COMMAND GUIDANCE SYSTEMS
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 of the guidance computer initiates the movement of the control surfaces and the missile responds.
There are (or were) various types of command guidance methods. Early examples included remote control by wire and by radio command. Generally command by (believe it or not) wire was limited to air-launched missiles. A pair of fine wires was unrolled from coils after the missile was launched. The airplane pilot mentally calculated and manually controlled the trajectory of the missile to the target. Radio command eliminated wires and extended the range of a missile.
However, one solution always leads to another problem. Radio command was effective as long as the operator could see the missile. After it flew beyond the range of normal vision . . . well, you can understand the problem if you have ever owned a remote-controlled model airplane. From wire, to radio, to the next logical method-radar.
In the radar command guidance method, 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. Sometimes a separate radio transmitter is used.
Figure 9-17 shows the basic arrangement of radar command guidance. As soon as radar #1 (target tracker) is locked on target, tracking information is fed to the computer. The missile is launched and tracked by radar #2 (missile tracker). Data from both target and missile radars, such as ranges, elevations, and bearings, are fed continuously into the computer. The computer analyzes the data and determines the correct flight path for the missile. The guidance signals or commands generated by the computer are routed to a command (radar or radio) transmitter and sent to the missile. The receiver of the missile accepts the instructions, converts them, and directs the control surfaces to make steering corrections.
Figure 9-17.-Basic radar command guidance.
HOMING GUIDANCE SYSTEMS
Homing guidance systems also rely on electromagnetic radiations for guidance information. The homing device is usually a small antenna located within the nose of the missile. It detects some type of distinguishing feature or radiation given off by or reflected from the target. This information is converted into usable data and positions the control surfaces. Three types of homing guidance systems are used by SMS missiles-active, semiactive, and passive.
Active Homing Guidance
In active homing guidance (view A of fig. 9-18), 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. These return "echoes" are picked up by the receiver antenna of the missile and fed to the guidance computer. The computer output generates steering corrections for the control system. Active homing guidance does not require a ships radar; the missile is entirely on its own after launch.
Semiactive Homing Guidance
In semiactive homing guidance (view B of fig. 9-18), the missile contains only a receiver (referred to as a seeker head or signal antenna). The ships fire control radar serves as the transmitting source and directs its radar energy to illuminate the target. As in active homing guidance, part of this energy is reflected or bounced from the target. The receiver of the missile picks up the reflected energy and uses it to generate its own steering commands.
Passive Homing Guidance
The passive homing guidance method (view C of fig. 9-18) 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. Steering corrections are made and the missile homes in on the target.
Passive homing guidance, like active homing, is completely independent of the launching ship. Passive homing normally is not used to guide the missile all the way (from launch to intercept). However, it is well adapted to serve as a secondary or backup guidance system. Should the enemy sense any radar illumination
Figure 9-18.-Homing guidance system: A. Active; B. Semiactive; and C. Passive.
(such as from active and semiactive homing methods), electronic jamming could be initiated. This jamming "mixes" up the guidance information to the missile. Sensing the jamming, circuits within the guidance system of the missile switch over to passive mode. The missile continues toward the target, homing on the jamming source. Other sources of energy used for passive homing can include light, sound, heat from a propulsion unit, and so forth.
COMPOSITE GUIDANCE SYSTEMS
There isn't any one type of guidance system (command or homing) best suited for all phases of guidance. Therefore, it is logical to design one guidance system that combines the advantages of the others. For example, a missile may ride a signal until it is within a certain range of the target. At this point, the signal is terminated and a type of homing guidance takes over until intercept.
Control of a particular guidance subsystem may come from more than one source. A signal is setup to designate when one phase of guidance is over and the next phase begins. This signal may come from a tape, an electronic timing device, or from a radio or radar command.
The device that switches guidance subsystems is often called a control matrix. It automatically transfers the correct signal to the guidance subsystem regardless of conditions. If the midcourse subsystem should fail, the matrix switches in an auxiliary subsystem. Should the original guidance subsystem become active again, the matrix switches back to the primary subsystem.