received contracts for

218-445: A counterforce strike could destroy a single US missile. If both forces had similar numbers of missiles, such an attack would leave both forces with few remaining missiles to launch a counterstrike. Adding Zeus would reduce the number of losses on the US side, helping ensure a counterstrike force would survive. The same would be true if the US built more ICBMs instead. The Air Force was far more interested in building its own missiles than

327-519: A MAR and its associated underground Defense Center Data Processing System (DCDPS). Because the Sprint was designed to operate at short range, a single base could not provide protection to a typical US city, given urban sprawl . This required the Sprint launchers to be distributed around the defended area. Because a Sprint launched from a remote base might not be visible to the MAR during the initial stages of

436-485: A SAC command and control center or an airfield on the outskirts of a city. The second study, HSD-II, considered the protection of isolated bases like missile fields. Most follow-up work focused on the HSD-II concept. HSD-II proposed building small Sprint bases close to Minuteman fields. Incoming warheads would be tracked until the last possible moment, decluttering them completely and generating highly accurate tracks. Since

545-585: A Zeus base by firing only four warheads at it. These did not even have to land close in order to destroy the base; an explosion within several miles would destroy its radars, which were very difficult to harden . If the Soviets did have hundreds of missiles, they could easily afford to use some to attack the Zeus sites. Additionally, technical problems arose that appeared to make the Zeus almost trivially easy to defeat. One problem, discovered in tests during 1958 ,

654-528: A complete deployment would have been extremely expensive, on the order of the total yearly budget of the Department of Defense . Robert McNamara , the Secretary of Defense, believed that the cost could not be justified and worried it would lead to a further nuclear arms race . He directed the teams to consider deployments where a limited number of interceptors might still be militarily useful. Among these,

763-399: A few seconds and could take place as low as 25,000 feet (7,600 m). To provide the needed speed and accuracy, as well as deal with multi-warhead attacks, Nike-X used a new radar system and building-filling computers that could track hundreds of objects at once and control salvos of many Sprints. Many dozens of warheads would need to arrive at the same time to overwhelm the system. Building

872-626: A hybrid approach, the benefits of AESA (e.g., multiple independent beams) can be realized at a lower cost compared to pure AESA. Bell Labs proposed replacing the Nike Zeus radars with a phased array system in 1960, and was given the go-ahead for development in June 1961. The result was the Zeus Multi-function Array Radar (ZMAR), an early example of an active electronically steered array radar system. ZMAR became MAR when

981-415: A less sophisticated radar could be used, one with accuracy on the order of a mile rather than feet, which could be built much less expensively using VHF parts. This Extended Range Nike Zeus, or Zeus EX for short, would be able to provide protection over a wider area, reducing the number of bases needed to provide full-country defense. Work on this concept continued throughout the 1960s, eventually becoming

1090-418: A much lighter system that would use only 1200 missiles. Technological improvements in warheads and missiles in the late 1950s greatly reduced the cost of ICBMs. After the launch of Sputnik, Pravda quoted Nikita Khrushchev claiming they were building them "like sausages". This led to a series of intelligence estimates that predicted the Soviets would have hundreds of missiles by the early 1960s, creating

1199-448: A much simpler radar whose primary purpose was to track the outgoing Sprint missiles before they became visible to the potentially distant MAR. These smaller Missile Site Radars (MSR) were passively scanned, forming only a single beam instead of the MAR's multiple beams. While MAR was ultimately successful, the cost of the system was enormous. When the ABM problem became so complex that even

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1308-441: A much wider range of frequencies, to the point of changing operating frequency with every pulse sent out. Shrinking the entire assembly (the transmitter, receiver and antenna) into a single "transmitter-receiver module" (TRM) about the size of a carton of milk and arraying these elements produces an AESA. The primary advantage of an AESA over a PESA is the capability of the different modules to operate on different frequencies. Unlike

1417-420: A narrow range of frequencies to high power levels. To scan a portion of the sky, the radar antenna must be physically moved to point in different directions. Starting in the 1960s new solid-state devices capable of delaying the transmitter signal in a controlled way were introduced. That led to the first practical large-scale passive electronically scanned array (PESA), or simply phased array radar. PESAs took

1526-456: A rotating antenna, or similar passive array using phase or amplitude comparison . Typically RWRs store the detected pulses for a short period of time, and compare their broadcast frequency and pulse repetition frequency against a database of known radars. The direction to the source is normally combined with symbology indicating the likely purpose of the radar – airborne early warning and control , surface-to-air missile , etc. This technique

1635-451: A signal and then listening for its echo off distant objects. Each of these paths, to and from the target, is subject to the inverse square law of propagation in both the transmitted signal and the signal reflected back. That means that a radar's received energy drops with the fourth power of the distance, which is why radar systems require high powers, often in the megawatt range, to be effective at long range. The radar signal being sent out

1744-415: A signal from a single source, split it into hundreds of paths, selectively delayed some of them, and sent them to individual antennas. The radio signals from the separate antennas overlapped in space, and the interference patterns between the individual signals were controlled to reinforce the signal in certain directions, and mute it in all others. The delays could be easily controlled electronically, allowing

1853-514: A single TACMAR along with about 20 Zeus EX missiles. In October 1965 the TACMAR was replaced by the upgraded MSR from the SCD studies. Since this radar had an even shorter range than TACMAR, it could not be expected to generate tracking information in time for a Zeus EX launch. PAR would thus have to be upgraded to have higher accuracy and the processing power to generate tracks that would be handed off to

1962-535: A single autonomous battery centered on a cut-down MAR called TACMAR (TACtical MAR), along with a simplified data processing system known as the Local Data Processor (LDP). This was essentially the DCDP with fewer modules installed, reducing the number of tracks it could compile and the amount of decluttering it could handle. To further reduce costs, Bell later replaced the cut-down MAR with an upgraded MSR,

2071-491: A small portion of the sky. The radar would have to survive the electrical effects of EMP , and significant effort was expended on this. It also meant that the threat tube trajectories would have to be calculated rapidly, before or between blackout periods, and the final tracking of the warheads in the 10 seconds or so between clearing the clutter and hitting their targets. This demanded a very high-performance computer, one that did not exist at that time. The centerpiece of

2180-636: A system like MAR could no longer deal with realistic attack scenarios, the Nike-X concept was abandoned in favor of much simpler concepts like the Sentinel program , which did not use MAR. A second example, MAR-II, was abandoned in-place on Kwajalein Atoll . The first Soviet APAR, the 5N65 , was developed in 1963–1965 as a part of the S-225 ABM system. After some modifications in the system concept in 1967 it

2289-456: A system would cost an estimated $ 40 billion to build ($ 398 billion in 2024, about half the annual military budget). This led to further studies of the system to try to determine whether an ABM would be the proper way to save lives, or if there was some other plan that would do the same for less money. In the case of Zeus, for instance, it was clear that building more fallout shelters would be less expensive and save more lives. A major report on

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2398-577: A warhead, which could be anywhere in the tube. The WSEG suggested that a single ICBM with decoys would almost certainly defeat Zeus. A mid-1961 staff report by ARPA suggested that a single large missile with multiple warheads would require four entire Zeus batteries, of 100 missiles each, to defeat it. The Advanced Research Projects Agency (ARPA, today known as DARPA ) was formed in 1958 by President Dwight Eisenhower 's Secretary of Defense, Neil McElroy , in reaction to Soviet rocketry advances. US efforts had suffered from massive duplication of effort between

2507-419: A wide band even in a single pulse, a technique known as a "chirp". In this case, the jamming will be the same frequency as the radar for only a short period, while the rest of the radar pulse is unjammed. AESAs can also be switched to a receive-only mode, and use these powerful jamming signals to track its source, something that required a separate receiver in older platforms. By integrating received signals from

2616-460: A wider angle of total coverage. This high off-nose pointing allows the AESA equipped fighter to employ a crossing the T maneuver, often referred to as "beaming" in the context of air-to-air combat, against a mechanically scanned radar that would filter out the low closing speed of the perpendicular flight as ground clutter while the AESA swivels 40 degrees towards the target in order to keep it within

2725-787: Is a more advanced, sophisticated, second-generation of the original PESA phased array technology. PESAs can only emit a single beam of radio waves at a single frequency at a time. The PESA must utilize a Butler matrix if multiple beams are required. The AESA can radiate multiple beams of radio waves at multiple frequencies simultaneously. AESA radars can spread their signal emissions across a wider range of frequencies, which makes them more difficult to detect over background noise , allowing ships and aircraft to radiate powerful radar signals while still remaining stealthy, as well as being more resistant to jamming. Hybrids of AESA and PESA can also be found, consisting of subarrays that individually resemble PESAs, where each subarray has its own RF front end . Using

2834-411: Is a simple radio signal, and can be received with a simple radio receiver . Military aircraft and ships have defensive receivers, called " radar warning receivers " (RWR), which detect when an enemy radar beam is on them, thus revealing the position of the enemy. Unlike the radar unit, which must send the pulse out and then receive its reflection, the target's receiver does not need the reflection and thus

2943-417: Is common on ships, for instance. Unlike the radar, which knows which direction it is sending its signal, the receiver simply gets a pulse of energy and has to interpret it. Since the radio spectrum is filled with noise, the receiver's signal is integrated over a short period of time, making periodic sources like a radar add up and stand out over the random background. The rough direction can be calculated using

3052-454: Is much less useful against a radar with a frequency-agile (solid state) transmitter. Since the AESA (or PESA) can change its frequency with every pulse (except when using doppler filtering), and generally does so using a random sequence, integrating over time does not help pull the signal out of the background noise. Moreover, a radar may be designed to extend the duration of the pulse and lower its peak power. An AESA or modern PESA will often have

3161-574: Is the APG-77(V)1, which draws heavily on APG-81 hardware and software for its advanced air-to-ground capabilities. In August 2005, the APG-81 radar was flown for the first time aboard Northrop Grumman's BAC 1–11 test aircraft. The radar system had accumulated over 300 flight hours by 2010. The first radar flight on Lockheed Martin's CATBird avionics test-bed occurred in November 2008. In June 2009,

3270-434: Is then disconnected and the antenna is connected to a sensitive receiver which amplifies any echos from target objects. By measuring the time it takes for the signal to return, the radar receiver can determine the distance to the object. The receiver then sends the resulting output to a display of some sort . The transmitter elements were typically klystron tubes or magnetrons , which are suitable for amplifying or generating

3379-442: Is used. Target motion analysis can estimate these quantities by incorporating many directional measurements over time, along with knowledge of the position of the receiver and constraints on the possible motion of the target. Since each element in an AESA is a powerful radio receiver, active arrays have many roles besides traditional radar. One use is to dedicate several of the elements to reception of common radar signals, eliminating

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3488-406: Is why AESAs are also known as low probability of intercept radars . Modern RWRs must be made highly sensitive (small angles and bandwidths for individual antennas, low transmission loss and noise) and add successive pulses through time-frequency processing to achieve useful detection rates. Jamming is likewise much more difficult against an AESA. Traditionally, jammers have operated by determining

3597-504: The Cold War . The X in the name referred to its experimental basis and was supposed to be replaced by a more appropriate name when the system was put into production. This never came to pass; in 1967 the Nike-X program was canceled and replaced by a much lighter defense system known as Sentinel . The Nike-X system was developed in response to limitations of the earlier Nike Zeus system. Zeus' radars could only track single targets, and it

3706-752: The Pacific Ocean . For full-scale tests, the Army built an entire Zeus base on Kwajalein Island in the Pacific, where it could be tested against ICBMs launched from Vandenberg Air Force Base in California. Test firings at Kwajalein began in June 1962; these were very successful, passing within hundreds of yards of the target warheads, and in some tests, low-flying satellites. Zeus had initially been proposed in an era when ICBMs were extremely expensive and

3815-402: The "Autonomous MSR". They studied a wide variety of potential deployments, starting with systems like the original Nike-X proposal with no SCDs, to deployments offering complete continental US protection with many SCD modules of various types and sizes. The deployments were arranged so that they could be built in phases, working up to complete coverage. One issue that emerged from these studies

3924-492: The 'building blocks' of an AESA radar. The requisite electronics technology was developed in-house via Department of Defense research programs such as MMIC Program. In 2016 the Congress funded a military industry competition to produce new radars for two dozen National Guard fighter aircraft. Radar systems generally work by connecting an antenna to a powerful radio transmitter to emit a short pulse of signal. The transmitter

4033-444: The 1960s, followed by airborne sensors as the electronics shrank. AESAs are the result of further developments in solid-state electronics. In earlier systems the transmitted signal was originally created in a klystron or traveling wave tube or similar device, which are relatively large. Receiver electronics were also large due to the high frequencies that they worked with. The introduction of gallium arsenide microelectronics through

4142-448: The 1980s served to greatly reduce the size of the receiver elements until effective ones could be built at sizes similar to those of handheld radios, only a few cubic centimeters in volume. The introduction of JFETs and MESFETs did the same to the transmitter side of the systems as well. It gave rise to amplifier-transmitters with a low-power solid-state waveform generator feeding an amplifier, allowing any radar so equipped to transmit on

4251-421: The ABM became what one historian calls a "technology in search of a mission". In early 1965, the Army launched a series of studies to find a mission concept that would lead to deployment. One of the original deployment plans for Zeus had been a defensive system for SAC . The Air Force argued against such a system, in favor of building more ICBMs of their own. Their logic was that every Soviet missile launched in

4360-465: The AESA system of a Raptor to act like a WiFi access point, able to transmit data at 548 megabits per second and receive at gigabit speed; this is far faster than the Link 16 system used by US and allied aircraft, which transfers data at just over 1 Mbit/s. To achieve these high data rates requires a highly directional antenna which AESA provides but which precludes reception by other units not within

4469-517: The AESA's 60 degree off-angle limit. With a half wavelength distance between the elements, the maximum beam angle is approximately ± 45 {\displaystyle \pm 45} °. With a shorter element distance, the highest field of view (FOV) for a flat phased array antenna is currently 120° ( ± 60 {\displaystyle \pm 60} °), although this can be combined with mechanical steering as noted above. The first AESA radar employed on an operational warship

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4578-593: The Army's, especially in the case of Zeus, which appeared to be easily outwitted. Things changed in the early 1960s when McNamara placed limits on the Air Force fleet of 1,000 Minuteman missiles and 54 Titan IIs . This meant that the Air Force could not respond to new Soviet missiles by building more of their own. An even greater existential threat to Minuteman than Soviet missiles was the US Navy 's Polaris missile fleet, whose invulnerability led to questions about

4687-468: The Army, Air Force, and Navy, and seemed to be accomplishing little in comparison to the Soviets. ARPA was initially handed the mission of overseeing all of these efforts. As the problems with Zeus became clear, McElroy also asked ARPA to consider the antimissile problem and come up with other solutions. The resulting Project Defender was extremely broad in scope, considering everything from minor Zeus system upgrades to far-out concepts like antigravity and

4796-597: The F-35, with production to run beyond 2035, and including large quantities of international orders. Capabilities of the AN/APG-81 include the AN/APG-77 's air-to-air modes, plus advanced air-to-ground modes, including high resolution mapping , multiple ground moving target indication and track, combat identification, electronic warfare , and ultra high bandwidth communications. The F-22 radar from Lot 5 aircraft onward

4905-676: The F-35s APG-81 active electronically scanned array radar was integrated in the Northern Edge 2009 large-scale military exercise when it was mounted on the front of a Northrop Grumman test aircraft. The test events "validated years of laboratory testing versus a wide array of threat systems, showcasing the extremely robust electronic warfare capabilities of the world's most advanced fighter fire-control radar ." Announced on 22 June 2010: The radar met and exceeded its performance objectives successfully tracking long-range targets as part of

5014-487: The Hardsite concept, by 1966 the Air Force came to oppose it largely for the same reasons it had opposed Zeus in the same role. If money was to be spent on protecting Minuteman, they felt that money would be better spent by the Air Force than the Army. As Morton Halperin noted: In part this was a reflex reaction, a desire not to have Air Force missiles protected by "Army" ABMs. ... The Air Force clearly preferred that

5123-718: The I-67 concept suggested building a lightweight defense against very limited attacks. When the People's Republic of China exploded their first H-bomb in June 1967, I-67 was promoted as a defense against a Chinese attack, and this system became Sentinel in October. Nike-X development, in its original form, ended. In 1955 the US Army began considering the possibility of further upgrading their Nike B surface-to-air missile (SAM) as an anti-ballistic missile to intercept ICBMs. Bell Labs ,

5232-683: The MSRs. During this same time, Bell had noted problems with long wavelength radars in the presence of radar blackout. Both of these issues argued for a change from VHF to UHF frequencies for the PAR. Further work along these lines led to the Nike-X Deployment Study, or DEPEX. DEPEX outlined a deployment that started out very similar to Nth Country, with a few bases primarily using Nike EX to provide lightweight cover, but which also included design features that allowed more bases to be added as

5341-407: The Nike-X system was MAR, using the then-new active electronically scanned array (AESA) concept to allow it to generate multiple virtual radar beams, simulating any number of mechanical radars needed. While one beam scanned the sky for new targets, others were formed to examine the threat tubes and generate high-quality tracking information very early in the engagement. More beams were formed to track

5450-498: The PESA, where the signal is generated at single frequencies by a small number of transmitters, in the AESA each module generates and radiates its own independent signal. This allows the AESA to produce numerous simultaneous "sub-beams" that it can recognize due to different frequencies, and actively track a much larger number of targets. AESAs can also produce beams that consist of many different frequencies at once, using post-processing of

5559-499: The Perimeter Acquisition Radar (PAR), which would operate cheaper electronics at VHF frequencies. The high-altitude explosions that had caused so much concern for Nike Zeus due to blackout had been further studied in the early 1960s and led to a new possibility for missile defense. When a nuclear warhead explodes in a dense atmosphere, its initial high-energy X-rays ionize the air, blocking other X-rays. In

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5668-520: The RVs once they had been picked out, and still more to track the Sprints on their way to the interceptions. To make all of this work, MAR required data processing capabilities on an unprecedented level, so Bell proposed building the system using the newly invented resistor–transistor logic small-scale integrated circuits . Nike-X centralized the battle control systems at their Defense Centers, consisting of

5777-438: The Soviets was to drop their warheads just outside the range of the defensive missiles, upwind of the target. Ground bursts would throw enormous amounts of radioactive dust into the air, causing fallout that would be almost as deadly as a direct attack. This would make the ABM system essentially useless unless the cities were also extensively protected from fallout. Those same fallout shelters would save many lives on their own, to

5886-475: The Soviets would have 400–700 ICBMs deployed by 1969, and their deployment eventually reached 1,601 launchers, limited by the SALT agreements . While Nike-X could be expected to attack these with a reasonable 1 to 1 exchange ratio, compared to Zeus' 20 to 1, it could only do so over a limited area. Most nationwide deployment scenarios contained thousands of Sprint missiles protecting only the largest US cities. Such

5995-517: The US believed that the Soviet fleet contained a few dozen missiles. At a time when the US deterrent fleet was based entirely on manned bombers, even a small number of missiles aimed at Strategic Air Command 's (SAC) bases presented a serious threat. Two Zeus deployment plans were outlined. One was a heavy defensive system that would provide protection over the entire continental United States, but require as many as 7000 Zeus missiles. McNamara supported

6104-511: The US wished to limit casualties to 10 percent. ABMs would only be cheaper than ICBMs if the US was willing to allow over half its population to die in the exchange. When he realized he was using outdated exchange rates for the Soviet rouble , the exchange ratio for the 30 percent casualty rate jumped to 20-to-1. As the cost of defeating Nike-X by building more ICBMs was less than the cost of building Nike-X to counter them, reviewers concluded that

6213-472: The Zeus program ended in favor of the Nike-X system in 1963. The MAR (Multi-function Array Radar) was made of a large number of small antennas, each one connected to a separate computer-controlled transmitter or receiver. Using a variety of beamforming and signal processing steps, a single MAR was able to perform long-distance detection, track generation, discrimination of warheads from decoys, and tracking of

6322-454: The antennas beamwidth, whereas like most Wi-Fi designs, Link-16 transmits its signal omni-directionally to ensure all units within range can receive the data. AESAs are also much more reliable than either PESAs or older designs. Since each module operates independently of the others, single failures have little effect on the operation of the system as a whole. Additionally, the modules individually operate at low powers, perhaps 40 to 60 watts, so

6431-496: The beam to be steered very quickly without moving the antenna. A PESA can scan a volume of space much quicker than a traditional mechanical system. Additionally, thanks to progress in electronics, PESAs added the ability to produce several active beams, allowing them to continue scanning the sky while at the same time focusing smaller beams on certain targets for tracking or guiding semi-active radar homing missiles. PESAs quickly became widespread on ships and large fixed emplacements in

6540-429: The capability to alter these parameters during operation. This makes no difference to the total energy reflected by the target but makes the detection of the pulse by an RWR system less likely. Nor does the AESA have any sort of fixed pulse repetition frequency, which can also be varied and thus hide any periodic brightening across the entire spectrum. Older generation RWRs are essentially useless against AESA radars, which

6649-657: The combined signal from a number of TRMs to re-create a display as if there was a single powerful beam being sent. However, this means that the noise present in each frequency is also received and added. AESAs add many capabilities of their own to those of the PESAs. Among these are: the ability to form multiple beams simultaneously, to use groups of TRMs for different roles concurrently, like radar detection, and, more importantly, their multiple simultaneous beams and scanning frequencies create difficulties for traditional, correlation-type radar detectors. Radar systems work by sending out

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6758-513: The construction funds allocated for Zeus would not be released, and the funding would instead be used for development of a new system using the latest technologies. The name Nike-X was apparently an ad hoc suggestion by Jack Ruina , the director of ARPA, who was tasked with presenting the options to the President's Science Advisory Committee (PSAC). With the ending of Zeus, the ZMAR radar effort

6867-412: The construction of an ABM system would simply prompt the Soviets to build more ICBMs. This led to serious concerns about a new arms race , which it was believed would increase the chance of an accidental war. When the numbers were presented to McNamara, according to Kent: [He] observed that this was a race that we probably would not win and should avoid. He noted that it would be difficult indeed to stay

6976-473: The course with a strategy that aimed to limit the damage. The detractors would proclaim that, with 70 percent surviving, there would be upwards of 60 million dead. Despite its technical capabilities, Nike-X still shared one seemingly intractable problem that had first been noticed with Zeus. Facing an ABM system, the Soviets would change their targeting priorities to maximize damage, by attacking smaller, undefended cities for instance. As one DoD official put it at

7085-503: The decoys to be picked out earlier. Used as radio receivers, they could also triangulate any radio broadcasts coming from the threat tube, which the enemy might use as a radar jammer . When the system was first being proposed it was not clear whether the phased-array systems could provide the accuracy needed to guide the missiles to a successful interception at very long ranges. Early concepts retained Zeus Missile Tracking Radars and Target Tracking Radars (MTRs and TTRs) for this purpose. In

7194-804: The development of the Multifunction Integrated RF System/Multifunction Array (MIRFS/MFA) in February 1996. Lockheed Martin and Northrop Grumman were selected as the winners of the Joint Strike Fighter competition; The System Development and Demonstration (SDD) contract was announced on 26 October 2001. The AN/APG-81 is a successor radar to the F-22's AN/APG-77 , and has an antenna composed of 1,676 transmit/receive modules. Over three thousand AN/APG-81 AESA radars are expected to be ordered for

7303-408: The early 1960s when it was believed they would have hundreds. The key concept that led to Nike-X was that the rapidly thickening atmosphere below 60 kilometers (37 mi) altitude disrupted the reflectors and explosions. Nike-X intended to wait until the enemy warheads descended below this altitude and then attack them using a very fast missile known as Sprint . The entire engagement would last only

7412-441: The end, the MAR proved more than capable of the required resolution, and the additional radars were dropped. Nike-X had been defined in the early 1960s as a system to defend US cities and industrial centers against a heavy Soviet attack during the 1970s. By 1965 the growing fleets of ICBMs in the inventories of both the US and USSR were making the cost of such a system very expensive. NIE 11-8-63, published 18 October 1963, estimated

7521-447: The entire US. The system would be unable to deal with large numbers of warheads, but that was not a concern for a system that would only be tasked with beating off small attacks. With only small numbers of targets, the full MAR was not needed and Bell initially proposed TACMAR to fill this need. This would have a shorter detection range, so a long range radar like PAR would be needed for early detection. The missile sites would consist of

7630-700: The first mission systems test flights of the F-35 Lightning II BF-4 aircraft. The AN/APG-81 team won the 2010 David Packard Excellence in Acquisition Award for performance against jammers. In January 2023 it was reported that the AN/APG-81 would be replaced by a new radar, the AN/APG-85 on Block 4 F-35s. The AN/APG-85 had been mentioned in a budgetary document in December 2022. Active electronically scanned array The AESA

7739-463: The funds for missile defense be used by the Air Force to develop new hard rock silos or mobile systems. During the project's development phase, the siting and size of the Nike-X bases became a major complaint of smaller cities. Originally intended to protect only the largest urban areas, Nike-X was designed to be built at a very large size with many missiles controlled by an expensive computer and radar network. Smaller sites were to be left undefended in

7848-460: The highest layers of the atmosphere, there is too little gas for this to occur, and the X-rays can travel long distances. Sufficient X-ray exposure to an RV can damage its heat shields . In late 1964 Bell was considering the role of an X-ray-armed Zeus missile in the Nike-X system. A January 1965 report outlines this possibility, noting that it would have to have a much larger warhead dedicated to

7957-577: The launch, Bell proposed building a much simpler radar at most launch sites, the Missile Site Radar (MSR). MSR would have just enough power and logic to generate tracks for its outgoing Sprint missiles and would hand that information off to the DCDPS using conventional telephone lines and modems . Bell noted that the MSR could also provide a useful second-angle look at threat tubes, which might allow

8066-493: The name Hardpoint . This led to the construction of the Hardpoint Demonstration Array Radar, and an even faster missile concept known as HiBEX. This proved interesting enough for the Army and Air Force to collaborate on a follow-up study, Hardsite. The first Hardsite concept, HSD-I, considered the defending of bases within urban areas that would have Nike-X protection anyway. An example might be

8175-515: The nature of the threat changed. The study described a four-phase deployment sequence that added more and more terminal defenses as the sophistication of the Nth Country missiles increased over time. In December 1966, the Army asked Bell to prepare a detailed deployment concept combining the light defense of Nth Country with the point defense of Hardsite. On 17 January 1967, this became the I-67 project, which delivered its results on 5 July. I-67

8284-562: The need for a large high-voltage power supply is eliminated. Replacing a mechanically scanned array with a fixed AESA mount (such as on the Boeing F/A-18E/F Super Hornet ) can help reduce an aircraft's overall radar cross-section (RCS), but some designs (such as the Eurofighter Typhoon and Gripen NG ) forgo this advantage in order to combine mechanical scanning with electronic scanning and provide

8393-464: The need for a separate radar warning receiver. The same basic concept can be used to provide traditional radio support, and with some elements also broadcasting, form a very high bandwidth data link . The F-35 uses this mechanism to send sensor data between aircraft in order to provide a synthetic picture of higher resolution and range than any one radar could generate. In 2007, tests by Northrop Grumman , Lockheed Martin, and L-3 Communications enabled

8502-489: The need for ground-based ICBMs. The Air Force responded by changing missions; the increasingly accurate Minuteman was now tasked with attacking Soviet missile silos, which the less accurate Navy missiles could not do. If the force was going to carry out this mission there had to be the expectation that enough missiles could survive a Soviet attack for a successful counterstrike. An ABM might provide that assurance. A fresh look at this concept started at ARPA around 1963–64 under

8611-433: The one to be used to jam. Most radars using modern electronics are capable of changing their operating frequency with every pulse. This can make jamming less effective; although it is possible to send out broadband white noise to conduct barrage jamming against all the possible frequencies, this reduces the amount of jammer energy in any one frequency. An AESA has the additional capability of spreading its frequencies across

8720-472: The operating frequency of the radar and then broadcasting a signal on it to confuse the receiver as to which is the "real" pulse and which is the jammer's. This technique works as long as the radar system cannot easily change its operating frequency. When the transmitters were based on klystron tubes this was generally true, and radars, especially airborne ones, had only a few frequencies to choose among. A jammer could listen to those possible frequencies and select

8829-562: The original Nike-X concept since the system was simply too expensive to build with only a few interceptors. These cities complained that they were not only being left open to attack, but that their lack of defenses might make them primary targets. This led to a series of studies on the Small City Defense (SCD) concept. By 1964 SCD had become part of the baseline Nike-X deployment plans, with every major city being provided some level of defensive system. SCD would consist primarily of

8938-472: The outbound interceptor missiles. MAR allowed the entire battle over a wide space to be controlled from a single site. Each MAR, and its associated battle center, would process tracks for hundreds of targets. The system would then select the most appropriate battery for each one, and hand off particular targets for them to attack. One battery would normally be associated with the MAR, while others would be distributed around it. Remote batteries were equipped with

9047-507: The point that the ABM seemed almost superfluous. While reporting to Congress on the issue in the spring of 1964, McNamara noted: I personally will never recommend an anti-ICBM program unless a fallout program does accompany it. I believe that even if we do not have an anti-ICBM program, we nonetheless should proceed with the fallout shelter program. Under any reasonable set of assumptions, even an advanced system like Nike-X offered only marginal protection and did so for huge costs. Around 1965,

9156-410: The primary contractor for Nike, was asked to study the issue. Bell returned a report stating that the missile could be upgraded to the required performance relatively easily, but the system would need extremely powerful radar systems to detect the warhead while it was still far enough away to give the missile time to launch. All of this appeared to be within the state of the art , and in early 1957 Bell

9265-430: The primary weapon in the following Sentinel system, and in the modified Sentinel system that was later renamed Safeguard . In February 1965 the Army asked Bell to consider different deployment concepts under the Nth Country study. This examined what sort of system would be needed to provide protection against an unsophisticated attack with a limited number of warheads. Using Zeus EX, a few bases could provide coverage for

9374-559: The problem with nuclear blackout. The lower edge of the extended fireball used to induce this effect extended down to about 60 km, the same altitude at which decluttering became effective. Hence, low-altitude intercepts meant that deliberate attempts to create a blackout would not affect the tracking and guidance of the Sprint missile . Just as importantly, because the Sprint's own warheads would be going off far below this altitude, their fireballs would be much smaller and would only black out

9483-475: The production of X-rays, and would have to operate at higher altitudes to maximize the effect. A major advantage was that accuracy needs were much reduced, from a minimum of about 800 feet (240 m) for the original Zeus' neutron-based attack, to something on the order of a few miles. This meant that the range limits of the original Zeus, which were defined by the accuracy of the radars to about 75 miles (121 km), were greatly eased. This, in turn, meant that

9592-474: The recently invented laser . Meanwhile, one improvement to Zeus was already being studied: a new phased-array radar replacing Zeus' mechanical ones would greatly increase the number of targets and interceptors that a single site could handle. Much more powerful computers were needed to match this performance. Additionally, the antennas were mounted directly in concrete and would have increased blast resistance. Initial studies at Bell Labs started in 1960 on what

9701-477: The signal drops off only as the square of distance. This means that the receiver is always at an advantage [neglecting disparity in antenna size] over the radar in terms of range - it will always be able to detect the signal long before the radar can see the target's echo. Since the position of the radar is extremely useful information in an attack on that platform, this means that radars generally must be turned off for lengthy periods if they are subject to attack; this

9810-578: The so-called " missile gap ". It was later shown that the number of Soviet missiles did not reach the hundreds until the late 1960s, and at the time they had only four. Zeus used mechanically steered radars, like the Nike SAMs before it, limiting the number of targets it could attack at once. A study by the Weapons Systems Evaluation Group (WSEG) calculated that the Soviets had a 90 percent chance of successfully hitting

9919-530: The targets' own radar along with a lower rate of data from its own broadcasts, a detection system with a precise RWR like an AESA can generate more data with less energy. Some receive beamforming-capable systems, usually ground-based, may even discard a transmitter entirely. However, using a single receiving antenna only gives a direction. Obtaining a range and a target vector requires at least two physically separate passive devices for triangulation to provide instantaneous determinations, unless phase interferometry

10028-409: The threat tube begins to reenter the denser portions of the atmosphere, at altitudes around 60 kilometers (37 mi). Nike-X intended to wait until the decluttering was complete, meaning the interceptions would take place only seconds before the warheads hit their targets, between 5 and 30 miles (8.0–48.3 km) away from the base. Low-altitude intercepts would also have the advantage of reducing

10137-437: The time: It was a very expensive terminal defense system which for a given amount of money could provide protection to some number of cities, but leaving many totally unprotected, and it suffered the flaw of any terminal defense system - namely that every piece contributes to the cost by the enemy can choose where to attack and only a small part of the system can be brought to bear to counter such an attack. Another solution for

10246-452: The topic by PSAC in October 1961 made this point, suggesting that Zeus without shelters was useless, and that having Zeus might lead the US to "introduce dangerously misleading assumptions concerning the ability of the US to protect its cities". This led to a series of increasingly sophisticated models to better predict the effectiveness of an ABM system and what the opposition would do to improve their performance against it. A key development

10355-455: The warheads had to land within a short distance of a missile silo to damage it, any warheads that could be seen to be falling outside that area were simply ignored – only those entering the "Site Protection Volume" needed to be attacked. At the time, Soviet inertial navigation systems (INS) were not particularly accurate. This acted as a force multiplier , allowing a few Sprints to defend against many ICBMs. Although initially supportive of

10464-400: Was also possible to deploy radar decoys to confuse the defense. Decoys are made of lightweight materials, often strips of aluminum or mylar balloons, which can be packed in with the reentry vehicle (RV), adding little weight. In space, these are ejected to create a threat tube a few kilometers across and tens of kilometers long. Zeus had to get within about 1,000 feet (300 m) to kill

10573-583: Was built at Sary Shagan Test Range in 1970–1971 and nicknamed Flat Twin in the West. Four years later another radar of this design was built on Kura Test Range , while the S-225 system was never commissioned. US based manufacturers of the AESA radars used in the F-22 and Super Hornet include Northrop Grumman and Raytheon. These companies also design, develop and manufacture the transmit/receive modules which comprise

10682-419: Was calculated that a salvo of only four ICBMs would have a 90% chance of hitting a Zeus base. The attacker could also use radar reflectors or high-altitude nuclear explosions to obscure the warheads until they were too close to attack, making a single-warhead attack highly likely to succeed. Zeus would have been useful in the late 1950s when the Soviets had only a few dozen missiles, but would be of little use by

10791-700: Was essentially Nth Country but with more bases near Minuteman fields, armed primarily with Sprint. The wide-area Zeus and short-range Sprint bases would both be supported by the PAR network. The basic outlines of these various studies were becoming clear by 1966. The heavy defense from the original Nike-X proposals would cost about $ 40 billion ($ 376 billion in 2024) and offer limited protection and damage prevention in an all-out attack, but would be expected to blunt or completely defeat any smaller attack. The thin defense of Nth Country would be much less expensive, around $ 5 billion ($ 47 billion in 2024), but would only have any effect at all under certain limited scenarios. Finally,

10900-529: Was given the go-ahead to develop what was then known as Nike II. Considerable interservice rivalry between the Army and Air Force led to the Nike II being redefined and delayed several times. These barriers were swept aside in late 1957 after the launch of the R-7 Semyorka , the first Soviet ICBM. The design was further upgraded, given the name Zeus, and assigned the highest development priority. Zeus

11009-722: Was little atmosphere to carry a shock wave , it mounted a 400  kiloton (kT) warhead. The search radar was a rotating triangle 120 feet (37 m) wide, able to pick out warheads while still over 600 nautical miles (1,100 km) away, an especially difficult problem given the small size of a typical warhead. A new transistorized digital computer offered the performance needed to calculate trajectories for intercepts against warheads traveling over 5 miles per second (8.0 km/s; Mach 24). The Zeus missile began testing in 1959 at White Sands Missile Range (WSMR) and early launches were generally successful. Longer range testing took place at Naval Air Station Point Mugu , firing out over

11118-416: Was politically unacceptable. This led to proposals for a new radar dedicated solely to the early warning role, determining only which MAR or SCD would ultimately have to deal with the threat. Used primarily in the first minutes of the attack, and not responsible for the engagements, the system could be considered disposable and did not need anything like the sophistication or hardening of the MAR. This led to

11227-611: Was renamed MAR, and plans for an even more powerful version, MAR-II, became the central part of the Nike-X concept. Decoys are lighter than the RV, and therefore suffer higher atmospheric drag as they begin to reenter the atmosphere. This will eventually cause the RV to move out in front of the decoys. The RV can often be picked out earlier by examining the threat tube and watching for objects that have lower deceleration. This process, known as atmospheric filtering , or more generally, decluttering , will not provide accurate information until

11336-447: Was similar to the two Nike SAM designs that preceded it. It used a long-range search radar to pick up targets, separate radars to track the target and interceptor missiles in flight, and a computer to calculate intercept points. The missile itself was much larger than earlier designs, with a range of up to 200 miles (320 km), compared to Hercules' 75 miles (121 km). To ensure a kill at 100,000 feet (30 km) altitude, where there

11445-431: Was that nuclear fireballs expanded to very large sizes at high altitudes, rendering everything behind them invisible to radar. This was known as nuclear blackout . By the time an enemy warhead passed through the fireball, about 60 kilometers (37 mi) above the base, it would only be about eight seconds from impact. That was not enough time for the radar to lock on and fire a Zeus before the warhead hit its target. It

11554-489: Was the Prim-Read theory , which provided an entirely mathematical solution to generating the ideal defensive layout. Using a Prim-Read layout for Nike-X, Air Force Brigadier General Glenn Kent began considering Soviet responses. His 1964 report produced a cost-exchange ratio that required $ 2 of defense for every $ 1 of offense if one wanted to limit US casualties to 30 percent of the population. The cost increased to 6-to-1 if

11663-839: Was the Japanese OPS-24 manufactured by Mitsubishi Electric introduced on the JDS Hamagiri (DD-155), the first ship of the latter batch of the Asagiri-class destroyer , launched in 1988. Nike-X Nike-X was an anti-ballistic missile (ABM) system designed in the 1960s by the United States Army to protect major cities in the United States from attacks by the Soviet Union 's intercontinental ballistic missile (ICBM) fleet during

11772-440: Was the problem of providing early warning to the SCD sites. The SCD's MSR radars provided detection at perhaps 100 miles (160 km), which meant targets would appear on their radars only seconds before launches would have to be carried out. In a sneak attack scenario, there would not be enough time to receive command authority for the release of nuclear weapons. This meant the bases would require launch on warning authority, which

11881-597: Was then known as the Zeus Multi-function Array Radar, or ZMAR. In June 1961, Western Electric and Sylvania were selected to build a prototype, with Sperry Rand Univac providing the control computer. By late 1962 a decision on whether or not to deploy Zeus was looming. Bell began considering a replacement for the Zeus missile that would operate at much shorter ranges, and in October sent out study contracts to three contractors to be returned in February. Even before these were returned, in January 1963 McNamara announced that

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