Semi-active radar homing
Encyclopedia
Semi-active radar homing, or 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. Semi-active missile systems use bistatic
continuous-wave radar
.
The NATO brevity code
for a semi-active radar homing missile launch is Fox One.
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 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.
(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 counter attack, 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.
) 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.
One of the more effective methods used to defeat semi-active homing radar is flying techniques
. This depends 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.
. USAF and US Navy fighters armed with AIM-7 Sparrow
attained a success rate of barely 10%, which tended to amplify the effect of deleting the gun on most F-4 Phantoms, which carried 4 Sparrows. 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 sea launched standard missile
.
Modern SARH delivery systems and rounds have combined reliability on the order of 90%, such as the following.
Missile guidance
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...
system, perhaps the most common type for longer-range air-to-air
Air-to-air missile
An air-to-air missile 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 fuelled but sometimes liquid fuelled...
and surface-to-air missile
Surface-to-air missile
A surface-to-air missile or ground-to-air missile is a missile designed to be launched from the ground to destroy aircraft or other missiles...
systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal
Passive radar
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...
– provided by an external (“offboard”) source — as it reflects off the target. Semi-active missile systems use bistatic
Bistatic radar
Bistatic radar is the name given to a radar system which comprises a transmitter and receiver which are separated by a distance that is comparable to the expected target distance. Conversely, a radar in which the transmitter and receiver are collocated is called a monostatic radar...
continuous-wave radar
Continuous-wave radar
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.Continuous wave radar uses Doppler, which renders the radar immune to interference from large stationary objects and slow moving...
.
The NATO brevity code
Brevity code
Multiservice tactical brevity codes are codes used by various military forces. The codes procedure words, a type of voice procedure, are designed to convey complex information with a few words.-American/NATO codes:...
for a semi-active radar homing missile launch is Fox One.
Concept
The basic concept of SARH is that since almost all detection and tracking systems consist of a radarRadar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
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
Angular resolution
Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...
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
Monopulse radar
Monopulse radar is an adaptation of conical scanning radar which sends additional information in the radar signal in order to avoid problems caused by rapid changes in signal strength. The system also makes jamming more difficult...
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
Analytical dynamics
In classical mechanics, analytical dynamics, or more briefly dynamics, is concerned about the relationship between motion of bodies and its causes, namely the forces acting on the bodies and the properties of the bodies...
using missile speed, target speed, and separation distance.
Techniques are nearly identical using jamming signals
Radar jamming and deception
Radar jamming and deception is the intentional emission of radio frequency signals to interfere with the operation of a radar by saturating its receiver with noise or false information...
, 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
Accelerometer
An accelerometer is a device that measures proper acceleration, also called the four-acceleration. This is not necessarily the same as the coordinate acceleration , but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame...
, gyroscopic data
Fibre optic gyroscope
A fibre optic gyroscope senses changes in orientation, thus performing the function of a mechanical gyroscope. However its principle of operation is instead based on the interference of light which has passed through a coil of optical fibre which can be as long as 5 km.Two beams from a laser...
, and global positioning data
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
. This maximizes distance by minimizing corrective maneuvers that waste flight energy.
Contrast this with beam riding
Beam riding
Beam-riding, also known as 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...
systems, like the RIM-8 Talos
RIM-8 Talos
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 for terminal guidance...
, 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 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.
Continuous-wave radar
Modern SARH systems use continuous-wave radarContinuous-wave radar
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.Continuous wave radar uses Doppler, which renders the radar immune to interference from large stationary objects and slow moving...
(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
Vympel R-33
The Vympel R-33 is a long-range air-to-air missile developed by the Soviet Union. It is the primary armament of the MiG-31 interceptor, intended to attack large high-speed targets such as the SR-71 Blackbird, the B-1 Lancer bomber, and the B-52 Stratofortress....
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
Missile lock-on
Missile lock-on refers to a scenario where the guidance system for a missile can accurately track a target, and a fire-control system can calculate the required flightpath for the missile to hit the target...
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 counter attack, 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
Spectral density
In statistical signal processing and physics, the spectral density, power spectral density , or energy spectral density , is a positive real function of a frequency variable associated with a stationary stochastic process, or a deterministic function of time, which has dimensions of power per hertz...
matched to the receive radar detection bandwidth is the limiting factor for maximum range.
Electronic counter-countermeasure (ECCM)
Recent-generation SARH weapons have superior electronic counter-countermeasure (ECCMElectronic counter-countermeasures
Electronic counter-countermeasures is a part of electronic warfare which includes a variety of practices which attempt to reduce or eliminate the effect of electronic countermeasures on electronic sensors aboard vehicles, ships and aircraft and weapons such as missiles. ECCM is also known as...
) capability, but the system still has fundamental limitations. Some newer missiles, such as the SM-2
Standard missile
Standard Missile can refer to a family of several different American missiles:* RIM-66 Standard , a medium range surface-to-air missile, the successor of the RIM-24 Tartar missile...
, 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.
One of the more effective methods used to defeat semi-active homing radar is flying techniques
Blue Angels
The United States Navy's Navy Flight Demonstration Squadron, popularly known as the Blue Angels, was formed in 1946 and is currently the oldest formal flying aerobatic team...
. This depends upon the pilot knowing that a missile has been launched. The global positioning system
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
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
Self-destruct
A self-destruct is a mechanism which causes a device to destroy itself under a predefined set of circumstances.Self-destruct mechanisms are also found on devices and systems where malfunction could endanger large numbers of people...
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.
- Pacific Missile Range FacilityPacific Missile Range FacilityThe Pacific Missile Range Facility, Barking Sands is a U.S. Naval facility and airport located five nautical miles northwest of the central business district of Kekaha, in Kauai County, Hawaii, United States....
- Naval Air Warfare Center, Weapon Division, Point Mugu, China Lake
- Atlantic Test Ranges
Combat record
The combat record of U.S. SARH missiles was unimpressive during the Vietnam WarVietnam War
The Vietnam War was a Cold War-era military conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of...
. USAF and US Navy fighters armed with AIM-7 Sparrow
AIM-7 Sparrow
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 various allied air forces and navies. Sparrow and its derivatives were the West's principal beyond visual...
attained a success rate of barely 10%, which tended to amplify the effect of deleting the gun on most F-4 Phantoms, which carried 4 Sparrows. 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
AIM-9 Sidewinder
The AIM-9 Sidewinder is a heat-seeking, short-range, air-to-air missile carried mostly by fighter aircraft and recently, certain gunship helicopters. The missile entered service with United States Air Force in the early 1950s, and variants and upgrades remain in active service with many air forces...
and guns.
Since Desert Storm, most F-15 Eagle
F-15 Eagle
The McDonnell Douglas F-15 Eagle is a twin-engine, all-weather tactical fighter designed by McDonnell Douglas to gain and maintain air superiority in aerial combat. It is considered among the most successful modern fighters with over 100 aerial combat victories with no losses in dogfights...
combat victories have been scored with the Sparrow at beyond visual range
Beyond Visual Range missile
A beyond-visual-range missile usually refers to an air-to-air missile that is capable of engaging at ranges beyond . This range has been achieved using dual pulse rocket motors or booster rocket motor and ramjet sustainer motor....
. Similar performance has been achieved with sea launched standard missile
Standard missile
Standard Missile can refer to a family of several different American missiles:* RIM-66 Standard , a medium range surface-to-air missile, the successor of the RIM-24 Tartar missile...
.
Modern SARH delivery systems and rounds have combined reliability on the order of 90%, such as the following.
- RIM-66 StandardRIM-66 StandardThe RIM-66 Standard MR is a medium range surface-to-air missile originally developed for the United States Navy . 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...
- RIM-161 Standard Missile 3RIM-161 Standard Missile 3The RIM-161 Standard Missile 3 is a ship-based missile system used by the US Navy to intercept short-to intermediate-range ballistic missiles as a part of Aegis Ballistic Missile Defense System. Although primarily designed as an anti-ballistic missile, the SM-3 has also been employed in an...
- RIM-174 Standard ERAM
- AIM-7 SparrowAIM-7 SparrowThe 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 various allied air forces and navies. Sparrow and its derivatives were the West's principal beyond visual...
- RIM-7 Sea SparrowRIM-7 Sea SparrowRIM-7 Sea Sparrow is a US ship-borne short-range anti-aircraft and anti-missile weapon system, primarily intended for defense against anti-ship missiles...