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Missile was used since along long time ago. It is very fearsome weapon because missiles are very rarely missed their target unless it is miss programmed. Missile can be categorized in many different ways. Each missile is using different type of sensor or seeker depending on the assign mission. The missile type of guidance and type of sensor are two concepts which often used interchangeable and it is important to understand their differences.
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. 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.
Figure 2.0: Phases of Missile Guidance
Phases of missile guidance
In many missiles, the guidance system is divided into three phases, which is boost, mid course and terminal. Boost phase is which the guidance system is disabled to allow the missile to safely travel away from the launch platform. Most of the time of the missile is flown using midcourse guidance, during which the missile makes slightly adjustments to its trajectory allowing it to reach the designated target. The final phase is terminal guidance where the missile uses a highly accurate tracking system to make rapid maneuvers for intercepting the target. Many missiles use a different type of guidance in the midcourse phase and in the terminal phase.
The Primary forms of Missile Guidance.
Beam Rider Guidance
The beam rider concept relies on an external ground or ship based radar station that transmits a beam of radar energy towards the target. The surface radar tracks the target and also transmits a guidance beam that adjusts its angle as the target moves across the sky.
Figure 3.0: Beam Rider Guidance
Beam rider guidance
The missile is launched into this guidance beam and uses it for direction. Scanning systems onboard the missile detects the presence of the beam and determine how close the missile is to the edges of the beam. This information is used to send command signals to control surfaces to keep the missile within the beam. The disadvantage of this guidance system is that it is inaccurate at long ranges.
Command guidance is similar to beam riding where the target is tracked by external radar. However, it has two radars, radar that detects missile and radar that detects target. The tracking data from both radars are fed into a ground based computer that calculates the paths of the missile and the target.
Figure 4.0: Command Guidance
This computer also determines what commands need to be sent to the missile control surfaces to direct the missile on an intercept course with the target. These commands are transmitted to a receiver on the missile allowing the missile to adjust its course.
This type of guidance system is commonly used now a day. Homing guidance can be divided into three types, semi active, active and passive.
Figure 5.0: Homing guidance
Semi-Active Homing Guidance
A semi-active system is similar to command guidance since the missile relies on an external source to illuminate the target. The energy reflected by the target is intercepted by a receiver on the missile. The difference between command guidance and semi-active homing is that the missile has an onboard computer. The computer uses the energy collected by the missile receiver to determine the target relative trajectory and send correcting commands to control surfaces so that the missile will intercepts the target.
Figure 6.0: Semi-active Homing Guidance
Semi-active homing guidance
The example shown above illustrates the guidance method used on an air-to-air missile like Sparrow. This missile relies on radar energy transmitted by the launch aircraft to track on the target. This system is also sometimes referred to as bistatic which is means that the radar waves that intercept the target and the reflected signals to the missile are at different angles.
However, it should be noted that semi-active guidance is used by other types of seekers besides radar. Laser-guided can also be considered as semi-active weapons because the laser energy is supplied by an external source. The source could be a laser designation pod on the launch aircraft, on a second aircraft, or aimed by a soldier on the ground.
Active Homing Guidance
Active homing works just like semi-active except that the tracking energy is now both transmitted by and received by the missile itself without using an external source. This is the reason that active homing missiles are often called "fire-and-forget" because the launch aircraft does not need to continue tracking the target after the missile is launched.
Figure 7.0: Active Homing Guidance
Active homing guidance
Active homing missiles use radar seekers to track their target. These seekers are also sometimes called mono static because the transmitted and reflected waves are at the same angle with respect to the line of sight between the missile and target. Examples of active homing missiles include the AMRAAM air-to-air and Exocet anti-ship missiles.
Passive Homing Guidance
A passive homing system is like active in that the missile is independent of any external guidance system and like semi-active in that it only receives signal and cannot transmit. Passive missiles are relying on some form of energy that is transmitted by the target and can be tracked by the missile seeker.
Figure 8.0: Passive homing guidance
Passive homing guidance
This energy could take many forms. For example, anti-radiation missiles like HARM track the radio frequency energy transmitted by ground-based radar stations.
Retransmission Homing Guidance
A more unusual example of homing guidance is the retransmission method. This technique is largely similar to command guidance but with a unique twist. The target is tracked via an external radar, but the reflected signal is intercepted by a receiver onboard the missile, as in semi-active homing. However, the missile has no onboard computer to process these signals. The signals received are then transmitted back to the launch platform for processing. The next commands are then retransmitted back to the missile so that it can deflect control surfaces to adjust its trajectory.
Figure 9.0: Retransmission Homing Guidance
Retransmission homing guidance
This method is also sometimes called "track via missile" (TVM) since the missile acts as a tracker and the information is transmitted back to the ground control station. The advantage of TVM homing is that most of the expensive tracking and processing hardware is located on the ground where it can be reused rather than be destroyed. Unfortunately, this method required excellent high-speed communication links between the missile and control station. Retransmission homing guidance is used on the Patriot surface-to-air missile.
Navigation guidance can be divided into several categories.
Inertial Navigation Guidance
Inertial navigation relies on devices onboard the missile that senses its motion and acceleration in different directions. These devices are called gyroscopes and accelerometers.
Figure 10: Mechanical, fiber optic, and ring laser gyroscopes
Mechanical, fiber optic, and ring laser gyroscopes
The purpose of a gyroscope is to measure angular rotation of the missile. A classic mechanical gyroscope senses the stability of a mass rotating on gimbals. More recent ring laser gyros and fiber optic gyros are based on the interference between laser beams. Current advances in Micro-Electro-Mechanical Systems (MEMS) offer the potential to develop gyroscopes that are very small and inexpensive.
The use of accelerometer is to measures linear motion. This device is translated into electrical signals for processing by the missile computer autopilots. When a gyroscope and an accelerometer are combined into a single device along with a control mechanism, it is called an inertial measurement unit (IMU) or inertial navigation system (INS).
Figure 11: Inertial Navigation Concept
Inertial navigation concept
The INS uses these two devices to sense motion relative to a point of origin. Inertial navigation works by informing the missile where it is at the time of launch and how it should move in terms of both distance and rotation. The missile computer uses signals from the INS to measure these motions and ensure that the missile travels along its proper programmed path. Inertial navigation systems are widely used on all kinds of aerospace vehicles, including weapons, military aircraft, commercial airliners, and spacecraft. Many missiles use inertial methods for midcourse guidance, including AMRAAM, Storm Shadow, Meteor, and Tomahawk.
Ranging Navigation Guidance
Ranging navigation depends on external signals for guidance. The earliest form of such navigation was the use of radio beacons developed primarily for commercial air service. These beacons transmit radio signals received by an aircraft in flight. Based on the direction and strength of the signals, the plane can calculate its location relative to the beacons and navigate its way through the signals.
Figure 12: Global Positioning System Used In Ranging Navigation Guidance
Global Positioning System used in ranging navigation guidance
The development of the Global Positions System (GPS) has largely replaced radio beacons in both military and civilian use. GPS consists of a constellation of 24 satellites in geosynchronous orbit around the Earth. If a GPS receiver on the surface of the Earth can receive signals from at least four of these satellites, it can calculate an exact three-dimensional position with great accuracy. Missiles like JSOW and the JDAM series of guided bombs make use of GPS signals to determine where they are with respect to the locations of their targets. Over the course of its flight, the weapon uses this information to send commands to control surfaces and adjust its trajectory.
Celestial Navigation Guidance
Celestial navigation is one of the earliest forms of navigation devised by humans. It is uses the positions of the stars to determine location especially latitude on the surface of the Earth. This form of navigation requires good visibility of the stars, so it is only useful at night or at very high altitude. As a result, celestial navigation is not very applicable to missiles, though it has been used on many ballistic missiles like Poseidon. The missile compares the positions of the stars to an image stored in the memory to determine its flight path.
Geophysical Navigation Guidance
Even older than celestial navigation is geophysical navigation, which relies on measurements of the earth for navigation information. Methods that fall under this category include the use of compasses and magnetometers to measure the earth's magnetic field as well as graviton meters to measure the earth's gravitational field.
These methods have not found much application in missiles, a more useful technique is terrain matching. This method typically requires a radar altimeter that uses radar waves to determine height above the ground. By comparing the contours of the terrain against data stored aboard the missile, the autopilot can navigate its way to a particular location.
A related but more accurate technique is called digital scene matching. The missiles are comparing the image seen below the weapon to satellite or aerial photos stored in the missile computer. If the scenes do not match, the computer sends command to control surfaces to adjust the missile courses until both images are identical. Digital scene matching is used on the Tomahawk cruise missile.