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Semi-active radar homing (SARH) is a common type of missile guidance system, perhaps the most common type for longer-range air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal—provided by an external ("offboard") source—as it reflects off the target(in contrast to active radar homing, which uses an active radar transceiver). Semi-active missile systems use bistatic continuous-wave radar.
The NATO brevity code for a semi-active radar homing missile launch is Fox One.
The basic concept of SARH is that since almost all detection and tracking systems consist of a radar system, duplicating this hardware on the missile itself is redundant. The weight of a transmitter reduces the range of any flying object, so passive systems have greater reach. In addition, the resolution of a radar is strongly related to the physical size of the antenna, and in the small nose cone of a missile there isn't enough room to provide the sort of accuracy needed for guidance. Instead the larger radar dish on the ground or launch aircraft will provide the needed signal and tracking logic, and the missile simply has to listen to the signal reflected from the target and point itself in the right direction. Additionally, the missile will listen rearward to the launch platform's transmitted signal as a reference, enabling it to avoid some kinds of radar jamming distractions offered by the target.
The SARH system determines the closing velocity using the flight path geometry shown in Figure 1. The closing velocity is used to set the frequency location for the CW receive signal shown at the bottom of the diagram (spectrum). Antenna offset angle of the missile antenna is set after the target is acquired by the missile seeker using the spectrum location set using closing speed. The missile seeker antenna is a monopulse radar receiver that produces angle error measurements using that fixed position. Flight path is controlled by producing navigation input to the steering system (tail fins or gimbaled rocket) using angle errors produced by the antenna. This steers the body of the missile to hold the target near the centerline of the antenna while the antenna is held in a fixed position. The offset angle geometry is determined by flight dynamics using missile speed, target speed, and separation distance.
Techniques are nearly identical using jamming signals, optical guidance video, and infra-red radiation for homing.
Maximum range is increased in SARH systems using navigation data in the homing vehicle to increase the travel distance before antenna tracking is needed for terminal guidance. Navigation relies on acceleration data, gyroscopic data, and global positioning data. This maximizes distance by minimizing corrective maneuvers that waste flight energy.
Contrast this with beam riding systems, like the RIM-8 Talos, in which the radar is pointed at the target and the missile keeps itself centered in the beam by listening to the signal at the rear of the missile body. In the SARH system the missile listens for the reflected signal at the nose, and is still responsible for providing some sort of “lead” guidance. The disadvantages of beam riding are twofold: One is that a radar signal is “fan shaped”, growing larger, and therefore less accurate, with distance. This means that the beam riding system is not accurate at long ranges, while SARH is largely independent of range and grows more accurate as it approaches the target, or the source of the reflected signal it listens for. Reduced accuracy means the missile must use a very large warhead to be effective (i.e.: nuclear). Another requirement is that a beam riding system must accurately track the target at high speeds, typically requiring one radar for tracking and another “tighter” beam for guidance.
The SARH system needs only one radar set to a wider pattern.
Modern SARH systems use continuous-wave radar (CW radar) for guidance. Even though most modern fighter radars are pulse Doppler sets, most have a CW function to guide radar missiles. A few Soviet aircraft, such as some versions of the MiG-23 and MiG-27, used an auxiliary guidance pod or aerial to provide a CW signal. The Vympel R-33 AA missile for MiG-31 interceptor uses SARH as the main type of guidance (with supplement of inertial guidance on initial stage).
SARH missiles require tracking radar to acquire the target, and a more narrowly focused illuminator radar to "light up" the target in order for the missile to lock on to the radar return reflected off target.The target must remain illuminated for the entire duration of the missile's flight. This could leave the launch aircraft vulnerable to counterattack, as well as giving the target's electronic warning systems time to detect the attack and engage countermeasures. Because most SARH missiles require guidance during their entire flight, older radars are limited to one target per radar emitter at a time.
The maximum range of a SARH system is determined by energy density of the transmitter. Increasing transmit power can increase energy density. Reducing the noise bandwidth of the transmitter can also increase energy density. Spectral density matched to the receive radar detection bandwidth is the limiting factor for maximum range.
Recent-generation SARH weapons have superior electronic counter-countermeasure (ECCM) capability, but the system still has fundamental limitations. Some newer missiles, such as the SM-2, incorporate terminal semi-active radar homing (TSARH). TSARH missiles use inertial guidance for most of their flight, only activating their SARH system for the final attack. This can keep the target from realising it is under attack until shortly before the missile strikes. Since the missile only requires guidance during the terminal phase, each radar emitter can be used to engage more targets. Some of these weapons, like the SM-2, allow the firing platform to update the missile with mid-course updates via datalink.
Some of the more effective methods used to defeat semi-active homing radar are flying techniques. These depend upon the pilot knowing that a missile has been launched. The global positioning system allows a missile to reach the predicted intercept with no datalink, greatly increasing lethality by postponing illumination for most of the missile flight. The pilot is unaware that a launch has occurred, so flying techniques become almost irrelevant. One difficulty is testing, because this feature creates public safety risks if a fault prevents datalink self-destruct signals when a missile is heading in the wrong direction. Most coastlines are heavily populated, so this risk exists at test centers for sea-based systems that are near the coastlines:
The combat record of U.S. SARH missiles was unimpressive during the Vietnam War. USAF and US Navy fighters armed with AIM-7 Sparrow attained a success rate of barely 10%, which tended to amplify the effect of removing the gun on most F-4 Phantoms, which carried 4 Sparrows. [ citation needed ].While some of the failures were attributable to mechanical failure of 1960s-era electronics, which could be disturbed by pulling a cart over uneven pavement, or pilot error; the intrinsic accuracy of these weapons was low relative to Sidewinder and guns
Since Desert Storm, most F-15 Eagle combat victories have been scored with the Sparrow at beyond visual range. Similar performance has been achieved with the sea-launched RIM-7 Sea Sparrow.
Soviet systems using SARH have achieved a number of notable successes, notably in the Yom Kippur War, where 2K12 Kub (NATO name SA-6) tactical SAM systems were able to effectively deny airspace to the Israeli Air Force. A 2K12 also shot down a U.S. F-16 in the Bosnian War.
SARH is a commonly used modern missile guidance methodology, used in multiple missile systems, such as:
The AIM-7 Sparrow is an American, medium-range semi-active radar homing air-to-air missile operated by the United States Air Force, United States Navy, and United States Marine Corps, as well as other various air forces and navies. Sparrow and its derivatives were the West's principal beyond visual range (BVR) air-to-air missile from the late 1950s until the 1990s. It remains in service, although it is being phased out in aviation applications in favor of the more advanced AIM-120 AMRAAM.
Semi-automatic command to line of sight (SACLOS) is a method of missile command guidance. In SACLOS, the operator has to continually point a sighting device at the target while the missile is in flight. Electronics in the sighting device and/or the missile then guide it to the target.
An air-to-air missile (AAM) is a missile fired from an aircraft for the purpose of destroying another aircraft. AAMs are typically powered by one or more rocket motors, usually solid fueled but sometimes liquid fueled. Ramjet engines, as used on the Meteor, are emerging as propulsion that will enable future medium-range missiles to maintain higher average speed across their engagement envelope.
Missile guidance refers to a variety of methods of guiding a missile or a guided bomb to its intended target. The missile's target accuracy is a critical factor for its effectiveness. Guidance systems improve missile accuracy by improving its Probability of Guidance (Pg).
The Convair RIM-2 Terrier was a two-stage medium-range naval surface-to-air missile (SAM), and was among the earliest surface-to-air missiles to equip United States Navy ships. It underwent significant upgrades while in service, starting with a beam-riding system with 10-nautical-mile (19 km) range at a speed of Mach 1.8, and ending as a semi-active radar homing system with a range of 40 nmi (74 km) at speeds as high as Mach 3. It was replaced in service by the RIM-67 Standard ER (SM-1ER).
RIM-7 Sea Sparrow is a U.S. ship-borne short-range anti-aircraft and anti-missile weapon system, primarily intended for defense against anti-ship missiles. The system was developed in the early 1960s from the AIM-7 Sparrow air-to-air missile as a lightweight "point-defense" weapon that could be retrofitted to existing ships as quickly as possible, often in place of existing gun-based anti-aircraft weapons. In this incarnation, it was a very simple system guided by a manually aimed radar illuminator.
Beam-riding, also known as Line-Of-Sight Beam Riding (LOSBR) or beam guidance, is a technique of directing a missile to its target by means of radar or a laser beam. The name refers to the way the missile flies down the guidance beam, which is aimed at the target. It is one of the simplest guidance systems and was widely used on early missile systems, however it had a number of disadvantages and is now found typically only in short-range roles.
The Hughes AIM-4 Falcon was the first operational guided air-to-air missile of the United States Air Force. Development began in 1946; the weapon was first tested in 1949. The missile entered service with the USAF in 1956.
A beyond-visual-range missile (BVR) is an air-to-air missile (BVRAAM) that is capable of engaging at ranges of 20 nmi (37 km) or beyond. This range has been achieved using dual pulse rocket motors or booster rocket motor and ramjet sustainer motor.
Continuous-wave radar is a type of radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects. Individual objects can be detected using the Doppler effect, which causes the received signal to have a different frequency from the transmitted signal, allowing it to be detected by filtering out the transmitted frequency.
Track-via-missile or TVM refers to a missile guidance technique which combines features of semi-active radar homing (SARH) and radio command guidance.
Command guidance is a type of missile guidance in which a ground station or aircraft relay signals to a guided missile via radio control or through a wire connecting the missile to the launcher and tell the missile where to steer in order to intercept its target. This control may also command the missile to detonate, even if the missile itself has a fuze.
Active radar homing (ARH) is a missile guidance method in which a missile contains a radar transceiver and the electronics necessary for it to find and track its target autonomously. NATO brevity code for an air-to-air active radar homing missile launch is fox three.
Passive radar systems encompass a class of radar systems that detect and track objects by processing reflections from non-cooperative sources of illumination in the environment, such as commercial broadcast and communications signals. It is a specific case of bistatic radar, the latter also including the exploitation of cooperative and non-cooperative radar transmitters.
The Bendix RIM-8 Talos was a long-range naval surface-to-air missile, and was among the earliest surface-to-air missiles to equip United States Navy ships. The Talos used radar beam riding for guidance to the vicinity of its target, and semiactive radar homing (SARH) for terminal guidance. The array of four antenna which surround the nose are SARH receivers which functioned as a continuous wave interferometer. Initial thrust was provided by a solid rocket booster for launch and a Bendix ramjet for flight to the target with the warhead serving as the ramjet's compressor.
The RIM-66 Standard MR (SM-1MR/SM-2MR) is a medium-range surface-to-air missile (SAM), with a secondary role as anti-ship missile, originally developed for the United States Navy (USN). A member of the Standard Missile family of weapons, the SM-1 was developed as a replacement for the RIM-2 Terrier and RIM-24 Tartar that were deployed in the 1950s on a variety of USN ships. The RIM-67 Standard (SM-1ER/SM-2ER) is an extended range version of this missile with a solid rocket booster stage.
The General Dynamics Mauler was a self-propelled anti-aircraft missile system designed to a late 1950s US Army requirement for a system to combat low-flying high-performance tactical fighters and short-range ballistic missiles.
Lock-on is a feature of many radar systems that allow it to automatically follow a selected target. Lock-on was first designed for the AI Mk. IX radar in the UK, where it was known as lock-follow or auto-follow. Its first operational use was in the US ground-based SCR-584 radar, which demonstrated the ability to easily track almost any airborne target, from aircraft to artillery shells.
In the field of weaponry, terminal guidance refers to any guidance system that is primarily or solely active during the "terminal phase", just before the weapon impacts its target. The term is generally used in reference to missile guidance systems, and specifically to missiles that use more than one guidance system through the missile's flight.
An inverse monopulse seeker is a type of semi-active radar homing that offers significant advantages over earlier designs. The system requires electronics that can compare three signals at once, so this design did not become practically possible until the early 1970s. One of the first such examples was the Soviet Union R-40 air-to-air missiles used in MiG-25P introduced in service in 1970 and RAF's Skyflash missile introduced in 1978, an adaptation of the AIM-7 Sparrow that replaced the original Raytheon seeker with a monopulse model from Marconi, followed by a very similar conversion by Selenia for the Italian Aspide. The USAF adopted similar technology in the M model of the AIM-7 Sparrow, and such designs are universal in semi-active designs today.