Submarines, as ships that can dive into underwater activities and fight, are the main assault ships of the modern navy. As submarines play an increasingly important role in naval warfare, submarine exploration technology has also developed rapidly.

Several reasons increase the likelihood of a submarine being detected. Mainly noise, the noise from thrusters and associated facilities is easily captured by sonar. There are also magnetism, electricity, heat, radiofrequency, etc. These are the main factors that affect the stealth of submarines in combat.

At present, there are many kinds of main submarine detection methods, which can be roughly divided into two categories: underwater acoustic detection equipment and non-acoustic detection equipment. The underwater acoustic detection equipment mainly includes sonar, underwater noise tester, sound ray locator, ballistic trajectory tester, underwater positioning tester, sound velocity meter, wavemeter, etc. The non-acoustic detection equipment mainly includes magnetic detectors and infrared detectors, low-visibility televisions, submersible radars, and temperature gradiometers.

Ocean surface temperature is usually detected by the infrared detector channel of an advanced high-resolution radiometer. Due to the absorption of infrared radiation by water molecules, the infrared radiation emitted by submarines operating underwater cannot be easily detected by the infrared thermal imaging system above water. However, all the underwater activities of the submarine, including the friction between the surface and the seawater in the submarine's propulsion, the operation of various equipment, and even the activities of personnel, cause energy consumption. According to the law of energy conversion and conservation, all the consumed energy is eventually converted into the form of thermal energy, which is inevitably dissipated into the surrounding environment, that is, seawater, thereby increasing the temperature of the seawater around the submarine. The warmer seawater reaches the surface with convection and remains there for a short period of time. In this way, in the place where the underwater submarine just passed, the water temperature on the sea surface is slightly different from the surrounding seawater temperature, so there is also a difference in its infrared radiation. The infrared thermal imaging system can convert this infrared radiation difference of the sea surface into an electrical signal and form a visible light image, which shows the submarine's track.

Of course, in order to detect this very small difference in water temperature on the sea surface, the infrared thermal imaging system must have extremely high sensitivity which allows for detection of this small difference in sea surface water temperature from a distance. In addition, the infrared thermal imaging system can quickly scan a large sea surface in an imaging manner, which is more suitable for the needs of anti-submarine detection. Nuclear submarines with special strategic significance are likely to form obvious infrared trajectories on the sea surface because of the need to regularly discharge the accumulated high-temperature hot water.

When a nuclear submarine sails, it always releases large quantities of warm wakes to cool the nuclear power plant and thus generates a temperature difference signal in the water. Foreign experts estimate that a nuclear submarine powered by a 190kW reactor releases as much as (4.5 x 4.19) x 107J of thermal energy per second into the ocean. The idea of detecting submarines by the abnormal temperature in the water was already proposed in the 1980s, and some satellites had been equipped with infrared microwave detectors as auxiliary equipment for detecting submarines.

The vertical distribution of fluid density in the actual marine environment is mostly nonlinear, but some can also be approximated by linear distribution analysis. In this regard, some domestic laboratories have had relevant experiments on submarine thermal wakes. Under the assumption that the density of seawater is approximately linear, the buoyancy law of submarine thermal wakes in density stratified fluids has been estimated. The approximate model of the wake buoyancy law was established, and the correctness of the model was verified through simulation experiments. The buoyancy characteristics and distribution characteristics of the thermal wake were specifically analyzed, which provided a scientific basis for the infrared detection of submarines on the surface of the thermal trajectory. Comprehensive theoretical analysis and analysis of experimental results can obtain the buoyancy law and exponent clusters of the thermal wake of ordinary submarines in the marine environment with uniform density.

(1) Without the influence of other factors, the thermal wake discharged by the submarine can surface unrestrictedly in a homogeneous marine environment.

(2) The attenuation of the thermal wake temperature difference signal intensity is relatively fast in the initial stage. As the altitude rises over a long distance, the thermal signal weakens extremely slowly. Thus, even if it rises several tens of meters, the thermal wake can still have a temperature difference signal of the order of 0.1 ℃.

(3) In the process of gradual buoyancy, the width of the thermal wake keeps expanding, but the speed of expansion gradually slows down. After reaching a certain height and a certain period of time, it basically does not change. The thermal wake of ordinary submarines The width of the thermal track signal on the water surface can reach about ten meters to several tens of meters.

(4) The thermal trajectory formed by the thermal wake on the water surface is not a uniform signal. Each part has the strongest signal point. The thermal trajectory of the surface signal can be described by connecting the strongest points, which can be a method of detecting submarines.

Infrared diving requires a temperature sensitivity of less than 0.2K. As analyzed above, the thermal energy emitted by a nuclear submarine can raise the temperature of the water behind it by about 0.2℃. According to the buoyancy law of submarine wakes, in a homogeneous marine environment, the thermal wakes emitted by submarines can surfaces without limit while the heat signature diminishes extremely slowly. Therefore, it is reasonable to select this temperature difference as the minimum temperature sensitivity index for submersible applications.

The thermal infrared thermal imaging system based on the focal plane detector is used to detect submarines. Since the thermal sensitivity of the system is greatly improved, the thermal trajectory of the submarine can be effectively found to meet the needs of submarine exploration applications. Among the NATO national military thermal imager systems investigated, the US military observers and the British IR-18 and LT1085 have a relatively high spatial resolution, but their temperature sensitivity is about 0.17K, close to 0.2K. Therefore, when the sea surface temperature difference is about 0.2K, they are not suitable for diving applications due to the low signal-to-noise ratio.

LASH-ASW is a visible light system with high spatial resolution, which is mainly used for submarine exploration missions in coastal waters or shallow waters. Submarines are detected by capturing the submarine's cone-shaped tracer wake on the ocean surface when the submarine is sailing at periscope depth or not submerged. To sum up, in modern submarine detection applications, acoustic equipment can be used to detect the noise of submarines, radars can be used to detect submarines on the surface. Now infrared imagers can be used to detect the thermal trajectory of submarines.

Although the temperature difference on the sea surface will be caused by the submarine sailing, this temperature difference is very small (usually less than 0.2K), and the infrared imager only has a small enough minimum distinguishable temperature difference to effectively extract useful signals from the background interference on the sea surface. This requirement can be met by reducing the spatial resolution of the system or increasing the effective aperture of the optical system. Therefore, the system can use two optical channels, a visible light channel with high spatial resolution and a thermal infrared channel. And the later signal analysis can be performed through multi-band data fusion.

If the absolute temperature of the seawater surface needs to be calculated, a high-precision calibration system is essential. Using the black body to calibrate an infrared system has become an effective method commonly used in the world, but the premise is that the calibration source has the radiation characteristics of a near black body to meet the high-precision calibration requirements.

In modern thermal imaging systems, the introduction of digital processing components plays an essential role. Digital processing can improve image quality and its matching with vision and can also use different adaptive processing signal forms, which greatly expands the possibility of applying thermal imaging systems in various tasks. During digital processing, it can ensure the establishment of isotherms, histograms, and temperature profiles, divide the area of interest and determine the extreme and median temperature of the thermal field.

As a new means of detecting submarines, infrared technology has incomparable advantages over other systems in certain application fields and drives the development of related technologies. In practical applications, the combination of infrared technology and other diving technologies will have greater advantages and a broader application space in diving.

After the end of the Cold War, the United States adjusted its global strategies. Among them, the U.S. military proposition for future operations that the global surveillance system and communication system and the synthesis and processing of related data could be concentrated in a certain war zone to form an information advantage, which would allow for the two combat capabilities of camouflaging and breaking through defense lines and of identifying and striking important fixed and mobile targets under all-weather, day and night conditions are closely related to infrared technology. Driven by military demands and the development of related technologies, infrared technology has developed from a tactical position in the past to a strategic position today.

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[1] Zhang Youwen, edited. "Infrared Optical Engineering", Shanghai Institute of Science and Technology, Chinese Academy of Sciences. Shanghai Science and Technology Press 

[2] Wang Xiangnan. "Dual-channel infrared radiometer for remote sensing temperature measurement", Institute of Ocean Technology, State Oceanic Administration. "Ocean Technology", 1999 No. 1, 217-219.

[3] Lu Xinping, Shen Zhenkang. "Analysis of Infrared Thermal Imaging System Applied to Anti-submarine Detection", ATR Laboratory, National University of Defense Technology. "Infrared and Laser Engineering", 2002.6, Vol.31, No.3.

[4] Wang Jiang, An Guoyan, Gu Jiannong. "Theoretical and Experimental Research on Infrared Detection of Latent Heat Wake", Naval Engineering University. "Laser and Infrared", 2002.6, Vol.32, No. 3, 159-162.

[5]Infrared technology XVIII, SPIE proceedings Vol.1762.

[6]Future European and Japanese Remote-Sensing Sensors and Programs, SPIE proceedings Vol. 1490.

[7]Sea Surface Temperature Estimation using Visible and Infrared Scanner (VIRS) Hiroshi Mu Takami, National Space Development Agency of Japan (NASDA), Earth Observation Research Center (EORC).