in military science, any method used to detect the situation or intention of an enemy so that warning can be given.

Because military tactics from time immemorial have stressed the value of surprise—through timing, location of attack, route, and weight and character of arms—defenders have sought to construct warning systems to cope with all these tactics. Many types of warning systems exist. Long-term, or political, warning systems employ diplomatic, political, technological, and economic indicators to forecast hostilities. The defender may react by strengthening defenses, by negotiating treaties or concessions, or by taking other action. Political warning, equivocal and incapable of disclosing fully an attacker's intention, often results in an unevaluated and neglected situation.

Medium-term, or strategic, warning, usually involving a time span of a few days or weeks, is a notification or judgment that hostilities may be imminent. Short-term, or tactical, warning, often hours or minutes in advance, is a notification that the enemy has initiated hostilities.

Warning and detecting are separate functions. The sensors or detection devices perceive either the attack, the possibilities of an attack, the nearness of the enemy, his location, his size, his activities, his weapon capability, or some changes in his political, economic, technical, or military posture.

Warning systems include detection devices but also imply the judgments, decisions, and actions that follow receipt of the sensor's information. Warning encompasses communications, analysis of information, decisions, and appropriate actions. Visual observation still remains important, supplemented by telescopes, cameras, heat-sensing devices, low-light-level devices, radar, acoustic, seismic, chemical, and nuclear detection devices. The product, or output, of these sensors is complicated and voluminous and requires computers to condense and summarize the data for the decision maker. Often, the most expensive portion and weakest link of the warning system is not the sensor but the communication and evaluation systems. Technology of all types is required in modern warning systems.

History abounds with examples of successful military surprises; examples of effective warning are difficult to find. Military training emphasized the value of surprise, stratagem, and deception, but the value of warning was long neglected. Flank and rear guards, to protect marching columns, patrols and scouts to locate the enemy, and sentries to guard camps, were of course used in war from earliest times. Animals were sometimes employed to detect the approach of an enemy; dogs and horses were especially favoured, though, according to the ancient historian Livy, the Romans used geese to detect the night attack of the Gauls on Rome in the 4th century BC. High ground, favourable for observation, was often supplemented by watchtowers, such as those placed along the Great Wall of China and on Hadrian's Wall in Britain.

The observation balloon was an important technological advance. First used in warfare by the French in the late 18th century, primarily for offensive reconnaissance on the battlefield, its defensive possibilities were demonstrated in the American Civil War; in May 1863 a balloon of the army of the Potomac detected Lee's army moving from its camp across the Rappahannock to commence the Gettysburg campaign. Aerial photography had already been pioneered by the French and used in the War of Italian Independence (1859).

A balloon observer in the Spanish-American War of 1898 is credited with discovering an alternate route up San Juan Hill during the battle there. A few other successes are ascribed to such observation before the balloon was supplemented by the far more valuable airplane in World War I. Nevertheless, the balloon never fulfilled its potential as a warning device.

In sea warfare, warning and detection were equally neglected. As far back as the Minoan civilization of Crete, patrol ships were used, but mainly for offensive purposes. In later centuries, raised quarterdecks and lookout posts atop sailing masts were provided, but the beginnings of serious maritime detection technology did not come until the advent of the submarine.

Binoculars, telescopes, the telegraph, and telephone were well established military equipment by 1914; the airplane, first used by the Italians in the Italo-Turkish War of 1911, showed its potential as an observation device at the Battle of the Marne. Radio communications provided the means to make air observations immediately available. Aerial combat became inevitable as each side tried to deny the other its aerial reconnaissance.

Searchlights (searchlight), first used in the Russo-Japanese War (1904), saw large-scale use in World War I to detect dirigibles and aircraft on night bombardment missions. Flares (flare) were used to illuminate the battlefield between trenches to detect raiding parties. Listening devices, using directional horns to detect and locate enemy aircraft, were also used with limited success.

Despite the novelties of World War I, World War II produced far more technological innovation. Radar made obsolete the slow and inaccurate older listening devices. Radio communications made great strides, particularly in the very high frequency range. The combination of radar and interference-free very high frequency communications was pivotal in permitting the RAF to resist Hitler's aerial attack and win the Battle of Britain.

Notwithstanding radar sophistication, ground spotters played an important role in filling the gaps between radar coverage. Their messages, forwarded to a plotting centre, were assembled to trace the progress of intruders (tracking).

The advent of nuclear weapons (nuclear weapon) (1945), especially when coupled later with the speed and range of intercontinental missiles, gave new dimensions to the value of surprise for the attacker. Long-term warning was suddenly of paramount importance. Not only did all forms of unequivocal warning become indispensable but the warning had to be made credible to an aggressor; that is, an assurance had to be given that the retaliatory weapons would not all be destroyed by a first strike. Bomber aircraft were kept in the air to avoid destruction on the ground and attempts were made to provide a degree of protection for the civilian population through shelters.

Practically all aspects of science and technology have been introduced into today's warfare and warning systems: airplanes, helicopters, submarines, earth satellites, television, lasers, and magnetic, acoustic, seismic, infrared, nuclear, and chemical detectors.

Binoculars and telescopes have changed very little. Where vibration and motion create interference, gyroscopically stabilized optics are used in surface vehicles, ships, and aircraft.

Newer in character are the image intensifiers used for nighttime detection. These devices receive the moonlight or starlight reflected from targets on a sensitive screen, amplify the image electronically, and present it at much higher light level on a small cathode-ray tube similar to that used in a television receiver. Typical of these devices is the starlight scope, resembling an oversized telescopic sight, with which riflemen can aim at night at 1,000–1,300 feet range. Artillery, tanks, helicopters, and aircraft use similar, larger devices having longer range. In aircraft the direct-viewing device is replaced by a cathode-ray tube in the instrument panel; this version is called low-light-level television.

Ordinary searchlights can often be used at night even in combat situations; but, to avoid drawing fire, invisible light, in the ultraviolet or near infrared range, can be used with appropriate viewing devices. Conventional photography, used in aerial reconnaissance and essential to long-term warning, must have high resolution despite temperature and vibration interference. To cover wide areas, panoramic cameras, scanning from side to side, record high-quality images. Frame cameras are also used, especially for mapping. At night, flares or flashing lights on aircraft are used.

Infrared sensors on the ground, or in aircraft or spacecraft, can detect such hot spots as motor-vehicle engines, hot jet engines, missile exhausts, even campfires. They have good location accuracy and high sensitivity to signals, without registering such false targets as sun reflections.

In the very near infrared region, infrared imaging detectors use specially sensitized photographic film to reveal forms hidden by camouflage. More important are the detectors used in the far infrared region; objects at room temperature radiate sufficient energy for detection at ranges of several miles. Infrared imagery can have longer range than image intensifiers and can operate without starlight. When the humidity is high, the effective range is reduced.

The sniperscope, an early device that used infrared illumination and an infrared viewer, has been largely replaced by the image intensifier and by laser illuminators.

Radar is used by ground forces for many purposes: in portable sizes, for infiltration detection; in intermediate sizes, for mortar and artillery shell tracking; and in large sizes, for early warning, search, and control of air-defense weapons (interceptors and surface-to-air missiles).

Radar is used in fighter aircraft for finding enemy aircraft and controlling air-to-air missiles, rockets, and guns. It is used in bombers to find surface targets, fixed or moving, and to navigate and avoid obstacles. It is used in large aircraft as an airborne warning and control system, searching the skies over great distances for enemy aircraft, tracking them, and controlling interceptors. It also is used to search the seas for surface vessels or surfaced submarines. Radar also can be used in spacecraft to locate patterns of activity.

In all applications of radar, clutter in the form of reflections from surface objects or the terrain, or the disturbed sea, competes with reflection from the targets and must be cancelled by appropriate circuitry. Side-looking radars are used to obtain higher resolution than conventional radar, improving the ability to recognize surface targets.

Conventional radar operates at microwave and ultrahigh frequencies that propagate in straight lines like light rays; consequently, they cannot ordinarily detect objects beyond the Earth's horizon. Because high-frequency waves reflect from the ionosphere, over-the-horizon radars have been built to operate in this region.

Radio receivers can be used to detect and locate enemy radio. Enemy radars can be located in much the same way. Messages can be intercepted. This form of warning has been combated by radio silence and by spoofing, the transmission of signals intended to deceive. In 1967 the Israelis transmitted voluminous radio messages from empty airfields to hide the fact that aircraft had been moved to other locations.

Radio direction finders (radio direction finder) can be used to locate nuclear bursts, because the explosion generates a large amount of energy in the radio frequency region.

While electromagnetic waves do not propagate well under water, acoustic waves do and can be used to detect submerged submarines. These detection systems, called sonar, may intercept propeller or other noise generated by the submarine or may send out sounds and receive echoes from the submarine hull. Sonar devices can be operated aboard surface ships, aboard submarines, on floating sonobuoys, or suspended by cables from helicopters and dunked in the ocean.

Sonar systems are limited in range by attenuation (weakening) of the sound energy in water, bending caused by temperature differences in water layers, and extraneous noises, including reflections from the sea bottom.

Acoustic receivers are also used on land in sensors deployed near trails to detect the presence of personnel or vehicles along roads. The sounds are sent by radio to listening posts. Acoustic sensors are also used in monitoring nuclear explosions.

Seismic detectors—as well as underground acoustic detectors called geophones (Geophone)—are used in sensors for infiltration and vehicle detection. Both types must be used, since either alone can yield false signals caused by the movement of animals.

Sensitive magnetic detectors (magnetometers (magnetometer)), flown in aircraft over the sea, are used to detect submarines because the large metallic mass of the submarine hull disturbs the earth's magnetic field. Buried on land, magnetometers are used to detect the passage of vehicles.

Underground nuclear explosions are detected by sensitive seismometers. To increase the sensitivity and reject natural earth tremors, seismometers are often used in large arrays extending for hundreds of miles.

For atmospheric or space explosions, radio-pulse receivers and light flash and acoustic detectors are used, as well as devices to measure fallout. Aircraft and rockets can be used to collect radioactive debris, while high-altitude satellites carry detectors for gamma rays and other emissions.

Concealed chemical sensors, sensitive to minute amounts of body products, are capable of detecting personnel from short distances.

Certain trends can be seen in sensor development for future warning systems. Infrared detectors of higher sensitivity and resolution are being developed. Higher-powered and smaller lasers (laser) will aid night warning systems. These and other lines of research, centred on lighter weight and more efficient optics and on more efficient detectors, should result in much cheaper systems with resolution approaching visual sensors. Perhaps most important are improvements in the resolution and brightness of the display—the chief limitation of most night viewing systems.

Photography (photography, history of) has already reached an advanced state of technology, yet improvements in resolution are being actively pursued. Lightweight optics, more sensitive and fine-grained film, film that can be developed quickly by heat, and better compensation for the motion of the aircraft are some of the areas where photography can be improved.

Developments in large ground radars centre around the phased array radar having electronically steered beams. The beams must be computer-controlled. Moving target discrimination and Doppler processing are built out of digital circuitry as used in digital computers. This permits sensitive discrimination and rapid response.

The advent of the transistor and solid-state microcircuits is making small radars for infantrymen and tank operators possible. The miniature components and high reliability of these devices makes extremely complex and sophisticated circuitry possible.

Airborne radar is in its early stages of development. Side-looking Doppler-processing radar has already yielded high resolution, but not quite as good as conventional photography. Developments in progress indicate that soon images comparable to photographic images will be obtained from airborne radar. Imperfections now common in radar imagery should be removed as a result of present research.

A great deal of effort in several countries has reduced the vulnerability of radars to electronic countermeasures; at the same time, however, similar improvements in electronic jamming and deception have taken place.

Nuclear propulsion enables submarines to remain submerged and escape detection by radar. This, plus its increased speed, makes the nuclear submarine a formidable threat. To combat this, sonar sensors for detection of submarines are now being formed into arrays. This increases the sensitivity and rejects extraneous noise, especially important in regions of turbulence.

The search for more sensitive systems of detection will go on. Measurement of the temperature change in the water in which a submarine lies and the magnetic anomaly observable when it is under the water are two directions in which study is being pursued. A range of such measurements may become possible. Testing of laser beams for underwater recognition capability has been proceeding for some time. The problem is extremely difficult, water being a medium quite different from air, and much work will be needed to overcome the obstacles.

The subject is closely linked with the study of undersea conditions generally; that is, oceanography. American efforts dwarf those of any other Western nation, though France, a pioneer in undersea exploration, is active. Underwater acoustic navigation enables ships to be used for missile or satellite tracking. Underwater communication over very long distances is essential for the control of nuclear submarines, to make the most of their almost unlimited radius of operation.

Radar and identification friend or foe (IFF) equipment constitute the forward elements of complex systems that have appeared throughout the world. Examples include the semiautomatic ground environment (SAGE), augmented by a mobile backup intercept control system called BUIC in the United States, NATO air defense ground environment (NADGE) in Europe, a similar system in Japan, and various land-mobile, airborne, and ship command and control systems. Little information concerning the Soviet systems is available, but they are known to be extensive, automated, and capable.

Air-defense systems require computers and communication nets to process the radar data. Position reports from the radars are formed into tracks of each detected aircraft. Height-finding radars add the third dimension. The IFF information, together with known flight plans, is correlated; clutter, false returns from clouds, and any electronic countermeasures are rejected. Decisions are made on whether to counter the attack with interceptors or surface-to-air missiles. The counterattack is controlled by guiding a missile or directing an intercept.

To avoid excessive centralization of equipment that would make the system vulnerable to nuclear attack, the computers and communication facilities are widely dispersed and supplemented by mobile facilities.

In addition to large conventional radars, small distributed radars (called gap fillers) are used to detect low-flying aircraft penetrating gaps in large radar coverage. Over-the-horizon radars and AWACS (airborne warning and control systems) are even more promising. The latter consist of large radar and computation, display, and control systems, housed in large aircraft. First introduced for naval defense, they have become potentially effective over land with new developments in clutter-rejection circuitry.

Large aircraft with powerful radars connected to sophisticated computer and display equipment can survive a nuclear attack and have a low-altitude surveillance capability. Their use, delayed because of problems caused by interference from land clutter, is growing.

A unique air-defense system is the U.S.-Canadian Distant Early Warning system stretching across the northern portion of North America. The radars are used strictly for early warning; no control of missiles or interceptors is provided. Elaborate communications to control centres to the south are part of the system.

Air-defense systems spread the warning to the civil population by sirens and radio alerts. Extensive communication nets are built for this purpose. Air-defense systems also select and assign the defensive weapons to particular threats. If interceptors are used, a control centre is assigned to send control information by digitally encoded radio messages.

If surface-to-air missiles are used, the target is designated to the missile control system, which has its own target-tracking and missile-control radar. Practically all surface-to-air missile systems have some autonomous capability of warning and target acquisition. Examples of these systems are the American Nike Hercules and Hawk, the British Thunderbird, Bloodhound, and Rapier, the French-German Roland, and the Italian Indigo. In sea warfare, such missiles as the U.S. Terrier and Talos, the British Sea Dart, and the French Masurca have autonomous radar capability.

At sea, air defense also uses large radars on ships, but more use is made of airborne radar and control systems. The weight and size of long-range radars restricts their installation to the larger ships; airborne radar over the ocean does not have severe land clutter to contend with, making it simpler than overland systems; the horizon limits are at a greater range; and the aircraft can patrol a large area. As in land defenses, extensive computer and display complexes, and communications between the ships, are used. In the U.S. Navy the Airborne Tactical Data System, consisting of airborne radar, computers, and memory and data links, is connected with the Naval Tactical Data System, located in fleet headquarters, which processes, organizes, and displays information of the overall picture of the tactical situation.

In the second half of the 20th century, warning against ballistic missiles with nuclear warheads has taken precedence over all other warning systems. Large ground radars, operating in the very high frequency (VHF) or ultrahigh frequency (UHF) range, are used. The radars search the skies and track detected objects. Computers calculate trajectory to determine if the target is a missile or an Earth-orbiting object. Depending on the trajectory, the number of objects, and other criteria, alerts, tentative warnings, or all-out warning signals are transmitted to command centres.

Surface-based radars have one serious flaw: they can detect an object only after it appears above the Earth's horizon. For earlier warning, over-the-horizon radars or satellite-borne infrared detectors can be used.

There are two types of over-the-horizon radars, operating in the high frequency range, which can reflect from the ionosphere. One system, called forward scatter, transmits from one location and receives the signal several thousand miles away on the other side of the launch point. The back-scatter system receives the signal from the same location as the transmitter, as is done in conventional radar. Both systems detect variations in the received signal due to fluctuations in the ionosphere caused by the missile's exhaust plume as it traverses the ionosphere.

Ballistic missile defense systems have their own warning and acquisition radar systems. These large radars are more sophisticated than the warning radars because they must form accurate tracks for the engagement radars. Decoy objects and lightweight metallic reflectors called chaff must be identified and rejected. To do this, the radars must be able to measure the velocity of all the objects, because lightweight objects decelerate more rapidly than heavy objects due to atmospheric drag and friction.

Closely allied to warning systems are space-object detection and tracking systems. It is likely that only the United States and the Soviets have developed and operate these systems. A variety of very large radars are used, although the newer installations are phased-array radars that have stationary antennas with electronically steerable multiple beams. The scanning is more rapid than that by a mechanically rotated antenna, and several objects can be tracked simultaneously. The radars used for ballistic missile early warning are connected into spacetrack nets.

To supplement radars, telescopes (telescope) have been designed for accurate tracking of comparatively low earth satellites. Telescopes, which can have cameras, have been adapted with varying degrees of success to pick up high-altitude satellites and extremely faint objects. The range depends on the size of the target, its reflectivity, and the solar aspect angle (angular position of the sun in the sky). Telescopes are not detection devices, but they can track objects if they are pointed in the correct direction by the ground radar net.

In 1963 a treaty banning nuclear weapon tests in the atmosphere, in outer space, and underwater was signed. Each signatory nation was to provide monitoring. A direct consequence was the development and construction of a wide variety of devices to monitor nuclear explosions.

Underground explosions, still permitted under the treaty, are monitored by seismometers, instruments that measure minute ground motions. Because of the high sensitivity required to measure at great distances the ground vibrations caused by nuclear explosions, the seismometers record many extraneous motions from natural sources; these are called noise. To reduce noise, a large number of seismometers arranged in arrays is used to reinforce the desired signal and exclude unwanted signals. Elaborate data processing, with the help of recorders and computers, further refines the output. Despite these measures, there is a limit to the sensitivity of underground and underwater systems, so that very small nuclear explosions at great distance from the receiving sites may not be detected or may be wrongly identified as a small earthquake.

Detection of explosions in the atmosphere and in space depends upon measuring the products of an explosion. Acoustic sensors are used to measure the sound waves created by the blast, aircraft and rockets to collect possibly radioactive debris samples, flash detectors to detect the light flash as well as the radio pulse generated by the explosion, and a number of radio-detection techniques to measure the considerable disturbance of the ionosphere. None of the techniques is adequate by itself, since each is disturbed by various background signals. Analyzed together, however, they yield positive results.

To detect explosions in space, high-altitude satellites are used. They carry detectors of X-ray emissions, gamma rays, and neutrons, all of which are generated by a nuclear explosion. They can be detected because there is essentially no atmosphere in space to absorb the emissions.

The growth of insurgency warfare has made necessary the development of a variety of sensors to detect vehicles and personnel in the jungle along trails or on roads. Acoustic, seismic, magnetic, infrared, radar, and Doppler radar (radars that detect movement by shift in frequency of received signal) are the sensors.

The sensors are connected to processing centres where the progress of an infiltrating column or truck convoy can be monitored. This process eliminates many false detections due to random noise or animals. Because the sensors are widespread and the processing quite sophisticated, the systems have become known as the instrumented battlefield or electronic barrier.

Aerial reconnaissance has grown in importance; it now encompasses all phases of warning. Visual observation from the air furnishes short-term information and warning. Direct receiving and image-recording infrared equipment in night reconnaissance, high resolution radar in bad weather, and conventional photography all contribute to medium and long-term warning by observing tactical preparations or discerning new military capabilities.

Manned aircraft are used more frequently than other platforms for these sensors. Unmanned aircraft, however, flying at low and high altitudes; helicopters, including small unmanned helicopters; and space vehicles are all used for various reconnaissance missions.

Photography from rockets was first undertaken in 1906. A model for military reconnaissance was built in 1912, but by this time photography from airplanes had been shown to be feasible. After the launching of the first Soviet satellite, Sputnik 1, in 1957, the potential of observations from space vehicles became obvious and various applications were developed.

Satellite (Earth satellite) platforms can carry a variety of sensors. Cameras in space can collect images on photographic film, infrared images, or television-type signals. Radars can be carried aloft for operation at night or through clouds that could otherwise obscure the images. Infrared sensors can be used to detect missiles, or space warnings. Sensors to detect nuclear explosions can also be used to monitor possible violations of the nuclear test treaty.

To be useful, the sensors must have high resolution. The large distances involved make this difficult. Cameras must have telescopic optics and must be quite large and heavy. As the ability to lift larger weights to orbital altitudes increases, the capabilities of the sensors will improve. Infrared sensors also need heavy equipment. Radar sensors are limited not only in resolution (generally much poorer than optical sensors) but by electrical power limitations, since quite powerful radar transmitters are necessary.

Photographic resolution of about one second of arc is achievable today. At 200 miles (320 kilometres) altitude, this would be equivalent to a resolution of 10 feet (three metres); that is, an object 10 feet in diameter could be clearly distinguished. Vibration and high speed reduce this resolution considerably.

The limited range of both active (echo-ranging) and passive (listening) sonar makes the use of many sensors necessary in submarine detection. To guard a shore, a line of sensors can be set on the ocean floor. In the broad ocean area, however, the sensors on ships and submarines leave vast spaces uncovered. To fill these gaps, sonobuoys, floating buoys with sonar sensors and radio transmitters, are used. The signals from the sonobuoys are received by patrolling aircraft; these then track the submarines.

Naval vessels use helicopters (helicopter) for submarine detection and warning. Each carries a sonar sensor at the end of a cable, lowering it into the water to detect submarines. Such sensors are called dunked sonar sensors.