Introduction
After World War II the term missile has been widely used to refer to guided projectiles. In unguided weapons, initial conditions (for example, launch train, elevation angle, gunpowder charge) and external ballistic effects, combined with normal distribution of those variables, determine the dispersion of impacts. Advances in technology have enabled significant improvements in terminal accuracy for military weapons. Automatic control is pervasive across missile technology, including torpedoes, aerodynamic surface-to-surface missiles, intercontinental ballistic missiles, air-to-air missiles, surface-to-air missiles, and guided projectiles.
Guidance and Control System Basics
Purpose and function
Each missile guidance system consists of an attitude control system and a flight-path control system. The attitude control system maintains the missile at the desired orientation by controlling pitch, roll, and yaw. The attitude control system functions like an autopilot, suppressing disturbances that tend to divert the missile from its commanded flight attitude. The flight-path control system determines the flight trajectory required to intercept the target and generates commands to the attitude control system to maintain that trajectory.
The concept of guidance and control includes not only maintaining a prescribed path from point A to point B, but also ensuring the vehicle behaves normally while following that path. A missile that reaches halfway to the target along a prescribed path and then becomes dynamically unstable will not remain on that path or may structurally fail under aerodynamic loads. To operate normally, the vehicle must be controllable and able to respond to control signals.
Guidance and control operation is based on feedback. When a guidance error exists, the control unit issues corrective adjustments to the missile control surfaces. The control unit also adjusts surfaces to stabilize roll, pitch, and yaw. Guidance and stability corrections are combined and applied as an error signal to the control system.
Sensors
The missile guidance system can be compared to a human pilot in an aircraft. When the pilot guides the aircraft to a landing site, the guidance system "sees" the target. If the target is distant or obscured, radio or radar beams can be used to locate it and direct the missile. Thermal, optical, television, the earth magnetic field, and Loran have all been found suitable for specific guidance tasks. When an electromagnetic source is used for guidance, antennas and receivers are installed in the missile to form the seeker or sensor. Sensors pick up or sense guidance information. Missiles that use means other than electromagnetic methods employ other sensor types, but every missile must have some means of receiving positional information.
The choice of sensor depends on factors such as maximum operating range, operating conditions, the type of information required, required accuracy, viewing angle, sensor weight and size, and the target type and speed.
Accelerometers and Inertial Measurement
Accelerometer arrays form the core of shipboard and missile inertial navigation systems. Accelerometers detect changes in vehicle motion. In basic form, an accelerometer measures acceleration. Simple devices, for example, a pendulum free to swing about a transverse axis, can measure acceleration along the missile fore-aft axis: when the missile accelerates forward, the pendulum lags, and its displacement is a function of the acceleration. Another simple device is a mass supported between two springs; when acceleration is applied, the mass moves opposite the applied force. The mass movement follows Newton's second law: acceleration is proportional to applied force and inversely proportional to mass.
If acceleration along the fore-aft axis is constant, velocity at any time is acceleration multiplied by elapsed time. However, acceleration may vary, requiring integration to obtain velocity. If velocity varies, a second integration is required to determine distance covered.
Moving elements in accelerometers can be linked to potentiometers, variable-inductor cores, or other devices that produce a voltage proportional to element displacement. Typically three double-integrating accelerometers continuously measure missile travel in three orthogonal directions—range, altitude, and azimuth. A double-integrating accelerometer is sensitive to acceleration and measures distance via two integrations. Measured distances are compared to desired preset distances loaded into the missile; if the missile deviates, correction signals are sent to the control system.
Accelerometers are sensitive to both gravitational acceleration and missile acceleration. Therefore, accelerometers used to measure range and azimuth must be fixed relative to gravity. This is achieved by mounting accelerometers on a platform stabilized by gyroscopes or a star-tracking telescope. Because the platform must remain oriented relative to gravity while the missile traverses Earth, the platform must be moved accordingly during flight. These factors cause inertial system accuracy to degrade with increasing flight time.
Dampers are included in accelerometer units to eliminate unwanted oscillations. Damping should be sufficient to prevent oscillation while allowing significant mass displacement so that mass motion remains proportional to vehicle acceleration. For example, if the case experiences acceleration in a given direction, the spring provides restoring force proportional to mass displacement while a viscous fluid damps oscillation.
In some designs, a mass (M) slides relative to a core (C); when the vehicle accelerates, a voltage proportional to mass displacement is sensed and amplified. A current (I), still related to displacement, is fed back around coils on the core. The resulting magnetic field produces a force on the mass, damping oscillations. Acceleration can thus be measured via mass displacement (X), voltage (E), or current (I).
Guidance Phases
Missile guidance is generally divided into boost, midcourse, and terminal phases. These names refer to different portions of the flight path. The boost phase may also be called the launch or initial phase.
Boost phase
Surface-to-air missiles accelerate to flight speed using booster components. The booster period lasts from launcher departure until the booster burns out. In missiles with separate boosters, the boosters detach when spent. The aim of this phase is to place the missile in a region of space where it can "see" the target or receive external guidance signals. In some missiles the guidance system and aerodynamic surfaces are locked during boost; in others, guidance continues during boost.
Midcourse phase
The midcourse phase is typically the longest in distance and time. During this portion, corrections are made to place and keep the missile on the desired flight path. Midcourse guidance can be provided in many ways. In most cases, midcourse guidance positions the missile near the target so that the terminal guidance system can take over. In some designs, the midcourse system also provides guidance for the later phases.
Terminal phase
The terminal phase requires high accuracy and rapid response to guidance signals. Missile performance becomes critical in this phase: the missile must execute the final maneuvers required for intercept within a rapidly decreasing available flight time. Maneuverability depends on speed and airframe design, so the terminal guidance method must be compatible with missile performance. Higher target accelerations increase the importance of the terminal guidance method. In some missiles, especially short-range types, all three guidance phases may be handled by a single guidance system; in others, each phase may use a different system.
Types of Guidance Systems
Missile guidance systems fall into two broad categories: those guided by artificial electromagnetic means and those guided by other means. The first category includes missiles controlled by radar or radio equipment, or those that use the target as an electromagnetic radiation source. The second category relies on electromechanical devices or natural electromagnetic references such as stars (self-contained guidance).
Missiles that maintain electromagnetic contact with an artificial source can be subdivided further into two subcategories: command-guided missiles and seeker-guided missiles.
Command guidance
Command-guided missiles are directed on the basis of direct electromagnetic contact between the missile and a friendly control point. Command guidance depends on a radar or radio link between the control point and the missile. Guidance information transmitted from the control point via radio or radar steers the missile’s flight path. Radar command guidance is the most common application of command guidance and serves as the discussion model here. The same principles apply to radio and television command guidance.
Radar command guidance may be divided into two categories: command guidance proper and beam-riding. In command guidance all steering commands originate from an external source. The missile contains a receiver that accepts commands from a ship, ground station, or aircraft. The missile flight-path control system converts these commands into guidance information that is fed to the attitude control system.
Typically one or two radars track both the missile and the target. Once the radar locks on the target, tracking data is input to a computer. After missile launch, the radar tracks the missile and target, continuously feeding range, elevation, and azimuth to the computer. The computer analyzes the data and computes an intercept flight path, then transmits appropriate guidance signals to the missile receiver. These signals may be sent by modulating the missile-tracking radar beam or via a separate radio transmitter. Radar command guidance can be used from ship, air, or ground launch platforms. Wired command guidance is also used in some short-range anti-armor weapons: an optical sight tracks the target, the weapon emits a characteristic infrared signature used by the missile’s IR sensor for tracking, and a direct line link carries steering commands to the weapon in flight. These systems are used in portable, short-range battlefield environments against armored targets.
Beam riding
Beam-riding differs from command guidance because the missile tracks a radar beam rather than receiving externally computed steering commands. The missile is designed to generate its own correction signals based on its position relative to the radar scan axis. After reviewing conical-scan tracking principles, this approach is easier to understand. The missile flight-path control unit is sensitive to deviations from the guidance radar scan axis and can compute the correct flight-path correction. Beam riding requires only one radar, but the radar must provide conical-scan capability to serve both target tracking and the missile flight-path reference axis. An advantage of beam riding is that multiple missiles can ride the same beam without complex multi-missile command systems because each missile generates its own steering commands.
As the radar beam diverges with range, beam-riding accuracy decreases with distance, and the missile becomes harder to keep near the beam center. If the target maneuvers rapidly, the missile must follow a changing path, which can subject it to large lateral accelerations.
Homing guidance (seeker guidance)
Homing guidance controls the weapon by using a seeker that responds to some characteristic of the target. Seekers can be sensitive to many energy forms, including RF, infrared, reflected laser, acoustic, and visible light. To lock onto a target, the missile or torpedo must determine target bearing and elevation using one of the angle-tracking methods described earlier. If necessary, active seekers also determine target range. Tracking is performed by a movable seeker antenna or a fixed electronically scanned array. Amplitude-comparison monopulse is generally preferred to older conical-scan systems due to higher data rates and faster response; however, phase-comparison monopulse or interferometer methods have advantages in some applications. Homing guidance methods are classified as active, semi-active, or passive.
Active homing
In active homing the weapon carries both a transmitter and receiver. Search and acquisition are performed like any tracking sensor. Single-site geometry is used so that the return echo follows the same path as the transmitted energy. Onboard computation calculates the intercept route and sends steering commands to the autopilot. The small size of missile seekers limits transmitter power and frequency choice, so seeker acquisition ranges are short.
Semi-active homing
In semi-active homing the target is illuminated by a radar at the launch platform or other control point. The missile carries a radar receiver only. The missile formulates its own correction signals from radar energy reflected from the target, but the return is a bistatic reflection because the illuminator and the missile receiver are at different locations. Target shape and composition may not reflect energy efficiently back toward the missile; in extreme cases the missile may lose the target and miss the intercept. This drawback can be mitigated by using higher power and more frequency diversity at the illuminator platform.
Passive homing
Passive homing depends solely on energy emitted by the target. This energy can be the noise radiated by ships or submarines in torpedo seekers, RF emissions from the target's own sensors (in anti-radiation missiles), thermal emissions from ship, aircraft, or vehicle exhaust, contrast in infrared or visible light, or microwave-region emissions from objects. As with other seeker types, the missile generates its own correction signals from target-originated energy rather than from control points. Passive homing reduces the detector's susceptibility to counter-detection and can use a wide range of energy forms and frequencies. Its disadvantages include vulnerability to decoys and deception and dependence on some level of target emissions.
Track-via-missile (TVM)
Track-via-missile (TVM) combines features of command guidance and semi-active homing. In command guidance steering commands are computed at the launch point from sensor-derived target and missile position data. In TVM the missile contains a semi-active seeker that determines bearing and elevation to the target and transmits that data back to the launch point via a data link. The fire-control system at the launch point can use its own target-tracking data, the missile’s tracking data, or both, along with missile position data, to compute steering commands which are then uplinked to the missile. TVM is used in some modern surface-to-air systems. Specific TVM implementations vary, but they all use missile-derived target-angle data to compute launch-point steering commands that are sent back to the missile.
Accuracy: Homing guidance is generally the most accurate of all guidance systems because it uses the target itself as the information source. Seeker devices control the missile path to a moving target in various ways; the most common are pursuit trajectories and commanded flight-path guidance. Because monopulse methods are advantageous in seeker heads and are becoming preferred in modern weapons, two basic monopulse types deserve mention: amplitude-comparison monopulse and phase-comparison monopulse (interferometer).
Amplitude-comparison monopulse requires a gimballed antenna within a radome on the weapon nose. Aerodynamic requirements often force a radome shape that is not optimal for radar performance. The antenna field of view is limited, so precise antenna commands are required for target acquisition. Antenna size limits seeker frequency range. The primary advantage is consistent performance across potential target speeds and maneuvers.
Interferometer (phase-comparison monopulse)
The interferometer eliminates the need for movable antennas by mounting fixed antennas on the airframe edges or wingtips, reducing complexity and widening the field of view. For each moving axis two antennas separated by a known distance are installed. Assuming the target is far enough away, incoming RF energy can be approximated as a plane wave, and the phase difference between antennas can be used to compute look angle. Interferometers offer a wide field of view, flexible airframe integration, unobstructed internal space, and wideband operation without antenna-size limits. Antenna spacing determines performance, typically separated by the missile diameter or fin span. A drawback is angular ambiguity when the wavelength is shorter than a specific separation-induced path difference: measured phase could correspond to multiple integer-2π ambiguities. This is typically a minor issue since the absolute angle is less important than its rate of change. In the same weapon size, an interferometer typically provides roughly double the range of an amplitude-comparison monopulse seeker, giving the missile twice the response time to transition from tracking a centroid to tracking a specific target, and thereby improving hit probability.
Composite and Hybrid Guidance
No single guidance system is optimal for every phase of flight. Combining a system with good midcourse characteristics with one having excellent terminal characteristics increases the probability of intercept. Such composite guidance systems combine methods; for example, a missile may use command guidance during midcourse until within a certain range of the target, then switch to a homing seeker for terminal guidance, or the command guidance may become a backup while the seeker takes primary control.
Hybrid guidance combining command and semi-active homing achieves advantages of both methods. It provides long-range capability by maintaining tracking sensors on the launch platform and transmitting data to the missile while simplifying fire-control mechanics by letting the missile compute its own attitude adjustments.
Autonomous Guidance Systems
Autonomous or self-contained systems carry all guidance and control equipment within the missile. Types include preset guidance, terrain- or ground-reference guidance, inertial guidance, and celestial navigation. These systems are often used for surface-to-surface missiles; they are relatively immune to electronic countermeasures because they do not emit or receive signals susceptible to jamming.
Preset guidance
Preset guidance means all control information is contained within the missile prior to launch. All information about target position and the trajectory the missile must follow is calculated and loaded before launch. The missile is then set to follow the planned route, maintain required attitude, measure its speed, and initiate terminal maneuvers at the correct time. A main advantage is relative simplicity; preset systems do not require tracking or line of sight after launch.
Early examples include the German V-2, where range and azimuth to the target were preset in the control mechanism. Early Polaris missile designs also used preset guidance in the first flight segment before being modified to allow more launch flexibility. Preset guidance is useful only against large, fixed targets such as land areas or cities because the guidance data are fixed before launch and cannot adapt to moving targets such as ships or aircraft.
Navigation-based guidance
When targets are far from the launch point, a navigation guidance method is required. Long-range accuracy requires rigorous path calculations that include control of pitch, roll, and yaw and account for external forces such as winds and missile inertia. Three navigation systems commonly used for long-range missiles are inertial, celestial, and terrain/ground-based navigation.
Inertial guidance
Inertial guidance relies on the laws of inertia. Inertial-guided missiles receive programming information before launch but have no electromagnetic link to the launch point after launch. They continuously correct flight path using accelerometers mounted on a gyroscopically-stabilized platform. All flight accelerations are continuously measured, and the missile’s attitude control produces corresponding corrective signals to maintain the correct trajectory. Inertial guidance removes much of the uncertainty from long-range delivery. Accelerometers detect unpredictable external forces and allow continuous path correction. In practice, inertial guidance has proven more reliable than any other long-range guidance method developed to date.
Celestial reference
Celestial navigation guidance uses continuous reference to fixed stars to adjust missile heading along a predesignated path. It is based on known apparent positions of stars or other celestial bodies relative to points on Earth at given times. Fixed stars and the sun are ideal references for long-range missiles because accuracy does not depend on range. Missiles must be equipped with a vertical or horizontal Earth reference, an automatic star-tracking telescope to determine star elevation angles relative to the reference, a time base, and a mechanical or electrical navigation star table. Onboard computation continuously compares star observations with the time base and navigation tables to determine the missile position and compute appropriate steering signals. These systems must fly above cloud cover to ensure star visibility. Celestial guidance (also called stellar guidance) is used in interplanetary probes and in some ICBM and SLBM systems.
Ground-reference methods
Prior to miniaturized computing, proposed ground-reference methods were limited. Early proposals included inertial reference systems combined with television cameras and film strips of planned flight paths, but image-matching proved too slow for most flight regimes. With compact memory and much greater onboard computing capability, ground-reference methods became practical. Small, accurate radar altimeters provide an alternative to optical photography and are less sensitive to weather and lighting. Radar altimeter returns provide coarse detection of surface features that can be compared with stored terrain profiles along the missile flight path. The system stores expected terrain elevations to the left and right of the planned ground track; by matching observed altitudes to stored profiles, the guidance computer can determine missile position, compute required heading corrections, and return the missile to the planned route. This technique is called terrain contour matching, or TERCOM.
TERCOM systems store a sequence of local terrain maps (TERCOM maps) along the route because even the most capable systems lack memory for continuous contour matching over hundreds of miles. The number and spacing of TERCOM maps depend on the quality of available data and inertial navigation accuracy. Data from various sources can support TERCOM without dedicated reconnaissance flights prior to engagement. TERCOM is accurate enough to find large targets such as military bases, but it cannot deliver conventional high-explosive warheads to specific locations inside a target area; thus TERCOM-guided missiles are typically armed with nuclear warheads.
Conventional high-explosive delivery requires higher accuracy available only in the terminal phase via some optical method. Advances in digital imaging allow storage of target-area grayscale scenes. Comparators can match missile TV camera images to stored grayscale scenes to determine position relative to the desired aimpoint and correct the flight path. This is called digital scene-matching area correlation, or DSMAC, and provides the precision needed to employ conventional warheads. DSMAC is used only in the final miles before the target, with TERCOM guiding most of the flight. Both TERCOM and DSMAC rely on the accuracy of the digital maps and scenes stored in missile memory, and preparing these data files requires significant support infrastructure.
Guided Flight Paths
A missile’s flight path is the vector sum of natural and applied forces. Natural forces include aerodynamic forces, gravity, and environmental effects; applied forces include thrust and control inputs. The instantaneous net force vector, a time-varying function, determines velocity vector changes. Planned missile paths are either preset or variable. Preset paths are planned and cannot change in flight; variable paths change according to conditions measured during flight.
Preset flight paths
Preset paths are either constant or programmed. Constant preset means the flight plan is predetermined and does not change after launch. In simple constant preset flight the missile has a single phase. The term can also include flights that, after a brief launch segment, maintain a constant configuration for the remainder of flight. Programmed presets are multi-phase: for example, a torpedo may be fired in an initial direction and use internal controls such as gyros and depth settings to orient toward the desired direction, then execute a search pattern in the second phase.
Variable flight paths
Variable flight paths adapt during flight. Weapon heading is usually a function of target position and velocity measured continuously; the missile assumes target motion remains constant until new tracking data are received. Common variable guidance laws include pursuit, constant-bearing (collision course), proportional navigation, and line-of-sight (beam riding).
Pursuit
The simplest procedure is pure pursuit: the missile always points directly at the target and flies along the instantaneous line of sight (LOS). The missile’s turn rate equals the LOS turn rate. Pure pursuit trajectories curve sharply near the end of flight and may demand more maneuverability than the missile can provide. Pursuit is often used against slow-moving targets or when the missile is launched from behind the target.
Constant-bearing (collision course)
At the opposite extreme is constant-bearing or collision-course guidance: the missile aims for a point ahead of the target such that missile and target arrive there simultaneously. The LOS to that intercept point does not rotate relative to the missile. For a constant-speed, non-maneuvering target, the collision path is linear and the missile’s lateral acceleration never exceeds the target’s lateral acceleration. The drawback is that the control system must forecast future target position, which requires sufficient data collection and processing.
Proportional navigation
More advanced seekers use proportional navigation. The missile guidance system measures the LOS rate (the rate at which the LOS rotates) and commands a turn rate proportional to that LOS rate. The proportionality constant, called the navigation constant, can be fixed or varied during flight to optimize performance. Early flight may use a navigation constant less than 1:1 to conserve speed and increase range; later it may increase to 2:1, 4:1, or higher to ensure sufficient agility to counter target maneuvering in the terminal phase.
Line-of-sight (beam-riding)
In line-of-sight or beam-riding guidance the missile is steered to travel along the LOS from the launch platform to the target. Beam-riding variants include beam riding with a constant lead angle, where the beam is offset ahead of the LOS. The main advantage of LOS guidance is its flexibility and the minimal complexity required on the missile since the launch platform bears most of the guidance burden.
Summary
Guidance is typically divided into three phases: boost, midcourse, and terminal. Distinctions between phases are based on flight path segmentation rather than discrete transitions between guidance methods, though the terminal phase demands peak system performance. Guidance systems split broadly into those using artificial electromagnetic means and those using other means, with further subcategories in each group. Missile trajectories may be preset or variable. Preset trajectories follow a preplanned routine and cannot be changed in flight; they may be single-phase (constant preset) or multi-phase (programmed preset). Variable trajectories can change during flight to intercept maneuvering targets via continuous reassessment of target position. Common variable guidance laws include pursuit, constant-bearing, proportional navigation, and line-of-sight.
Interception of moving targets depends on predicting future target positions and requires assumptions. For projectiles, ballistic missiles, or preset-guided missiles, the assumption is that measured target motion remains constant during missile flight. For variable-guided missiles, the assumption is that target motion measured nearly continuously will remain unchanged for short intervals while new data are collected and used to update guidance.
