randombio.com | science commentary
Sunday, May 27, 2018

Radar is becoming obsolete

Future wars will be fought by invisible soldiers on invisible ships launching invisible missiles at targets they can't see.


R adar helped Britain to prevent the Germans from invading in WWII. It helped the United States defeat the Japanese in the Pacific. Now new forms of radar are changing how battles are fought. Within a few decades, radar itself may become obsolete.

What our top military planners really think is a closely guarded secret, but we can deduce what they might be thinking by looking at the technology. They probably recognize, for instance, that the aircraft carrier model, on which their naval strategy is based, is in deep trouble.

Yes, carriers have been pronounced dead many times before. Planners consider them too useful to abandon, so we'll be seeing intensive efforts to counter the latest threats, and it's a truism that no asset is ever perfectly secure. But we can read between the lines. One commentator, for example, writes:

Although the Navy has changed it tactics to deal with the proliferation of fast anti-ship missiles and the growing military power of China in the Western Pacific, large-deck aircraft carriers remain among the most secure and useful combat systems in America's arsenal.

The new tactic he's refering to is to keep the carriers as much as 1000 miles away from the target and use drones to extend the range of their fighters, keeping the ships out of the danger zone—which is, in fact, the purpose of the missiles. More than anything else it's a tacit acknowledgment of how serious the threat of supersonic missiles really is.

Ocean
Artist's conception of the U.S. Navy's future cloaked aircraft carrier

Some of these missiles fly close to the waves, and pop up for only a few seconds to get a bearing. They're very hard to detect because they use a type of radar known as LPI (low probability of intercept) radar.[1] In the last few seconds they jump up and accelerate toward the ship from above at Mach 12, while weaving at high G to avoid kinetic defenses. And, according to electronic warfare expert James Genova in an important new book titled Electronic Warfare Signal Processing[2], our radar jamming technology is becoming increasingly powerless to stop them.

Until recently, it's been possible to jam the radar of these missiles. To interfere with a missile's radar seeker, you simply detect the radar signal, process it, and transmit a customized false echo to convince the missile that the target has, for example, suddenly jumped to Mars—or at least to someplace not where the missile thinks it will be.

To do that, you need details about the enemy's radar. Traditionally, the strategy has been to fly aircraft near the country's borders—and let's be specific, we're talking about China here—in order to get tracked by their radar. It's dangerous work, but it gives you priceless information about the waveforms and frequencies that they might use.

FFT of phase-coded sine wave
Phase-coded sine wave. The waveform is encoded with coded phase shifts (two examples shown A, B). If the echo doesn't have the correct code, which only the attacker knows, the attacker rejects it. This not only makes it very hard to send a fake return echo, it makes it more resistant to noise jamming. The phase coded modulation also automatically adds an element of frequency hopping (C), making it harder to intercept. If this is added to conventional coded frequency hopping, interception and jamming become nearly impossible.

But as Genova points out, powerful microproscessors, sophisticated new computer algorithms, and LPI radar are making this increasingly difficult. The details of those algorithms, which the Chinese have published in technical papers, are very complicated, but the message is clear: without major technological innovation, America's dominance of the oceans could come to a swift and unexpected end.

Traditional defenses like noise jamming and chaff don't work well against these new radars. Kinetic defenses (which means computer-targeted rapid fire antiaircraft guns) and surface-to-air missiles are rapidly exhausted. For a hypothetical missile-carrier battle, if 100 missiles costing $1m apiece can take out a $15b carrier (including its planes), the attacker has a 150:1 potential tactical advantage.

There are, of course, things a ship could do. It could, for example, use directed energy weapons; or it could synthesize a false radar image indistinguishable from seascatter. This would take advantage of the missile's two big weaknesses: it has to be compact and cheap. But infrared imaging sensors are getting cheaper as well, and they're nearly impossible to jam. And cloaking could make directed energy weapons useless.

Cloaking

Genova's warnings are serious for the present. But an old (2010) nontechnical report titled The Cheshire Jet put it in layman's terms: the next war may be fought between invisible missiles and invisible ships launching invisible airplanes.

The authors say cloaking will at first be useful against old-fashioned fixed frequency radar, IR lasers, and IR seekers, but eventually it will expanding to broadband frequency ranges from infrared to ultraviolet. They cite an article saying that in the 21st century, 90% of all aircraft lost in combat were due to infrared SAMs. These missiles, too, could become obsolete.

You might think cloaking is just another form of stealth. In both cases, you don't get a radar return from the target. The advantage of cloaking will be that the target will be invisible not just to radio waves, but also to infrared and visible light, and even sound waves.

Cloaking works by using an array of small structures that interact with the wave nature of radiation. These structures, called metamaterials, have a diameter close to the wavelength of the energy coming in. Instead of reflecting the energy, they redirect it around the object.

It works with any kind of waves—even sound waves and mechanical vibrations. In a recent book[3], Sébastien Guenneau and Richard Craster describe how an array of simple metal cylinders with small ridges inside, resembling more than anything else the aftermath of a frat beer party, can cloak an object against sound. The applications are mind-boggling: silent highways, undetectable submarines, and buildings designed to be invisible to earthquake energy.

Imagine the possibilities: you can take empty beer cans, an abundant resource on any college campus, and arrange them in such a way that the neighbors can no longer hear you burp.

At the moment cloaking materials for infrared and visible light aren't tough enough to survive the stresses that a fighter plane or a missile encounters. Researchers are trying to make metamaterials cheap, compact, and durable, and to make them tunable and effective against a wide range of wavelengths.

Contrary to what you might think, it would also make the receivers onboard more sensitive: an antenna or microphone trailing outside the cloaked region would no longer be blocked by the massive, noisy objects around it.

Cloaking technology isn't just for the military; it will find uses in architecture, transportation, and in dozens of other aspects of life. Someday we may indeed find, as we used to recite in school, that we're one nation, invisible, after all.


1. Pace PE 2004. Detecting and classifying low probability of intercept radar. Artech House, Boston

2. Genova J (2018). Electronic warfare signal processing. Artech House, Boston

3. Guenneau S, Craster RV (2013) Fundamentals of acoustic metamaterials. In Acoustic Metamaterials: Negative Refraction, Imaging, Lensing and Cloaking Springer Series in Materials Science 166


may 27 2018, 3:58 pm; edited for clarity jun 24 2018, 9:22 am


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Book Reviews

Electronic Warfare Signal Processing by James Genova

Acoustic Metamaterials: Negative Refraction, Imaging, Lensing and Cloaking by Craster & Guenneau

Detecting and Classifying Low Probability of Intercept Radar by P.E. Pace

Modern Communications Jamming Principles and Techniques by R.A. Poisel


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