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.

Science Illustrated - - CONTENTS -

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.

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