What is Guidance System? | Types of guidance system in Missile

Guidance System

In this lecture, we are going to learn about the guidance system used in missiles and the types of guidance systems used in missiles.

A guidance system is used to keep the missile on its proper flight path (trajectory) and headed toward the target.

The guidance system can be thought of as the brain of the missile. 

The guidance and control function of a missile is often confused as being the same.

A guidance system is used to keep the missile on its proper trajectory while the control system performs two distinct tasks.

First, it maintains the missile at proper flight altitude. Second, the control system responds to orders from the guidance system and steers the missile toward the target.

Therefore, the guidance system and control system work together to:

1. Determine the flight path of the missile and
2. Maintain the missile in proper flight attitude(stability)

So, there are four processes that are involved in these combined operations:

1. Tracking: The positions of the target and missile are continuously determined.
2. Computing: The tracking information is used to determine the directions necessary for control.
3. Directing: The directions or correcting are applied to the controlling units.
4. Steering: Using the correcting signals to direct movements of the control surfaces.

Block Diagram of Guidance System

• First, we will be discussing the basic control system and followed by a block diagram of guidance system.
• A basic guidance system is very similar to a basic control system as shown in the below figure.
• The two systems interrelate and interact in their operations.

• The free gyroscope provides inertial references from which missile attitude can be determined.
• For any particular attitude, gyro signals are sent from the gyroscope sensors to the summing network of the computer.
• These signals are proportional to the amount of roll, pitch, and yaw at any given instant.

What are the types of Guidance Systems?

There are various types of guidance systems, which are defined into four main categories as follows:

1. Command guidance system
2. Homing guidance system
3. Composite guidance system, and
4. Self-contained guidance system

Command Guidance system:

• Command guidance missiles are those, which are guided on the basis of direct electromagnetic radiation contact with a friendly source(i.e. ship, ground, or aircraft).
• All guidance instructions or commands, come from outside the missile.
• The guidance sensors detect this information and convert it to a usable form.
• The output guidance computer initiates the movement of the control surfaces and the missile response.
• There are various types of command guidance methods. Early examples included remote control by wire and by radio command. Generally, command by wire was limited to air-launched missiles.
• A pair of fine wires was unrolled from coils after the missile was launched. Radio command eliminates wires and extended the range of a missile.
• In the radar command guidance system, radar is used to track the missile and the target. Guidance signals are sent to the missile by varying the characteristics of the missile radar-tracking beam. 

Homing Guidance system:

• Homing guidance system also relies on electromagnetic radiation for guidance information.
• The homing device is usually a small antenna located within the node of the missile. It detects some type of distinguishing or radiation given off or reflected from the target.
• This information is converted into usable data and positions the control surfaces.
• There are three types of homing guidance systems that are used by SMS missiles:
  1. Active 
  2. Semi-active and 
  3. Passive.

1. Active Homing Guidance:

• In active homing guidance, the missile contains an onboard transmitter and receiver. The transmitter sends out radar signals in the general direction of the target.  These signals strike the target and reflect or bounce back to the missile. 
• Active homing guidance does not require a ship’s radar; the missile is entirely on its own after launch.
• Active homing uses a missile that contains both a power source and an illuminator.

2. Semiactive Homing Guidance:

• In semi-active homing guidance, the missile contains only a receiver. The ship’s free control radar serves as the transmitting and directs its radar energy to illuminate the target.
• Semi-active homing is the name given to the system where the target is illuminated or designated by directing a beam of light, IR, or radio energy at it.
• The great advantage of semi-active homing s the greatly increased power that can be brought to bear on the target without adding to the weight and size of the missile.

3. Passive Homing Guidance:

• The passive homing guidance method depends on the missile’s detecting some form of energy emitted by the target.
• A receiver antenna inside the missile picks up this “signal” and computes all necessary guidance information.
• Passive homing guidance, like active homing, is completely independent of the launching ship. 
• Passive homing relies on natural energy, which is emitted or reflected from the target.

Composite Guidance system:

• There is not any one type of guidance system(command or homing) best suited for all phases of guidance. Therefore, it is logical to design on guidance system that combines the advantage of the others.
• For example, a missile may ride a signal until it is within a certain range of the target.
• Control of a particular guidance subsystem may come from more than one source. 
• The device that switches guidance systems s often called a control Matrix.
• It automatically transfers the correct signal of the guidance subsystem regardless of the condition

Self-contained Guidance system:

• Certain guided missiles have self-contained guidance systems. All guidance and control functions are performed totally within the missile.
• They neither transmit nor receive any signals during flight.
• Therefore, jamming or other electronic countermeasures are ineffective against them.
• Generally, self-contained guidance systems are used in surface-to-surface or shore applications.