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.

Infiltration and base defense systems

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

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 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.

Antisubmarine systems

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 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.