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

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 sensors

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 can be used to locate nuclear bursts, because the explosion generates a large amount of energy in the radio frequency region.

Acoustic techniques

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

Seismic detectors—as well as underground acoustic detectors called geophones—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.

Magnetic detectors

Sensitive magnetic detectors (magnetometers), 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.

Nuclear detectors

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.

Chemical sensors

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