MAGNETS VS SUBMARINES
Modern spy submarines are hard to spot with sonar, so a network of sensitive magnets are to make it impossible to sneak into foreign waters undetected.
Today’s submarines are stealthy and almost silent. They’re all-but-invisible to sonar, but they’re still made of metal. That’s a problem.
Ping ... Ping .. Ping. The time lapse between the highfrequency sonar signals and the resulting echoes becomes ever briefer. The submarine is approaching a rock formation, trying to avoid an enemy torpedo. The captain listens intently to the pauses between ping and echo, which indicate the distance between the submarine and the rocks. Finally, the sounds are almost heard as one, and at the very last moment, he gives his order, so the submarine steers clear. The torpedo continues into the rocks and explodes.
The scene, which is from the Hunt for Red October film from 1990 illustrates how much sound means in submarine warfare. In the abyss, there is zero visibility, and radio waves from radar, are slowed down in water. Sound waves, on the other hand, travel faster under the water than through air, and so, sonar with the familiar ping sounds has been the primary method for navigating under the water, tracking down enemy vessels in the abyss. But as the sense of hearing is the only way to size up the situation below the surface, a silent submarine is impossible to spot. Today, military stealth submarines have become so silent that they are almost impossible to detect with sonar, and so, they play an ever more important role in nations’ efforts to keep and gain control of the waters of the world.
One of the most controversial waters is the South China Sea, where several nations claim the right of the same territories. The tense situation has made the Chinese navy initiate the Great Underwater Wall, which will consist of sonar on the ocean floor, patrolling submarine drones, and buoys with s e n s i t i ve underwat e r microphones, that will make it difficult for vessels to pass undetected into Chinese waters.
China has even developed a brand new super weapon, which no submarine can hide from: a quantum magnetometer. Instead of listening for weak noise signals from submarine mechanics, it can detect the magnetic field of the metal hull at depths of several hundred metres.
Wi-Fi on the Ocean Floor
Sonar remains the most reliable method for detecting stealth submarines, and it comes in two versions: active and passive. Active sonar is the method that is often seen in films such as the Hunt for Red October. If the characteristic pings strike large objects, the sound waves will be reflected back to their starting points. Based on the number of sound waves reflected and the speed, it is possible to figure out which objects have been hit. However, active sonar is not so efficient, when it comes to searching for uninvited guests. The technology is directional, so it is vital to know in which direction to send the ping sound. Moreover, the enemy submarine’s own sonar systems will immediately detect the ping, reducing the tactical advantage of monitoring the enemy. So, China will particularly use passive sonar.
Passive sonar does not involve any pings. Instead, the sensor listens for deviating sounds in the ocean. The weakness is that background noise from marine animals, waves, and vibrations on the ocean floor can easily interrupt, increasing the risk of false positives, by which the system believes it has detected an enemy submarine, which turns out to be an innocent marine animal. Passive sonar is also directional, and for the same reason, the Chinese navy is going to use hundreds of meters, which will each cover a small area in all directions. They are to sit on the ocean floor, be fastened to buoys 400-500 m under the surface, and be mounted on ships and Chinese submarines, so all ocean depths are covered. With a finemeshed sonar network, which will continuously collect data, the
calculations of what is noise in any given area will be much more accurate. Passive sonar can listen for weaker, deviating sounds without risking a false positive, making it harder for enemy submarines to pass undetected.
In order for the network to be efficient, all permanent and mobile sonar units must be able to exchange data quickly and flexibly via a wireless network. Wireless communication is challenging under the water, as it usually uses radio waves, i.e. electromagnetic radiation. The salty ocean water conducts electricity, which will slow down all radio waves across even short distances. But with sound, it is the other way around. Sound waves are mechanical waves, which push through a material in the shape of pressure variations. As the molecules of ocean water are located closer together than those of the air, the sound waves can push ahead more efficiently, and so, they travel four times faster in salt water than in air.
Silent submarines are detected
But even a sensitive and sophisticated sonar network is rather inefficient, when it comes to the most recent generations of military submarines, which have been developed to cause the least possible noise, when they enter into stealth mode.
Approaching enemy territory, they will switch to electric propulsion, which is much more silent than mechanics. Many military submarines are powered by small nuclear reactors and will often deactivate the cooling of their reactors temporarily and slow down – both reduces the noise of the vessel considerably. Modern submarines are also typically lined with anechoic tiles made of rubber or plastic with thousands of microscopic cavities, which ensure that sound waves are absorbed and cast about the cavities, until they have shed all their energy. In this way, sound from inside the submarine has more difficulties escaping, and a ping from an active sonar does not cause a marked echo.
No matter how silent a submarine is, it will not be able to travel through the water without leaving magnetic evidence. Submarines are made of ferrugineous metal, which contains unpaired electrons. This makes the material magnetic, and so, the submarine’s metal hull will bend the field lines of Earth’s magnetic field slightly towards itself. Salt water is also full of electrically charged sodium and chlorine ions, and when the submarine pushes through the water, they are set in motion, producing small magnetic fields in the wake of the submarine.
For decades, submarine hunters have used magnetometers to detect submarines – often mounted on the tails of planes, which can quickly search large areas. But as the electromagnetic waves emitted by the submarine are slowed down in salt water, a submarine needs not dive very low, before an ordinary magnetometer cannot detect it. So, China has developed one of the world’s first SQUID ( superconduc t i ng quantum interference device) magnetometers, which take advantage of the qualities of superconductive materials, which are characterized by the fact that they can conduct an electric current without any resistance. SQUID magnetometers are so sensitive that microscopic variations in Earth’s magnetic field will make a current pass trough them, destroying their superconductivity. The result is electric reistance, which can be read very accurately. According to experts, SQUID magnetometers can detect a fridge magnet interrupting Earth’s magnetic field by moving slightly from a distance of 6 km.
However, the extreme sensitivity makes SQUID magnetometers more vulnerable to erroneous measurements and false positives. China aims to solve the problem by installing many quantum magnetometers on planes, so background noise measurements become more accurate – like with the network of passive sonars. If China succeeds in extending the “SQUID net”, it will be the first nation to take advantage of the sensitive technology to detect submarines.
In the Hunt for Red October, the submarine’s ultimate stealth technology is a worm drive, which uses magnetism for silent propulsion. The fictive technology has never been realized, but even if it were, magnetometers would still be efficient. So, engineers must be very creative, if the submarines of the future are to be able to sneak past the Great Underwater Wall.
SQUID magnetometers are more vulnerable to false positives
China has developed a quantum magnetometer, from which even stealth subs cannot hide.
A stealth sub is often lined with noise-absorbing tiles, which deaden noise from the inside, reducing the echo, when a ping sound from a sonar hits the hull from the outside.