MISSILES NAVIGATIONINTRODUCTIONWHAT IS A MISSILE

Missile is an object that can be project or thrown at target. This definition includes stone, arrows, bullets, bombs, torpedoes, and rockets. In current military application the word missile is becoming synonymous to ‘guided missile’ due to wide range of impact on the weapons field. In the unguided case, initial conditions (such as train, elevation, power charge in naval guns) and exterior ballistic effects are parameters that, along with normal distribution, affect the ‘fall of shot’. As advances in technology permitted (paralleled by increasing threat complexity), the development of guided missiles made possible a significant increase in terminal accuracy of military weaponry.

These missiles are classified on the basis of guidance

(1) Guided missiles

(2) Unguided missiles

Guided Missile:- In the guided class of missiles belong the aerodynamic guided missiles. That is, those missiles that use aerodynamic lift to control its direction of flight. An aerodynamic guided missile can be defined as an aerospace vehicle, with varying guidance capabilities, that is self-propelled through the atmosphere for the purpose of inflicting damage on a designated target. Stated another way, an aerodynamic guided missile is one that has a winged configuration and is usually fired in a direction approximately towards a designated target and subsequently receives steering commands from the ground guidance system (or its own, onboard guidance system) to improve its accuracy.Examples: - BrahMos…Unguided Missiles: - Unguided missiles, which includes ballistic missiles, follow the natural laws of motion under gravity to establish a ballistic trajectory. The important thing to note is that an unguided missile is usually called a rocket and is normally not a threat to airborne aircraft. Examples:- Rockets..

CLASSIFICATION OF GUIDED MISSILES:-

A number of different classifications of guided missiles are possible. Presently, there are many types of guided missiles. They can be broadly classified on the basis of their features such as type of target; range; mode of launching; system adopted for control, propulsion or guidance; aerodynamics; etc. They are also termed in a broad sense as strategic or tactical, defensive or offensive.On the basis of target they could be called

Anti-tank/anti-armor, Anti-personnel, Anti-aircraft/ helicopter, Anti-ship/anti-submarine, Anti-satellite or Anti-missile.

Another classification of missiles which is very popular is based on the method of launching. The following list will clarify this

S urface-to-Surface-Missiles (SSM), S urface-to-Air Missiles (SAM), A ir-to-Air Missiles (AAM), A ir-to-Surface Missiles (ASM).

Difference between Missile and Rocket:-

A rocket is a flying device with its engines using self-contained energy rather than relying on external fuel (such as oxygen) which accelerates matter and expels it to derive thrust for its movement. The matter can be accelerated by chemical reaction (most common), nuclear reaction (rarely) or other forms of energy (say magnetic or ionization).

A missile is self-guided object used as a weapon, generally using rockets or jet engines for propulsion. Missiles usually have explosive warheads.

“All missiles are rockets, but not all rockets are missiles”

MISSILE COMPONENTS:-

GUIDANCE SYSTEM WARHEAD

PROPULSION FUEL TANK

GUIDANCE SYSTEM:- Guidance system is the brain of a missile. It consist of two

separate systems, which are altitude control and flight path control. Altitude control system maintains the missile in the desired altitude by controlling it in pitch, roll and yaw. Flight path control system guides the missile to its designated targets.

PHASE GUIDANCE:-

Missile guidance is generally divided into three phases. Those names refers to different parts of the flight path.

1. Boost phase.2. Midcourse phase.3. Terminal phase.

BOOST PHASE :-

The objective of this phase is to place the missile at a position in space from where it can either "see" the target or where it can receive external guidance signals. Missiles are boosted to flight speed by booster component of propulsion system. This booster period lasts from the time the missile leaves the launcher until the booster burns its fuel.

Missiles aimed in specific direction on orders from fire control computer. This establishes line of fire along which missile must fly during boosted period. At the end of boost, missile must be at pre-calculated point. During the boost phase of some missiles, the guidance system and the aerodynamic surfaces are locked in position. Other missiles are guided during the boost phase. Later we will discuss completely about guidance in the study of INS.MIDCOURSE PHASE:-

The second, or midcourse, phase of guidance is often the longest in both distance and time. During this part of the flight, changes may be required to bring the missile onto the desired course and to make certain that it stays on that course. During this guidance phase, information can be supplied to the missile by any of several means. In most cases, the midcourse guidance system is used to place the missile near the target, where the system to be used in the final phase of guidance can take over. In other cases, the midcourse guidance system is used for both the second and third guidance phases.TERMINAL PHASE:-

The last phase of missile guidance must have high accuracy as well as fast response to guidance signals. Missile performance becomes a critical factor during this phase. The missile must be capable of executing the final maneuvers required for intercept within the constantly decreasing available flight time. The maneuverability of the missile will be a function of velocity as well as airframe design. Therefore, a terminal guidance system must be compatible with missile performance capabilities. The greater the target acceleration, the more critical the method of terminal guidance becomes. Suitable methods of guidance will be discuss later. In some missiles, especially short-range missiles, a single guidance system may be used for all three phases of guidance, whereas other missiles may have a different guidance system for each phase.

TYPES OF GUIDANCE SYSTEM:- Missile guidance systems can be divided into four groups.

1. Self-contained Guidance 2. Command Guidance

3. Beam-rider Guidance 4. Homing Guidance

No one system is best suited for all phases of guidance. It is logical then to combine a system that is excellent for midcourse guidance with one that is excellent for terminal guidance. Combined systems are known as composite guidance systems or combination systems. A particular combination of command guidance and semi-active homing guidance is called hybrid guidance. When a missile changes from one type of guidance to another while in flight, it must also contain some type of switching device to make the change. This device is called a control matrix, a highly sophisticated equipment found in modern missiles.1. SELF-CONTAINED:-

The self-contained group falls in the second category of guidance system types. All the guidance and control equipment is entirely within the missile. Some of the systems of this type are: preset, terrestrial, inertial, and celestial navigation. These systems are most commonly applicable to surface-to-surface missiles, and electronic countermeasures are relatively ineffective against them since they neither transmit nor receive signals that can be jammed.

I. Preset Guidance:- The term preset completely describes one guidance method. When preset guidance is used, all of the control equipment is inside the missile. This means that before the missile is launched, all information relative to target location as well as the trajectory the missile must follow must be calculated. After this is done, the missile guidance system must be set to follow the course to the target, to hold the missile at the desired altitude, to measure its air speed and, at the correct time, cause the missile to start the terminal phase of its flight and dive on the target. A major advantage of preset guidance is that it is relatively simple compared to other types of guidance; it does not require tracking or visibility. The preset method of guidance is useful only against stationary targets of large size, such as land masses or cities. Since the guidance information is completely determined prior to launch, this method would, of course, not be suitable for use against ships, aircraft, enemy missiles, or moving land targets.

II. Inertial Navigation Systems(INS) :-

Inertial navigation has been a key element of missile system design since the 1950s.Traditionally, the focus has been on strategic and precision-strike systems. In these applications, terminal-position accuracy is the primary objective of the navigation system. In guided missile systems in which a terminal seeker is used to sense and track an air or ballistic missile threat, a critical function of the inertial navigation system (INS) is to provide accurate seeker-attitude information and, therefore, allow accurate pointing of the seeker for acquisition-of a target. In addition, the navigation system provides essential data for guidance and flight-control functions. Definition of INS:- An inertial navigation system is a navigation aid that uses motion sensors to continuously track the position, orientation, and velocity (direction and speed of movement) of a vehicle without the need for external references. The operations of inertial navigation system depends on Newton’s law of classical mechanics.Main parts in INS

1. IMU2. Instrument support electronics3. Navigation computers.

There are mainly two types of INS systems are there, based on different designs with different performance characteristics.

Principle of Inertial Navigation:-

The principle of inertial navigation is based upon Newton's first law of motion, which statesA body continues in its state of rest, or uniform motion in a straight line, unless it is compelled to change that state by forces impressed on it.Put simply, this law says that a body at rest tends to remain at rest and a body in motion tends to remain in motion unless acted upon by an outside force. The full meaning of this is not easily visualized in the Earth's reference frame. For it to apply, the body must be in an inertial reference frame (a non-rotating frame in which there are no inherent forces such as gravity).Newton's second law of motion shares importance with his first law in the inertial navigation system, and states Acceleration is proportional to the resultant force and is in the same direction as this force.

This can be expressed mathematically asF = mawhereF is the forcem is the mass of the bodya is the acceleration of the body due to the applied force F.

The physical quantity pertinent to an inertial navigation system is acceleration, because both velocity v and displacement s can be derived from acceleration by the process of integration. Conversely, velocity and acceleration can be estimated by differentiation from displacement, written mathematically

Differentiation is the process of determining how one physical quantity varies with respect to another. Integration, the inverse of differentiation, is the process of summing all rate-of-change that occurs within the limits being investigated, which can be written mathematically as

An inertial navigation system is an integrating system consisting of a detector and an integration. It detects acceleration, integrates this to derive the velocity and then integrates that to derive the displacement. By measuring the acceleration of a vehicle in an inertial frame of reference and then transforming it to the navigation frame and integrating with respect to time. it is possible to obtain velocity, attitude and position difference. Measurement of the vehicle's rotation is needed for the transformation from the inertial to the navigation frame and for the computation of the attitude of the vehicle.

Physical Implementation of INS:-

There are two implementation approaches to an INS:a) Gimbaled or stabilized platform techniquesb) Strap down.

The original applications of INS technology used stable platform techniques. In such systems, the inertial sensors are mounted on a stable platform and mechanically isolated from the rotational motion of the vehicle. Platform systems are still in use, particularly for those applications requiring very accurate estimates of navigation data, such as ships and submarines.

Modern systems have removed most of the mechanical complexity of platform systems by having the sensors attached rigidly, or “strapped down”, to the body of the host vehicle. The potential benefits of this approach are lower cost, reduced size, and greater reliability compared with equivalent platform systems. The major disadvantage is a substantial increase in computing complexity.

Gimbaled or stabilized platform system:-

A gimbal is a rigid with rotation bearings for isolating the inside of the frame from external rotations about the bearing axes. At least three gimbals are required to isolate a subsystem from host vehicle rotations about three axes, typically

labeled roll, pitch, and yaw axes. The gimbals in an INS are mounted inside one another. Gimbals and torque servos are used to null out the rotation of stable platform on which the inertial sensors are mounted. This platform is driven by gyros to always maintain its alignment with these axes regardless of any movement of the vehicle. Analogue feeds can be taken directly from the accelerometers and gyros that are in direct proportion to acceleration, and changes in velocity and direction. Stabilized platforms have some disadvantages. The main design issue is “Gimbal Lock”. Gyros are usually mounted in three gimbals on bearings; this allows the aircraft (and gimbals) to rotate around the gyros without moving the platform. However, when two of the three gimbals align, and are effectively operating around the same axis, they can become locked together and be directly affected by movement around the remaining third axis. The solution is to complicate the system further by adding a fourth motorized gimbal, which is continuously driven to avoid alignment with the other three. How Gimbal Works :-

The integrity and availability of position updates benefit from inertial navigation technology, which is independent of external signals. The

purchase and correct installation of an inertial navigation system, known as INS, is a process that requires considerable knowledge of such high-tech sensors. Such as Accelerometers and Gyroscopes.

The gyros of a type known as “integrating gyros” give an output proportional to the angle through which they have been rotated. Output of each gyro connected to a servomotor driving the appropriate gimbal, thus keeping the gimbal in a constant orientation in inertial space. The gyros also contain electrical torque generators which can be used to create a fictitious input rate to the gyros. Applications of electrical input to the gyro torque generators cause the gimbal torque motors/servos to null the difference between the true gyro input rate and the electrically applied bias rate. This forms a convenient means of cancelling out any drift errors in the gyro.Inertial Measurement Unit (IMU):-

The Inertial Measurement Unity (IMU) is an integrated sensor package that combines multiple accelerometers and gyros to produce a three dimensional measurement of both specific force and angular rate, with respect to an inertial reference frame, as for example the Earth-Centered Inertial (ECI)

reference frame. Specific force is a measure of acceleration relative to free-fall. Subtracting the gravitational acceleration results in a measurement of actual coordinate acceleration. Angular rate is a measure of rate of rotation. It is important to note that in recent years the term IMU has become an umbrella term used to describe a wide assortment of inertial systems including Attitude Heading Reference Systems and Inertial Navigation Systems. In the context of this writing we will use the term IMU in accordance with its classical meaning to describe the combination of only an3-axis accelerometer combined with a 3-axis gyro. A onboard processor, memory, and temperature sensor may be included to provide a digital

interface, unit conversion and to apply a sensor calibration model. The IMU by itself does not provide any kind of navigation solution (position, velocity, attitude). It only actuates as a sensor, in opposition to the INS (Inertial Navigation System), which integrate the measurements of its internal IMU to provide a navigation solution. For instance an Inertial Navigation System (INS) uses an IMU to form a self-contained navigation system which uses measurements provided by the IMU to track the position, velocity, and orientation of a object relative to a starting point, orientation, and velocity.

Working of Gyros:- Gyroscope is a mechanical

instrument which uses a rapidly rotating mass to maintain a stable axis. The gyroscopes are mounted on the same platform as accelerometer platform to prevent movement of the accelerometers from the established reference axis. For simplicity it would suffice to state that the gyroscopic properties of rigidity and precision are used in inertial guidance systems to provide space stabilized platform for the accelerometers. Which must measure missile acceleration along a predetermined axis only. The gyroscope maintains its level of effectiveness by being able to measure the rate of rotation around a particular axis. When gauging the rate of rotation around the roll axis of an aircraft, it identifies an actual value until the object stabilizes out. Using the key principles of angular momentum, the gyroscope helps indicate orientation. In comparison, the accelerometer measures linear acceleration based on vibration.

Gyroscopes are used in various applications to since either the angle turned through by a vehicle or structure or more commonly its angular rate of turn about some defined axis. The sensors are used in a variety of roles such as:

Stabilization, Autopilot feedback, flight path sensor or platform stabilization , Navigation.

Working of Accelerometers:-

An accelerometer is a compact device designed to measure non-gravitational acceleration. When the object it’s integrated into goes from a standstill to any velocity, the accelerometer is designed to respond to the vibrations associated with such movement. It uses microscopic crystals that go under stress when vibrations occur, and from that stress a voltage is generated to create a reading on any acceleration. Accelerometers are important components to devices that track fitness and other measurements in the quantified self movement.

strapdown inertial navigation technology :-

The gimbals are mechanical frames, each of which is free to rotate about a single-axis which is nominally perpendicular to the free axis of its neighboring gimbal(s). Torque motors are used to rotate the gimbals with respect to one another and angular pick-offs provide a measure of their relative orientation. The system is configured so that the three gimbal pick-off angles correspond to the roll, pitch and yaw orientations of the host vehicle with respect to the platform/reference frame. Sometimes, it is necessary to add a fourth gimbal; this is required for very agile vehicle applications to allow the platform to remain isolated from the vehicle irrespective of its orientation. For example, a vertical launch guided weapon test vehicle was produced some years ago which had a four-axis gimbal system to avoid 'gimbal lock' during the dynamic turn-over manoeuvre.

The platform configuration (Figure) illustrated here minimizes the amount of computing needed to implement the navigation function of providing position, velocity and attitude of the host vehicle with respect to the designated navigation reference frame. Since the platform, and hence the accelerometer triad, is held in alignment with the reference frame, typically coincident with the local geographic frame (north, east, down), it is simply required to sum the accelerometer outputs with the gravity terms and to integrate the navigation equations to obtain estimates of velocity and position in the reference frame. Any rotational motion of the platform would

be detected by a gyroscope, the output of which would be fed back via a torque motor to rotate the appropriate gimbal (and hence the platform) in the opposite sense, so maintaining its initial (and fixed) orientation in space. For systems required to navigation around the Earth, it is required to torque the platform at Earth's rate plus any rate caused by the velocity of the system with respect to the Earth (transport rate) to allow it to remain aligned with the local level frame.

In a strapdown inertial navigation system the accelerometers are rigidly mounted parallel to the body axes of the vehicle. In this application the gyroscopes do not provide a stable platform; they are instead used to sense the turning rates of the craft. Double numerical integration, combining the

measured accelerations and the instantaneous turning rates, allows the computer to determine the...

As gimbaled inertial navigation systems evolved, they necessarily became increasingly based on intricate mechanical designs. In recent decades, technology has progressed more in the area of electronics than mechanics. This is reflected in the evolution of the strapdown inertial navigation system. With less moving parts and mechanisms than gimbaled systems, strapdown inertial navigation systems have strongly benefited from the advance of computer technologies, being built upon electronics, optics, and solid state technology. The majority of commercially available accelerometers and gyros take advantage of these modern technologies and are manufactured on micro-machined silicon.

Strapdown inertial navigation systems are rigidly fixed to the moving body (Figure 1.3). Therefore strapdown INUs move with the body, their gyros experiencing and measuring the same changes in angular rate as the body in motion. The strapdown INU's accelerometers measure changes in linear rate in terms of the body's fixed axes. The body's fixed axes is a moving frame of reference as opposed to the constant inertial frame of reference. The navigation computer uses the gyros' angular information and the accelerometers' linear information to calculate the body's 3D motion with respect to an inertial frame of reference.

There are two fundamentally different types of inertial navigation systems: gimbaling systems and strapdown systems. A typical gimbaling inertial navigation system, such as might be used on board a missile, uses three gyroscopes and three accelerometers. The three gimbal-mounted gyroscopes establish a frame of reference for the vehicle’s roll (rotation about the axis running from the front to the rear of the vehicle), pitch (rotation about the axis running left to right), and yaw (rotation about the axis running top to bottom). The accelerometers measure velocity changes in each of these three directions. The computer performs two separate numerical integrations on the data it receives from the inertial guidance system. First it integrates the acceleration data to get the current velocity of the vehicle, then it integrates the computed velocity to determine the current position. This information is compared continuously to the desired (predetermined and programmed) course.

In a strapdown inertial navigation system the accelerometers are rigidly mounted parallel to the body axes of the vehicle. In this application the gyroscopes do not provide a stable platform; they are instead used to sense

the turning rates of the craft. Double numerical integration, combining the measured accelerations and the instantaneous turning rates, allows the computer to determine the craft’s current velocity and position and to guide it along the desired trajectory.

In many modern inertial navigation systems, such as those used on commercial jetliners, booster rockets, and orbiting satellites, the turning rates are measured by ring laser gyroscopes or by fibre-optic gyroscopes. Minute errors in the measuring capabilities of the accelerometers or in the balance of the gyroscopes can introduce large errors into the information that the inertial guidance system provides. These instruments must, therefore, be constructed and maintained to strict tolerances, carefully aligned, and reinitialized at frequent intervals using an independent navigation system such as the global positioning system (GPS).