Missile Technology 5 - Guidance

2/15/2017

Guidance is that aspect of a missile system which helps it to decide the direction in which the missile should move. Generally this decision has to be taken at very short intervals of time (1/50th of a second) during the flight of the missile.
For a specific mission, particular guidance technique is used. The different types of guidance are

Command guidance
Homing or seeker guidance
Beam rider guidance
Inertial guidance
Stellar guidance.

Some missiles need more than one system of guidance. The requirement depends on the phase of guidance. The various guidance phases are the launch phase, the mid-course phase and the terminal phase.

The aerodynamic requirements of the missile are different during the boosted launch phase. The total weight of the missile also varies as the large amount of propellant gets rapidly consumed during the launch phase. Due to this weight reduction and a consequent shift in the centre of gravity, the load, and the various parametric requirements of the missile are altered. During the mid-course phase, a guidance system is required that can hold the missile on course and at required altitude for a long time.

During the terminal phase, another guidance system is needed that can bring the missile accurately to the target and make up for possible inaccuracies or deviations which might have crept in during the flight.

Homing guidance would be a good choice for the terminal phase. The mid-course may have beam rider guidance and during the launch phase, some form of command guidance may be used.
Thus, it is seen that sensing the development status and system integration feasibility, a composite guidance system may be employed.

Command Guidance

In this method, the guidance signal is transmitted from launch site to the missile, giving the missile its deviation from 'the path line -pointing from launcher to the target, also called the line of sight (LOS). The missile has logic on board to actuate its control mechanism to turn it towards the LOS. The signal from the ground is transmitted by different means.

One method is a wire link between the launcher and the missile and this has been widely applied in surface-to-surface anti-tank missiles up to 4 Km range. In this a thin wire is wound on a spool on the missile and is unreeled as the missile travels. Another method is by radio link which is used by relatively faster moving anti-aircraft missiles. The third method is by fibre optic link. Wire and fibre optic link are used where the velocity of missile is below the speed of sound (Mach l), say about 300 m/sec. The advantage of fibre optic system: that it is also used sometimes to aim at targets beyond visible line of sight. A TV camera in the nose of the missile transmits the picture through optical fibre link back to the launch site, based on which suitable commands are passed through the same link.

The deviation of the missile from target to launch line of sight is computed on the ground at short intervals (30-50 ms) and then updated commands are transmitted to the missile. To compute errors, instantaneous positions of missile and target are found out. This is done by means of radar, TV or infrared sensors located on the launcher. Most of the anti-tank missiles and some of surface-to-air missiles use command guidance.

An important advantage of command guidance systems is that very little guidance equipment need be carried in the missile itself. Because target tracking and flight path computation are carried out by tracking radars and the associated computers on the ground, the missile need carry only its control system and a receiver to accept the signals. Reduction in the amount of guidance equipment carried in the missile means more room for a larger warhead. Alternatively, smaller body can be used thereby reducing the overall cost.

The disadvantage of command guidance is that it cannot be used against a situation of multiple targets. The system can guide only a limited number of missiles at one time.

Homing Guidance

Homing guidance is generally used for short-range missiles. In this system the missile receives the signals reflecting / emanating from the target and generates the command to direct its motion along the instantaneous Line of Site formed between the missile and the target. Active, semi-active and passive homings are the main types of homing guidance systems.

In the active homing guidance system, the missile itself carries the transmitter and the receiver. The signal, generally electromagnetic radiation, is transmitted at the target and the reflected signal is received. In this system, the missile is not dependent on the ground launcher. Active homing can be used for guidance in all phases, from launch up to target interception. It can also be used in terminal guidance in conjunction with other modes of guidance for the initial phases.

Where homing guidance is used alone, the range is limited because the system is bulky and needs a lot of force. It has instruments called homing head, also called seeker head, which are locked on to the target in tracking mode before launch. Such a system is also called the 'fire and forget' type of guidance. When used in terminal guidance, the homing head is provided with search capability to locate the target and then lock on to it till interception.

In semi-active guidance, the source for target illumination is located in the launcher and the missile has only the receiver. The rest of the process is identical to active type.

In passive homing type, the missile has only a receiver and detects signals emanating (not reflected) from the target. The signals could be electromagnetic or infrared or both. 'The missile has in its homing head detectors sensitive to infrared or electromagnetic radiation. The missile where infrared homing used is also called heat-seeking missiles.

Beam Rider Guidance

In this method, the guidance system is to illuminate the target by radiation of a beam of energy from a radar antenna pointed at the target. The missile is fired into this beam and thereafter gets guided over the beam till it hits or misses the target. The sensitivity is lesser at the commencement of the flight and towards the end as the missile approaches the target.

In a beam rider guidance system, equipment in the missile measures the displacement of the missile from the centre of the radar beam then appropriate action by the control system steers the missile back into the centre of the beam. If the missile is flying in the centre of the beam, no signals are sent to the control system, indicating that no corrective action is necessary.

The guidance beam that guides the missiles is formed by the radar antenna, which sends out electromagnetic energy in the form of lobes. The antenna is rotated in such a manner that the tips of the lobes describe a circle, resulting in a cone of radiation in space with its origin at the radar antenna. The missile is guided along the axis of this cone.

A few launching considerations are to be taken care of in this system. The missile must be launched in such a manner that it flies as nearly parallel to the beam axis as possible when it first enters the cone of radiation. Otherwise, it might fly right through the beam without being captured by its guidance signals. At this time the missile might not be up to full operational velocity, and its aerodynamic control system would not be as effective in controlling the missile as it would at the operating speed for which it is designed. Launching the missile as closely as possible to the beam axis eliminates sharp turns and sudden manoeuvres.

This type of guidance system is relatively simple, less complex with increased reliability and lower cost. The limitation is that the trajectory requires high lateral acceleration (latax) during the terminal phase.

Inertial Navigation System

Inertial navigation is another type of guidance which is used for short as well as long ranges. It is a method of dead reckoning. The measuring instruments in this system are accelerometers which measure translational acceleration .Since acceleration the rate of change is of velocity, it is possible by performing integration to obtain the velocity from the acceleration. A second integration gives the distance travelled. These mathematical integrations are performed by electronic circuits in the inertial guidance, system. The accelerometer thus makes it possible to keep track of distance travelled from the launching pad and simultaneously the distance from the target.

There are two basic types of inertial system: stable platform inertial measuring system and strap-down inertial system. In the first, the accelerometers are mounted on a stabilized platform which maintains its reference axes in flight with the help of gyroscopes. Whereas in the second type, the accelerometers are mounted fixed to the body axes and though measurements are made for acceleration in instantaneous body axes reference they are transformed to the reference axes system using gyroscopes rate data and computational equipment onboard. However for accuracy in long range systems, stable platform system is more suitable.

Gyroscope is a mechanical instrument which uses a rapidly rotating mass to maintain a stable axis. The gyroscopes are mounted on the same platform as accelerometer platform to prevent movement of the accelerometers from the established reference axis. For simplicity it would suffice to state that the gyroscopic properties of rigidity and precision are used in inertial guidance systems to provide space-stabilized platform for the accelerometers. Which must measure missile acceleration along a predetermined axis only.

Accelerometers are suitably mounted for measuring the distance travelled along the longitudinal axis and transverse deviations from the preset course. In ballistic missiles the values of instantaneous height, velocity and inclination angle are used to calculate the moment of engine's cut off in final phase of propulsion so as to achieve the preset target position. Lateral deviations from the flight due to winds, misalignment errors or changes in rocket motor thrust are sensed by accelerometers; any corrections required are attained by generating guidance commands and implemented through control system.

Thus, unlike other guidance systems, the inertial guidance system does not rely on any outside reference like heat, light or electromagnetic radiation which are susceptible to conditions like weather, atmospheric disturbances, range of the missile from the launching point, low cloud formations, radio-horizon limitations or target position. Therefore this system is self-contained and needs no ground equipment for guidance and cannot be easily detected. Jamming and countermeasures against this are a very far feasibility.

However, the system calls for a high accuracy of the individual components. Certain errors during actual run of gyros which build up with flight time, thereby leading to measurement of erroneous accelerations, are possible. Thus, inertial guidance system is essentially a dead reckoning system that measures the distance between two points over a period of time. It cannot be used by itself alone against moving targets but only by missiles used to attack a large and fixed target such as a city. All ballistic missiles employ inertial guidance.

Stellar Guidance

These systems are used by some strategic missiles and use star constellations as points of reference for guidance. Stellar guidance is combined with an inertial guidance system on the TRIDENT D-5 (also called TRIDEN'I' II) built by Lockheed Missiles and Space Co of USA.

A few other guidance systems are hypergolic guidance, acoustic guidance and optical guidance. Some of these are also used in satellite applications which are a direct outgrowth of missile technology.