The strategic defence against a foreign military attack is of paramount importance to any country. During the 20th century the concept of strategic defence has been revolutionized by new weapons, especially by intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs) equipped with nuclear warheads. One of the most important inventions of mankind in recent history, however, was the development of aircraft. These weapons have made defensive strategies based on frontier and coastal defence obsolete and have brought about a strategy based on a deterrence rather than one based on the destruction of the attacking forces. Active defence against nuclear-armed ballistic missiles and aircraft has been discarded in favour of the threat of retaliation, that is, the ability of the offensive forces, or counterforce, of the attacked nation to survive
a first-strike attack and deliver a second-strike counter-attack. The ability to launch a second strike theoretically acts to deter an aggressor from launching an initial attack.

This strategy of deterrence, which has been the basis of Pakistani defence policy since the 1980s, depends on an effective air force, an effective nuclear delivery force and on the protection of that force from a first-strike attack. The nuclear delivery force currently consists of a triad of two weapons systems: the deep penetration strike aircraft such as the PAF F-16s, the short and
medium-range surface-to-surface missiles (SSMs) such as the Hatf I, II, III and Ghauri, launched from mobile units. The defence of this force is more complex and consists almost entirely of passive defence systems designed to provide early warning of a missile strike, as well as to protect missiles by concealing them and dispersing them on mobile launchers, or fortifying them in underground silos.

Counterforce:

The strategic forces of the Pakistan incorporate both active offensive elements and passive defensive measures. Land-based SSMs and manned bombers, part of the PAF Central Air Command, consisted of SSMs and bombers. The Pakistan Air Force strategic bomber force, made up mainly of F-16s and A-5s are also considered vulnerable to missile attack and depend on early warning systems to get away from unprotected airfields, which are all within 15 minutes air time for Indian
bombers. Preservation of the national and military decision-making authorities in the event of a nuclear attack rests on both the escape of special command aircraft and on a system of dispersed underground shelters.

Passive Defence Systems:

Protection of the Pakistani counterforce relies almost entirely on passive defence systems for surveillance, assessment, and warning of a missile attack. This effort begins with a vast intelligence-collecting and analyzing programme to determine the nature of the threat. Information is gathered from worldwide sources ranging from sophisticated satellites to the press releases of foreign governments. Information about the threat might include the size and capabilities of a potential attacking force or any passive measures that could nullify the effect of the Pakistani counterforce. On the basis of an analysis of the threat, Pakistani executive and military leaders all contribute to the establishment of an effective defence programme within existing constraints. The role of the Pakistan Institute for Air Defence Studies (PIADS) would be to further strengthen this system.

Operation of air defence is delegated to the Air Force's three commands which are charged with overlooking the air defence of the three districts, the Northern Command, the Central Command and the Southern Command and is supported by the Pakistan Air Force Central Air Command (air defence radar, control centres, interceptors, and missile-warning and space-surveillance sensors).

The PAF employs a variety of means to monitor the strategic strike forces of neighbouring countries, in particular India. The detection and tracking devices installed by the PAF include radar and sonar; laser beams; high-resolution optical devices using natural or artificial illumination; and magnetic, thermal, chemical, and acoustical sensors. This equipment may be located on ground, on sea, on aircraft, or on space satellites, and, when linked together by a central control for a specific set of
tasks, it constitutes a defensive system. The most important of these systems is a classified complex of warning and tracking radar based at various sites. This equipment can detect missiles and provide a 15-minute advance warning of an attack on Pakistan from anywhere.

Active Defence Systems:

Belts of radar stations still provide notice of a conventional air attack, while the newer defence systems co-ordinates the surveillance and tracking of all objects in Pakistani and surrounding airspace through various regional control centres. The aircraft tracking system is operated jointly by the Pakistan Air Force and the Civil Aviation Authority (CAA) for both air traffic control and air defence. One significant addition to air defence should be the Airborne Warning and Control System
(AWACS) aircraft, in effect a flying radar station.

Pakistan has no active antiballistic missile (ABM) system in operation. While, perhaps, Pakistani ABMs may increase the protection of radar and strategic sites, the parallel deployment of ABMs by India may reduce any defensive advantage, and in both countries their construction, maintenance, and continued improvement would prove extremely costly.

Some active defensive systems are those employed by the Pakistan Navy to detect and destroy ballistic-missile-launching submarines. Ship-mounted sonars on destroyers and frigates are now supplemented by towed, underwater sonar arrays that disregard distorting thermal currents and eliminate false reverberations and other clutter. Shipborne helicopters capable of "dunking" small sonar sets, expendable sonar buoys, and high-speed, sonar-equipped submarines also enhance
detecting capabilities. Sensitive shipborne and airborne radar can locate small objects such as periscopes on the surface of the sea, and aircraft equipped with magnetic anomaly detectors (MAD) are able to sense minute variations in the earth's magnetic field caused by large metal objects such as underwater vessels. Once detected, submarines can be attacked with a variety of weapons, from nuclear depth charges delivered by missiles to wire-guided torpedoes launched from ships or helicopters. Without adequate warning of an impending attack, however, these forces could do little to stop such submarines from launching a first-strike missile attack.

Guided Missiles:

Guided missiles are self-propelled aerial projectiles, guided in flight toward a target either by remote control or by internal mechanisms. Guided missiles vary widely in size and type, ranging from large strategic ballistic missiles with nuclear warheads to small, portable rockets carried by foot soldiers. Although most are military weapons with explosive warheads, others may carry scientific instruments for gathering information within or above the earth's atmosphere.

Guided missiles consist of three separate systems: power source, guidance and control mechanism, and warhead or payload. Power sources normally are either self-contained rocket motors or air-breathing jet engines , but may also be airfoils or outside booster charges from ramp or tube launchers. The type of guidance and control system employed depends on the type of missile and the nature of the target. Inertial guidance systems sense the position of the flight path in relation to a fixed
target; other guidance systems use a variety of more active sensors to help direct the missile toward a moving objective. Payloads are generally warheads designed for specific missions, from piercing armour plate to destroying entire urban areas.

Before World War II guided missiles were limited to experimental, pilotless aircraft controlled by radio. During the WWII, however, rapid technological advances in such fields as aerodynamics, electronics, jet and rocket propulsion, radar, servomechanisms, inertial guidance and control systems, and aircraft structures, coupled with the intensive search for better weapons, led to the construction, testing, and finally mass production of the modern guided missile.

Guided missiles today are grouped into four launch-to-target categories: surface-to-surface, surface-to air, air-to-surface, and air-to-air. "Surface" in each case signifies on as well as below the surface of the land or sea. Missiles may also be grouped by their area of operation: Tactical missiles are used by military forces in direct combat on and above the battlefield; support missiles are employed behind the main battle area; and strategic missiles are designed for intercontinental warfare. Missiles may also be separated by their flight characteristics; Aerodynamic missiles are supported in flight by air pressure around their wing and body surfaces similar to conventional piloted aircraft; ballistic missiles depend solely on their internal power source, usually a rocket engine, to remain airborne. Aerodynamic missiles normally travel on a straight-line or flat trajectory toward their target, and ballistic missiles are usually surface-to-surface weapons that follow curved or arched trajectories similar to that of an artillery projectile.

Surface-to-Surface Missiles:

The first successful guided missiles were the German V-1 and V-2 Vengeance weapons launched against Antwerp and London during World War II. The V-1, or buzz bomb, was an aerodynamic missile powered by a pulse jet engine with a pre-set guidance system that could sense the correct deviations in altitude and direction. Its average range was about 240 km (about 150 mi), after which the missile automatically sent itself into a steep dive and detonated its 1-metric-ton high-explosive
warhead upon impact. The V-2, on the other hand, was a true ballistic missile fuelled by a mixture of alcohol and liquid oxygen that produced a 25,000-kg (55,000-lb) thrust for about one minute after takeoff. With a maximum range of about 320 km (about 200 mi), the V-2 carried its 730-kg (1600-lb) warhead through an arching trajectory, reaching a maximum altitude of 95 to 110 km (about 60 to 70 mi) and a speed of more than 1.6 km (more than 1 mi) per second. Both missiles
were relatively inaccurate and were used only against large cities. The V-1 could be destroyed easily in flight by conventional fighters, but no defence existed against the V-2. Allied attempts to destroy their mobile launching sites proved ineffective. Germany managed to launch about 4000 V-2s before the end of the war. Wire-guided antiaircraft and antitank missiles and air-to-air guided missiles were tested by German scientists but were never produced.

Appreciating the great potential of the German efforts in guided and ballistic missiles, Allied intelligence teams scoured Germany in 1945 for technical data, design drawings, and missiles, and they interrogated key German scientists and engineers. During the war the Allied powers had made little progress in these fields; they were quick, however, to integrate German research and
technicians into their own development programs, and most post-war research was based on the work done in wartime Germany. The German V-2, in fact, served as the prototype for all large space and missile rockets built in the United States and the Soviet Union.

Strategic Missiles:

After World War II, the defensive policy of strategic deterrence depended on a large fleet of long-range bombers that could deliver accurate nuclear strategic attacks. Defence planners also experimented with air-breathing subsonic missiles similar to the V-1. Three developments in the mid-1950s, however, led to the intercontinental ballistic missile (ICBM): (1) development of thethermonuclear bomb with a much greater destructive power than the original atomic bomb; (2) the rapid refinement of inertial guidance systems for ballistic missiles; and (3) the development of powerful booster engines for multistage rockets, greatly increasing their size and range. As a result, ballistic missiles became sufficiently accurate and powerful to destroy targets 8000 km (5000 mi) away. Atlas, the U.S.'s first successful ICBM, was tested in 1959 and was followed one year later
by the Titan. Both were multistage liquid-fuelled rockets using extremely low temperature propellants that had to be added just before launching. The U.S. Minuteman ICBM that went into service in 1961 used solid fuels stored within the missile, could be launched on short notice, and was sheltered in underground concrete silos. The U.S. Missile X, or MX, which was under consideration in the early 1980s, was designed to carry a larger warhead and to have a mobile launching site capability.

Strategic delivery systems went through the same evolution in the U.S. Navy, as slow air-breathing missiles, liquid-fuelled rockets, and carrier-based piloted bombers were abandoned for the two-stage, solid-propellant Polaris ICBM. Carried in two parallel rows of eight aboard large nuclear-powered submarines, these missiles are launched from underwater. The U.S. Navy is replacing Polaris with the longer range Poseidon ICBM and developing an entirely new ICBM compatible with the large Trident submarines under construction. The newer ICBMs of both the U.S. Air Force and Navy carry several nuclear warheads and have the capability to attack a series of different targets.

Recently the U.S. has also resurrected the air-breathing cruise missile for both strategic and tactical missions. The experimental Tomahawk and the Boeing AGM-86 can be launched from ground, ship, aircraft, or submarine against short-range tactical targets such as ships, or against strategic targets several thousand kilometres away. The anti-ship version travels a few meters above sea level toward the general area of a target. It then climbs, locates the target through its own sensing devices, and
dodges before making a final, high-speed diving attack. The land version of this missile also travels at a low altitude to avoid radar detection, guided by an internal terrain contour navigation system. Some strategists argue that the air-to-ground cruise missile would greatly increase the effectiveness of the U.S. strategic piloted bomber force.

Tactical Missiles:

Tactical surface-to-surface guided missiles range from hand-portable antitank rockets to large ballistic missiles able to attack airfields, supply lines, and communications hundreds of kilometres behind the battlefield. Small missiles often employ line-of-sight guidance systems that relay corrections in the flight path of the missile by means of trailing wires or infrared signals. In newertypes, like the U.S. Copperhead guided projectile, the target is marked by a laser beam on the battlefield. The missile is then launched several kilometres to the rear and searches for the marked target while in flight. On a larger scale are battlefield support missiles that vary between ballistic weapons such as the U.S. Pershing to tactical cruise missiles such as the U.S. Lance. All are mobile, carry nuclear or high-explosive warheads, and have a range of 160 to 640 km (about 100 to 400
mi). Since 1970, similar types of midrange cruise missiles such as the U.S. Harpoon have replaced artillery as the major armament aboard warships, although the continued development of rocket-assisted and guided projectiles may reverse this trend in the future.

Today, the SSM inventory of Pakistan constitutes the 100 km range Hatf-1 BRBM, the 300 km Hatf-2 SRBM, the 600 km Hatf-3 SRBM and the 1,500 km Ghauri MRBM. Future missile projects include the 700 km Shaheen-1 MRBM and the 2,500 km Ghaznavi IRBM.

Air Launched Missiles:

Air-to-air and air-to-surface guided missiles are generally short-range, light, rocket-powered projectiles with sophisticated internal guidance systems. Both types were tested in wartime Germany, and German radio-controlled glide bombs were responsible for sinking numerous Allied ships. Although these early missile weapons relied on optical tracking and control from parent aircraft, most current air-to-ground guided missiles depend on their own target-sensing mechanism once launched.
The U.S. Walleye and Maverick use a miniature television homing system, the Shrike follows radar emissions from hostile positions, and others, such as the unpowered "smart bombs" employed in Southeast Asia, use laser or infrared target designators to help guide the missile to its objective. The Soviet Union developed several versions of large, air-breathing missiles similar to U.S. cruise missiles, designed to be used against shipping or as a nuclear delivery system for strategic bombers.Air-to-air guided missiles are used to destroy hostile aircraft and are generally smaller, lighter, and faster than air-to-ground projectiles. Most employ infrared or radar homing devices, and they have replaced automatic gunfire as the main armament of fighter aircraft. Like almost all tactical missiles, however, their success depends on the skill of the operator, in this case the aircraft pilot, to identify hostile targets visually or by radar and to manoeuvre the launching platform in order to place themissile in the vicinity of the moving target.

Surface-to-Air Missiles:

This type of missile was developed to protect ground areas from hostile air attack, especially from high-altitude bombers flying above the range of conventional antiaircraft artillery. During the 1950s and '60s, batteries of Nike-Ajax and the improved Nike-Hercules provided strategic air defence for the U.S. against long-range Soviet bombers. Subsequently, with the replacement of piloted bombers by the ICBM as the Soviet Union's primary nuclear delivery system, and with the signing of an
agreement between the U.S. and the USSR to limit the deployment of antiballistic missiles, most research and development went into shorter-range, tactical surface-to-air missiles to protect ground combat units and warships against low-flying aircraft and hostile missiles. Most air defence missiles employ separate radar to locate, track, and guide the missile toward hostile aircraft; final interception is accomplished by the internal guidance system of the missile itself. Some of these missiles are
air-to-air weapons adapted for ground units; others, such as the U.S. Navy Standard, can also be used against ground targets. Since 1970, almost all the major industrial nations have developed many different types of these weapons to cover overlapping areas above the battlefield. A new addition has been the hand-portable antiaircraft missile, sighted optically and using an internal infrared homing device. More important has been the development of integrated fire-control systems for ground units, enabling them to separate friendly from hostile aircraft and to engage them with the most appropriate air defence systems available.

Warheads:

Military guided missiles carry either high-explosive or nuclear warheads. Short-range tactical missiles employ high-explosive charges that produce damage through their force of impact and blast or through fragmentation. Antitank missiles, for example, normally depend on a concentrated blast effect to penetrate or splinter armour; warheads used against less protected targets such as aircraft rely more on fragmentation to produce the greatest damage. Nuclear warheads are weapons of mass
destruction carried primarily by ICBMs. To enhance the effectiveness of these long-range delivery systems, several new types of warheads have been developed: The multiple independently targeted re-entry vehicle (MIRV) dispatches several nuclear warheads from a single missile while in flight; the U.S. experimental rocket-powered re-entry vehicle enables an individual warhead to change course as it falls. The Soviet-built fractional orbit bombardment system (FOBS) allows missiles or warheads to remain in earth orbit before beginning their descent. FOBS gave the USSR the ability to launch a
mass attack against the United States from any direction rather than just depending on a ballistic pathway arching over the North Pole. Nuclear warheads were also used in the now defunct U.S. antiballistic missile system and in several U.S. air defence missiles with the intention of breaking up mass bomber formations. Most large tactical missiles also carry nuclear warheads to destroy military concentrations on or behind the battlefield. In the 1980s, Soviet airborne or shipboard
nuclear-tipped cruise missiles posed a threat to U.S. Navy carrier task forces.

Guidance and Control:

Missiles are guided toward targets by remote control or by internal guidance mechanisms. Remote control missiles are linked to a human or mechanical target locator through trailing wires, wireless radio, or some other type of signal system; internal guidance mechanisms have optical, radar, infrared, or some other type of sensor that can detect heat, light, or electronic emissions from the target. Most missiles have some type of movable fins or airfoil that can be used to direct the course
of the missile toward the target while in flight. The inertial guidance systems of ballistic missiles are more complex. Missile velocity, pitch, yaw, and roll are sensed by internal gyroscopes and accelerometers, and course corrections are made mechanically by slightly altering the thrust of the rocket exhaust by means of movable vanes or deflectors. In larger rockets, small external jets are also used to alter direction.

Military Aircraft:

Since the beginning of the 20th century, the military aircraft has evolved from a frail contraption of wood, wire, and fabric into a sophisticated weapons system of enormous complexity that revolutionized the conduct of warfare. Air power has provided military commanders with new means of gathering intelligence, dominating a battlefield, striking the enemy over great distances, and forging global lines of supply and communication. Aviation redefined old notions of war, rendering civilians on the home-front as vulnerable to attack as soldiers on the battlefield.