Precision has always been one of the five most important attributes of a weapon, with operational range, striking power, volume of fire and portability being the other four. The state-of-the-art precision-guided munitions (PGMs) combine all these attributes to make them a potent force multiplier in the contemporary battlefield. Emergence and subsequent maturity of PGMs, made possible mainly due to advances in electronics, optoelectronics and optics, is one of the most significant developments of modern warfare. Let us discuss the various guidance techniques employed in this class of weapons.
PGMs, also called smart munitions, belong to the group of advanced fire-power munitions which mainly include projectiles fired from land or ship based military platforms, surface-to-air and air-to-air missiles and aerially-delivered bombs. These weapons employ one or more guidance techniques to hit the target more precisely with minimised collateral damage than would be possible with conventional unguided weapons. Launched from a variety of military platforms, including land vehicles, aircraft, ships and submarines, these weapons exemplify the concept of a low-cost threat, forcing a high-cost and complicated defence mechanism.
Though the concept of PGMs was first envisaged during World War I, the scientific and technological capability of that time did not allow it to become a practical reality. This weapon arrived on the battlefield in a rudimentary yet significant form during World War II. It was the success and experience gained during the Korean and Vietnam conflicts, and then the Gulf war, that established the efficacy of precision-strike weapons.
This class of weapons has come a long way from the era of World War II, when the guided-weapon was characterised by a circular error probable (CEP) of about 1000 metres to the state-of-the-art precision-strike weapon with CEP of less than a metre.
New guidance mechanisms are being developed to further enhance the precision of these weapons. More than one guidance technologies are being employed to make them all-weather weapons and eliminate the possibility of a mis-hit. Use of GPS/INS in conjunction with other guidance technologies, such as radar or electro-optic guidance, is an example.
Joint direct attack munitions (JDAM) is an important manifestation of the use of multiple-guidance technologies. The capability of a modern precision weapon has progressed from targeting a specific building to hitting a specific room. Soon the next generation technologies will turn the foot soldier, with an advance chip in his boot, into a precision strike weapon, able to navigate without GPS and fire guided bullets at targets like would-be snipers before they have a chance to fire at him.
Types of guided weapons
Different types of guided weapons mainly include anti-radiation weapons, radar-guided weapons, laser-guided weapons, infra-red-guided weapons, wire-guided weapons, beam-riding weapons and GPS/INS-guided weapons.
Anti-radiation weapons. Anti-radiation weapons (ARMs) are designed to target ground based radars. These do so by detecting radio emission from the radars and then homing onto the source of radio emission. These weapons can also be used to target jammers and radios used for communication. Air-to-surface, surface-to-surface, surface-to-air and air-to-air variants of ARMs are in use. A common deployment of these weapons is in specialist aircrafts meant to target ground based radars in suppression of enemy air defence (SEAD) role.
AGM-88 anti-radiation missile (HARM) of the USA is an example of air-to-surface anti-radiation weapon (Fig. 1). Surface-to-surface ARM, such as MM40 Exocet (Fig. 2), employs an active radar seeker, whose receiver component is used to home onto the radar. These missiles are extremely hard to defeat with electronic countermeasures.
Surface-to-air ARMs are used to target airborne early warning (AEW) and airborne warning and control systems (AWACSs). FT-2000 system of People’s Republic of China is an example. More recently, air-to-air ARM designs have also begun to appear on the scene, such as the anti-radiation Russian Vympel R-27P. Such missiles do not trigger any radar warning receivers and therefore are relatively immune to countermeasures.
Radar-guided weapons. Radar guidance, which includes semi-active radar guidance and active radar homing, is commonly used in long-range surface-to-air and air-to-air missiles. In the case of semi-active radar guidance, external radar irradiates the target, and the missile-seeker head makes use of the signal reflected off the target to home on to the target.
Active radar-homing missile has a radar transceiver on board the missile, which finds and continuously tracks the target till it hits it. MBDA MICA short- and medium-range surface-to-air and air-to-air missiles, MBDA EXOCET anti-ship missile of France and DRDO-Astra BVRAAM of India are some examples.
Laser-guided weapons. These weapons make use of a laser beam to guide the weapon (bomb, projectile or missile) to precisely hit the target. Most laser-guided munitions, with the exception of laser beam riding, operate on the principle of semi-active laser homing, similar to semi-active radar homing. A laser beam irradiates the intended target—a process called laser designation. It bounces off the target and gets scattered in all directions. The laser seeker in the munition detects the direction of arrival of laser energy and guides it to the target. Targets employ laser-absorbing paints, smoke screens and active protection systems as countermeasures.
Laser-guided munitions are available as canon-launched surface-to-surface projectiles, aerially-delivered bombs, and surface-to-surface, surface-to-air and air-to-surface missiles. Copperhead and Krasnopol (both canon-launched projectiles), Paveway-II (aerially-delivered bomb) and Hellfire (surface-to-surface and air-to-surface missile) are some examples. Fig. 3 shows Hellfire missile, depicting its inner parts.
Infra-red-guided weapons. Infra-red-guided weapons make use of electromagnetic radiation emitted from the target, predominantly in the infra-red part of the spectrum, to track the target and then home onto it. Such missiles are also referred to as heat-seeking missiles. The seeker head in this case is an infra-red sensor, located on the tip or head of the missile. The infra-red seekers are designed to be sensitive to either 3-5 micron band, in which case these are called single-colour seekers, or 3-5 micron and 8-12 micron bands and referred to as two-colour seekers. Infra-red missiles using two-colour seekers are far more immune to countermeasures like flares.
Another variant of the infra-red-guided missile is the one employing an imaging infra-red (IIR) seeker head, which uses IR/UV focal plane sensor array. Missiles employing IIR seeker heads are far more resistant to countermeasures and are less likely to be fooled into locking onto the Sun’s radiation.
IRIS-T manufactured by Diehl BGT Defence as part of Germany-led multinational program is an advanced air-to-air guided missile employing IIR seeker. It has three variants including IRIS-T (air-to-air guided missile), IRIS-TSL (surface-launched medium-range-guided missile) and IRIS-T (surface-launched short-range-guided missile).
Wire-guided weapons. In this case, the missile is guided by electrical signals sent to it through a bundle of wires connected between the missile and the guidance mechanism located near the launch site. The wires reel out behind the weapon as it flies. Wire guidance is commonly used in anti-tank missiles, where its suitability in limited line-of-sight availability is particularly advantageous. Also, the missile’s limited range, imposed by length of the wire, is not an issue in anti-tank operations. Tube-launched optically-tracked and wire-guided (TOW) of the USA with an operational range of 3750 metres and MILAN of France (Fig. 4) with an extended operational range of 3000 metres (MILAN ER) are well-known in-service wire-guided missiles.
TOW family of missiles, including TOW 2A, TOW 2B Aero and TOW Bunker Buster missiles, is the premier long-range, heavy-assault-precision anti-armour, anti-fortification and anti-amphibious-landing weapon system. TOW missiles have been integrated on ground, vehicular and helicopter platforms. MILAN is a portable medium-range anti-tank missile, with later versions equipped with tandem heat warheads, making it more effective against reactive armour.
Beam-riding weapons. Beam-riding weapons make use of a radar or laser beam for guidance. A narrow radar or laser beam is directed at the target, usually a tank or an aircraft. The missile is launched in the direction of the target and sometimes, after it is launched, it flies into the radar or laser beam. With the help of sensors and a computer onboard the missile, it keeps itself within the beam. The aiming station keeps the beam pointed at the target till it hits it. The inherent shortcoming of the radar-beam-riding guidance is that the beam spreads as it moves away from the aiming station. Laser beam riders do not have this limitation.
Beam-riding guidance is generally used for short-range air-defence and anti-tank-guided missile applications. LAHAT (short-range anti-tank missile manufactured by Israel Aerospace Industries) (Fig. 5), Starstreak (short-range air-defence system manufactured by Thales Air Defence) and RBS-70 (short-range anti-tank guided missile manufactured by SAAB Bofors Dynamics) are known laser beam riders.
LAHAT is an advanced laser-homing attack missile that makes use of semi-active laser guidance. The target in this case can be designated either directly from the launching platform or by another land based or aerial platform. It has an operational range of eight kilometres when fired from a ground platform and 13km when fired from an aerial platform, and has a hit accuracy of 0.7m CEP. It is in use in Germany, Israel and India.
Starstreak is a man-portable/vehicle-mounted high-velocity missile with 3.5Mach velocity, designed to counter threats from conventional air threats and fast pop-up strikes by helicopter attacks. It has an operational range in excess of seven kilometres and is currently in the inventory of armed forces of the UK, South Africa, Indonesia and Thailand.
RBS-70 is not susceptible to any deception by countermeasures employed by the target aircraft in the form of chaff or flares. It has an operational range of zero to six kilometres and a speed up to 1.6Mach. RBS-70 new-generation (RBS-70NG) includes an improved sighting system capable of night vision. RBS-70 Mk-2 upgrade is called Bolide missile. It is faster with a speed of 2Mach against 1.6Mach of standard RBS-70, and a range of eight kilometres as against six kilometres in the case of standard RBS-70. Fig. 6 shows RBS-70NG.
GPS/INS-guided weapons. GPS/INS-guided weapons make use of a multi-channel GPS receiver to provide information on the weapon’s location and an inertial measurement unit (IMU) to monitor its attitude to adjust its flight path to precisely hit the target. This is a low-cost means of precision targeting that is unaffected by weather and target concealment and is immune to countermeasures. These weapons are primarily used against fixed targets or relocateable targets whose location is likely to remain static for the duration of planning and attack.
In another role, GPS/INS guidance is also used to adjust a weapon’s free fall to hit a predefined point fed into the weapon, prior to launch. Yet another application of GPS guidance is in mid-course correction of guided missiles and cruise missiles. Precision in the basic weapons is characterised by a CEP in the range of one to ten metres. However, CEP is considerably improved when GPS is used together with a semi-active laser (SAL) or imaging infra-red (IIR) seeker head.
Guided weapons may use more than one guidance mechanisms for improved performance. The different guidance techniques include:
1. Beam rider guidance
2. Command guidance
3. Homing guidance
4. Navigation guidance
Homing guidance further comprises: semi-active homing guidance, active homing guidance, passive homing guidance and re-transmission homing guidance.
Navigation guidance further comprises: inertial navigation guidance, ranging navigation guidance, celestial navigation guidance and geophysical navigation.
Beam rider guidance. Beam-riding guidance of munitions is based on a radar beam or a laser beam, constantly pointed toward the target throughout the flight time of the munitions. After the missile is launched, it attempts to keep itself inside the beam, while the aiming station keeps the beam pointed at the target.
The missile’s flight path control functions as follows: The missile’s guidance sensors located at the rear of the missile receive information about the position of the missile within the beam. The missile interprets this information and generates its own correction signals. These correction signals are used to send command signals to the control surfaces of the weapon to keep the missile in the centre of the beam. The launch station keeps the beam pointed at the target throughout the engagement period and the missile rides the beam to the intended target. Both radar and laser beam rider guidance have been successfully employed for surface-to-surface, surface-to-air and air-to-ground weapons.
Fig. 7 illustrates the concept of beam riding for a surface-to-air weapon using a radar beam. Fig. 8 shows a laser beam rider missile launched from a helicopter against a tank target. As the beam moves farther away from the launcher and towards the target, it spreads out and it becomes difficult to keep the beam in the centre of the target. That makes the beam rider concept effective only for short to medium operational ranges.
Laser beam riding guidance became more attractive particularly for short-range anti-air and anti-tank missiles in the 1980s and 1990s with the introduction of low-cost and highly-portable laser designators. Laser beam riding also allows the designer to encode additional information in the beam using digital means. Laser beam rider missiles are inherently more accurate. Also, a narrow laser beam guiding the weapon makes it more difficult to be detected and therefore immune to countermeasures.
Command guidance. In this case, the missile is commanded on an intercept course with the target. Conventionally, this is achieved by using two separate radars to continuously track the target and missile. Tracking data from these radars is fed to a computer that computes the trajectories of the two vehicles. The computer, in turn, sends appropriate command signals over a radio link to the missile. A sensor onboard the missile decodes the commands and operates the control surfaces of the missile to adjust its course so as to intercept the target in flight.
Fig. 9(a) shows the block schematic representation of command guidance. Fig. 9(b) shows the deployment scenario.
Wire-guided missiles are an example of command guidance. Here, command signals are sent to the missile through a conventional wire or a fibre-optic cable that actually reels out from the rear of the missile up to the launch platform. The missile trajectory in this case is controlled with the help of command signals transmitted via a wired link rather than a radio link. These missiles are commonly used for short-range anti-tank operations launched from either land based platforms or helicopters. In many cases, even torpedoes fired from submarines use wire guidance.
TOW is a popular example of a wire-guided missile. Manufactured by Hughes Aircraft Company, it is primarily used in anti-tank warfare and is a command to line-of-sight weapon. Current versions are capable of penetrating 76.2cm (30 inches) of armour at a maximum range of three kilometres. It can be fired from a vehicular platform, a helicopter and even by infantrymen using a tripod stand. Fig. 10 shows TOW missile being launched from a land platform.
Command guidance can be classified as command line-of-sight (CLOS) and command off line-of-sight (COLOS) guidance.
CLOS systems are further sub-divided into four groups. First is manual command to line-of-sight (MCLOS), where target tracking, missile tracking and control functions are all performed manually. Second type is semi-manual command to line-of-sight (SMCLOS). Here, target tracking is automatic but missile tracking and control functions are performed manually.
The third category is called semi-automatic command to line-of-sight (SACLOS). Here, target tracking is manual and missile tracking and control functions are automatic. SACLOS is the most common form of guidance in use against ground targets such as bunkers and tanks. Hellfire from Lockheed Martin is a helicopter-launched fire-and-forget anti-armour air-to-ground weapon of SACLOS category.
First three generations of the weapon use laser seeker, while the fourth generation uses radar seeker. The fourth sub group is known as automatic command to line-of-sight (ACLOS), where all three functions are automatic.
COLOS system, unlike CLOS system, does not depend on angular coordinates of the missile and the target. The guidance system ensures missile interception of the target by locating both missile and target in space for which distance coordinate is needed. This can be possible only if both missile tracker and target tracker were active. In the case of COLOS system, missile and target tracker can be oriented in different directions.
Homing guidance. Homing guidance is most commonly used in surface-to-air and air-to-air guided weapons. It is further sub-divided into four groups: semi-active homing, active homing, passive homing and track-via-missile homing (also known as re-transmission homing).
In the case of semi-active homing guidance, the target is illuminated by an external source, which could be a radar or laser. The electromagnetic energy reflected by the target is intercepted by the seeker head of the guided weapon. An onboard computer processes the intercepted signal and determines the target’s relative trajectory. It sends appropriate command signals to the control surfaces of the weapon to make it intercept the target.
Fig. 11 illustrates the concept of semi-active homing guidance in the case of an air-to-air missile. Semi-active homing is similar to command guidance, except for the fact that in the case of former, the command computer is onboard the weapon. The type of seeker head, whether it is radar seeker or laser seeker, depends on the type of external source designating the target. Both radar as well as laser-guided semi-active homing weapons are in use. Sparrow air-to-air missile and laser-guided weapons of the Paveway family are examples of semi-active homing guidance.
In the case of active homing guidance, the source of target designation is also onboard the weapon, with the result that this methodology does not require an external source. These features put it in the category of fire-and-forget missiles as the launch platform does not need to continue to illuminate the target after the missile has been launched. Active homing guidance weapons are usually radar-guided.
Also, in the case of active homing guidance, transmitted and reflected waves are at the same angle with respect to the line-of-sight between the target and the missile. This is different from semi-active homing mechanism in which transmitted and reflected waves are at an angle. It is because of this reason that semi-active and active homing guidance systems are sometimes called bi-static and mono-static systems, respectively.
RBS-15 anti-ship missile from Saab Bofors Dynamics, MBDA Exocet anti-ship and MBDA MICA surface-to-air and air-to-air missiles from MBDA, AS-34 Kormoran anti-ship missile from EADS and Indian DRDO-Astra BVRAAM air-to-air missile (Fig. 12) are some examples of missiles that use active radar homing in the terminal phase.
Passive homing guidance makes use of some form of energy emitted by the target. This energy is intercepted by the missile seeker, which is processed to extract guidance information to guide the missile to home on to the target. This energy could be in the form of heat energy generated by the target, which is made use of by the seeker in an infra-red-guided missile. Infra-red-guided missiles constitute an important category of electro-optically-guided precision-strike weapons.
Anti-radiation missiles, such as AGM-88 HARM air-to-ground missiles, track the RF energy emitted by ground based radar stations to generate guidance signals. Passive torpedoes make use of sound waves generated by the engines of ships or sonars to attack their targets. There are missiles such as AGM-65 Maverick that are equipped with electro-optic sensors that rely on visual images to guide the weapon to the target.
In the case of re-transmission homing guidance, the target is illuminated by external radar. The energy reflected by the target is intercepted by the missile sensor. In this case, the missile does not have an onboard computer to process the sensor signal and generate guidance command.
Instead, the sensor signal is transmitted back to the launch platform for processing. Command signals generated at the launch platform are re-transmitted back to the missile for use by the missile’s control surfaces to guide the missile to home on to the target.
The advantage with this guidance technique, also called track-via-missile, is that the expensive tracking and processing hardware is reusable and does not get destroyed along with the missile. But, it requires a high-speed communication link between the missile and launch station. MIM-104 Patriot surface-to-air missile system (Fig. 13) of Raytheon Company of the USA is an example.
Navigation guidance. The term guidance not only refers to the determination of the desired path of travel (also called trajectory) from the vehicle’s current location to an intended target, it also refers to the desired changes in velocity, rotation and acceleration needed to be executed for following the desired path.
The term navigation refers to the determination, at a given time, of the vehicle’s present-state vector defined by location, velocity and attitude. The term control refers to the manipulation of the forces, by way of steering controls, thrusters, etc, needed to track guidance commands while maintaining vehicle stability. These three functions are collectively known as navigation guidance.
Navigation guidance is further sub-divided into inertial navigation, ranging navigation, celestial navigation and geophysical navigation.
In the case of inertial navigation guidance, the vehicle uses onboard sensors to determine its motion and acceleration with the help of inertial measurement unit (IMU) or inertial navigation sensor (INS). This system works by telling the vehicle where it is at the time of launch, and the vehicle’s computer uses the signals from the inertial measurement unit to ensure that the vehicle travels along the programmed path. Inertial navigation systems are widely used on a range of aerospace vehicles, which include commercial airliners, military aircraft and spacecraft.
With reference to precision-guided munitions, navigation guidance is used for mid-course correction of guided missiles. Long-range all-weather subsonic cruise missile Tomahawk and medium-range all-weather beyond-visual-range air-to-air missile AMRAAM (Fig. 14) are some examples that use inertial navigation for mid-course guidance.
While inertial navigation guidance technique makes use of onboard sensors, ranging navigation depends on external signals for guidance, which are usually provided by radio beacons. Based on the direction and strength of the signals received by the aircraft, it navigates its way along the desired trajectory.
Ranging navigation guidance has been largely rendered obsolete with the arrival of global positioning system (GPS). GPS based navigation has largely replaced radio beacons in both military and civilian applications. GPS is a key enabling technology for existing and future military precision navigation applications.
Joint direct attack munitions (JDAM) series of guided bombs make use of integrated INS and GPS guidance techniques to determine where these are with respect to the locations of their targets. INS-GPS combination gives the precision-guided weapon a kind of all-weather capability and largely overcomes the vulnerability to adverse ground and weather conditions of weapons employing laser and imaging infra-red seekers. State-of-the-art precision-strike weapons use a combination of guidance technologies, including inertial navigation, global position sensing and laser/infra-red, seeking to achieve higher performance levels.
Celestial navigation is one of the oldest navigation techniques that uses the positions of stars to determine location, especially latitude, on the surface of the Earth. This form of navigation guidance requires good visibility of the stars, which makes it particularly useful at night or at very high altitudes. In celestial navigation, the missile compares the positions of stars to an image stored in its memory to determine its flight path.
Submarine-launched ballistic missile (SLBM) Poseidon of Lockheed Martin, carrying multiple independent re-entry technology and having an operational range in excess of 4500km, is an example of a ballistic missile using celestial navigation.
Geophysical navigation guidance depends for operation on the measurements made on the surface of the Earth. It uses compasses and magnetometers to measure the Earth’s magnetic field and gravitometers to measure the Earth’s gravitational field. This technique has not found much application in missile guidance.
Yet another guidance technique makes use of terrain contour matching (TERCOM). It uses a radar altimeter to measure height above ground. By comparing the contours of the terrain against data stored aboard the missile, the missile’s autopilot navigates its way to the destined location. TERCOM is a navigation system used primarily by cruise missiles.
A related technique to terrain matching is called digital scene matching, but is far more accurate. This technique relies for guidance on comparing the image seen below the weapon to satellite or aerial images stored in the missile computer. If the scenes do not match, the computer sends commands to control surfaces to adjust the missile’s course until the images match to a certain acceptable level.
Digital scene matching is used on Tomahawk cruise missile. In fact, Tomahawk’s guidance system uses a combination of INS, GPS, TERCOM and digital scene matching techniques.
In subsequent parts, we will discuss the different types of PGMs in terms of involved technologies, capabilities and limitations, deployment configurations, state-of-the-art and future trends.
Next in Part 2
Dr Anil Kumar Maini is former director, Laser Science and Technology Centre, a premier laser and optoelectronics research and development laboratory of Defence Research and Development Organisation of Ministry of Defence
Nakul Maini is currently pursuing Masters at University of Bristol, UK. He was working as a technical editor with Wiley India Pvt Ltd