Missile Guidance

Introduction

Missile guidance concerns the method by which the missile receives its commands to move along a certain path to reach a target. On some missiles, these commands are generated internally by the missile computer autopilot. On others, the commands are transmitted to the missile by some external source. The missile sensor or seeker, on the other hand, is a component within a missile that generates data fed into the missile computer. This data is processed by the computer and used to generate guidance commands. Sensor types commonly used today include infrared, radar, and the global positioning system. Based on the relative position between the missile and the target at any given point in flight, the computer autopilot sends commands to the control surfaces to adjust the missile's course.

Missiles are the one of the major weapon in the wars. The word missile comes from the Latin verb mittere, meaning "to send". The first missiles to be used operationally were a series of missiles developed by Nazi Germany in World War II. Most famous of these are the V-1 flying bomb and V-2.

Missiles are self-propelled aerial projectiles containing explosives. For the efficient elimination of target we want to direct it to the target accurately. For this purpose we are using guidance technologies. Missiles are classified according to three basis. One is based on application, which are surface to surface, surface to air, air to surface and air to air. The second one is based on area of operation, which are tactical, support and strategic. The third one is based on flight characteristics, which are aerodynamic and ballistic.

Guided missiles have a number of different system components, which are guidance, control, armament and propulsion sections. Guidance system directs its manoeuvres. Control system execute the manoeuveres. Armament system carries the explosive charge and fusing & firing sections. Propulsion system propels the missile. In all of these components guidance system is the brain of the missile. Every missile guidance system consists of an attitude control system and a flight path control system. A novel formulation of sliding mode control (SMC) based proportional navigation (PN) guidance law is presented. Unlike conventional SMC-based guidance laws, the law presented here does not need any knowledge of bounds of target acceleration. The target acceleration is estimated using the so-called inertial delay control (IDC). Closed- loop stability for the guidance loop is established. Simulations are carried out by considering highly-maneuvering targets and constant as well as varying missile velocity and the results are presented to demonstrate the effectiveness of the proposed formulation.

 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.

3.1 PHASES OF GUIDANCE

Missile guidance is generally divided into three phases. Those names refers to different

parts of the flight path.

 Boost phase.

 Midcourse phase.

 Terminal phase.

3.2 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. In missiles with separate boosters, the booster drops away from the missile at

burnout.

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.

3.3 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 itstays 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.

3.4 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 discussed in later sections. 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.

4.1 TYPES OF GUIDANCE SYSTEM

Missile guidance systems can be divided into four groups.

 Self-contained

 Command

 Beam-rider

 Homing

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.

4.2 SELF-CONTAINED

The self-contained group consists of the guidance systems in which all the guidance

and control equipment is inside the missile. It is grouped in to two.

1. Preset guidance

The term "preset" completely describes this guidance method. All the control

equipment is inside the missile, and all the information relative to the target location

and the trajectory the missile must follow are calculated and set into the missile before

it is launched. One disadvantage is that the trajectory cannot be changed once the

missile is launched. For this reason it is used against stationary targets and large land

masses; it cannot be used against a moving target. It is a relatively simple type of

guidance system. A completely preset system probably will not be used in missiles of

the future, but some features of the preset system will be combined with other systems.

An early example of a preset guidance system was the German V-2, where

range and bearing of the target were predetermined and set into the control mechanism.

The earliest Polaris missile was also designed to use preset guidance during the first

part of its flight, but this was soon modified to permit greater launch flexibility. 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.

2. Inertial navigation guidance

The inertial guidance method is used for the same purpose as the preset method and is

actually a refinement of the preset method. The simplest principle for guidance is the

law of inertia. In aiming a basketball at a basket, an attempt is made to give the ball a

trajectory that will terminate in the basket. However, once the ball is released, the

shooter has no further control over it. If he has aimed incorrectly, or if the ball is

touched by another person, it will miss the basket. However, it is possible for the ball

to be incorrectly aimed and then have another person touch it to change its course so

it will hit the basket. In this case, the second player has provided a form of guidance.

The inertial guidance system sup-plies the intermediate push to get the missile back

on the proper trajectory.

4.3 COMMAND GUIDANCE

The term command is used to describe a guidance method in which all guidance

instructions, or commands, come from sources outside the missile. The guidance

system of the missile contains a receiver that is capable of receiving instructions from

ship or ground stations or from air-craft. The missile flight path control system then

converts these commands to guidance information, which is fed to the attitude control

system.

In the command guidance method, one or two radars are used to track the

missile and target. As soon as the radar is locked on the target, tracking information is

fed into the computer. The missile is then launched and is tracked by the radar. Target

and missile ranges, elevations, and bearings are continuously fed to the computer. This

information is analyzed and a missile intercept flight path is computed. The appropriate

guidance signals are then transmit-ted to the missile receiver. These signals may be

sent by varying the characteristics of the missile-tracking radar beam, or by way of a

separate radio transmitter. The radar command guidance method can be used in ship,

air, or ground missile delivery systems. A relatively new type of command guidance

by wire is now operational in some short-range antitank-type weapons. These systems

use an optical sight for tracking the target while the weapons emits a characteristic

infra-red signature, which is used for tracking the weapon with an IR sensor. Deviation

of the weapon from the line of sight (LOS) to the target is sensed, and guidance

commands are generated that are fed to the weapon control system in flight via a direct

wire link. Each weapon contains wire spools that pay out as the warhead flies out the

line of sight to the target. Current usage of these systems is in relatively lightweight,

portable, short-range battlefield environments against armored targets where their high

accuracy and substantial warheads are most efficiently employed.

Remote control by radio

A guidance system based on remote control of the missile by radio was a

natural step in the development of missile guidance systems. Using this technique, the

control link could be stretched many miles, and any physical contact between

launching platform and the missile eliminated.

A simple radio remote control system is shown in figure. In this system the

operator visually observes the drone (tracking) and mentally decides the changes

necessary in course, speed, and altitude (computing). Guidance commands such as up

- down, right - left, and slow down - speed up are then sent to the drone by radio

(directing) where a receiver in the missile picks them up. The guidance commands are

then sent to the missile's flight control system to execute the desired maneuver

(steering).

Remote control by radar

The basic missile guidance system contains two radars and a computer. These

three units replace the human operator we needed in the radio remote control system

we talked about a moment ago. One radar tracks the target, the other tracks the missile.

Both radars are located at the launching platform. And so is the computer which takes

the two sets of tracking data and issues commands so that the missile will either collide

with the target or pass within lethal range of it. The command signals are sent to the

missile by the missile tracking radar beam. Notice that here, as in the radio remote

control system, guidance signals are developed in a source outside the missile. The two

systems we have just talked about come under the broad category of command

guidance.