Missile Control and Guidance

Missile Guidance and Controls

Introduction

The term missile in the post- World War II era has generally been used synonymously with "guided missile," due to the wide impact of guided missile technology upon the weapons field.

        FUNDAMENTALS OF GUIDANCE SYSTEMS

        Purpose and Function

               Every missile guidance system consists of an attitude control system and a flight path control system. The attitude control system functions to maintain the missile in the desired attitude on the ordered flight path by controlling the missile in pitch, roll, and yaw. The attitude control system operates as an auto-pilot, damping out fluctuations that tend to deflect the missile from its ordered flight path. The function of the flight path control system is to determine the flight path necessary for target interception and to generate the orders to the attitude control system to maintain that path.

       TYPES OF GUIDANCE SYSTEMS

              Missile guidance systems may be classified into two broad categories: missiles guided by man-made electromagnetic devices, and those guided by other means.

All of the missiles that maintain electromagnetic radiation contact with man-make sources may be further subdivided into two subcategories.

Control guidance missiles:

           Control guidance missiles are those that are guided on the basis of direct electromagnetic radiation contact with friendly control points.

Radar Control Guidance: Radar control guidance may be subdivided into two separate categories. The first category is simply referred to as the command guidance method. The second is the beam-rider method, which is actually a modification of the first, but with the radar being used in a different manner.

Command guidance:The term command is used to describe a guidance method in which all guidance instructions, or com-mands, come from sources outside the missile. The guidance sys- tem of the missile contains a receiver that is capable of re- ceiving 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.

Beam-rider Method:The main difference between the beam-rider method and the radar command guidance method is that the characteristics of the missile-tracking radar beam are not varied in the beam-rider system. The missile has been designed so that it is able to formulate its own correction signals on the basis of its position with respect to the radar scan axis.

Homing guidance missiles:

Homing guidance systems control the flight path by employing a device in the weapon that reacts to some distinguishing feature of the target. Homing devices can be made sensitive to a variety of energy forms, including RF, infrared, reflected laser, sound, and visible light. In order to home on the target, the missile or torpedo must determine at least the azimuth and elevation of the target by one of the means of angle tracking mentioned pre-viously. 

Active Homing: In active homing, the weapon contains both the transmitter and receiver. Search and acquisition are conducted as with any tracking sensor. The target is tracked employing monostatic geometry in which the returning echo from the target travels the same path as the transmitted energy

Semi active Homing: In semiactive homing, the target is illuminated by a tracking radar at the launching site or other control point. The missile is equipped with a radar receiver (no transmitter) and by means of the reflected radar energy from the target, formulates its own correction signals as in the active method.

Passive Homing: Passive homing depends only on the target as a source of tracking energy.

 Retransmission Homing or Track Via Missile (TVM). Re-transmission homing is a blending of the characteristics of both command and semiactive homing guidance. In command guidance, missile steering commands are computed at the launch point using target position and missile position data derived from launch point sensors.

GUIDED FLIGHT PATHS

                A guided missile is usually under the combined influence of natural and man-made forces during its entire flight. Its path may assume almost any form. Man-made forces include thrust and directional control as shown in figure 16-14. The vector sum of all the forces, natural and man-made, acting on a missile at any instant, may be called the total force vector. 

Preset Flight Paths.

Preset flight paths are of two types: constant and programmed.

Constant: A preset guided missile path has a plan that has been fixed beforehand. This plan may include several different phases, but once the missile is launched, the plan cannot be changed. The phases must follow one another as originally planned. The simplest type of preset guided missile path is the constant preset. Here, the missile flight has only one phase.

Programmed: A missile could be guided in a preset path against a fixed target; the joint effect of missile power and gravity would then cause the path to become a curve. A missile following a preset path may be guided in various ways--for in-stance, by an autopilot or by inertial navigation. The means of

propulsion may be motor, jet, or rocket.

Variable Flight Paths:

The guided flight paths of greatest interest are those that can vary during flight. In general, the heading of the weapon is a function of target position and velocity. These parameters are measured by continuous tracking, and the resultant missile flight path is determined, assuming that the target motion will remain unchanged until new data is received.

Pursuit: The simplest procedure for a guided missile to follow is to remain pointed at the target at all times. The mis-sile is constantly heading along the line of sight from the mis-sile to the target, and its track describes a pursuit path with the rate of turn of the missile always equal to the rate of turn of the line of sight. Pure pursuit paths are highly curved near the end of flight, and often the missile may lack sufficient

Constant Bearing: At the opposite extreme to a pursuit path is a constant-bearing or collision path. The missile is aimed at a point ahead of the target, where both the missile and target will arrive at the same instant. The line of sight to this point does not rotate relative to the missile. The missile path is as linear as the effect of gravity and aerodynamic forces allow. If the target makes an evasive turn or if the target's velocity changes, a new collision course must be computed and the missile flight path altered accordingly.

Proportional Navigation: The more advanced homing mis-siles will employ some form of proportional navigation. The mis-sile guidance receiver measures the rate of change of the line of sight (LOS) (bearing drift, if you will) and passes that informa-tion to the guidance computer, which in turn generates steering commands for the autopilot. The missile rate of turn is some fixed or variable multiple of the rate of change of the LOS. This multiple, called the navigation ratio, can be varied during mis-sile flight to optimize performance.

Line of Sight: Defined as a course in which the missile is guided so as to remain on the line joining the target and point of control. This method is usually called "beam riding."