Radar and sonar technologies came into their own as pivotal capabilities in World War II, during the Battle of Britain, the Battle of the Atlantic, and many other battles that helped shape the latter half of the 20th Century. Research in the 1930s led to widespread deployment of active radar and sonar systems across three operational domains - air, land, and sea - during and after World War II.
For the past half century, efforts to defeat both have spawned generations of stealth technology and a renewed focus on passive radar and sonar, which actually predate active systems by three decades.
Radar, short for radio detection and ranging, bounces radio waves off objects to calculate their distances from the RF transmitter. Sonar, which stands for sound navigation and ranging, bounces sound waves off objects to calculate their distances from the sound transmitter. Most often sonar is for detecting and locating surface vessels, submarines, or other maritime objects for detection and tracking or to assist navigation and obstacle avoidance.
The biggest drawback to both is they are the equivalents of flashlights in the dark: While they help the user illuminate targets of interest, they also help others see the location of the user. That has led to development of less visible approaches that involve RF and sound transmitters that appear to be causing random noise, rather than conducting a determined search.
"There are spread-spectrum transmissions for radar that is an active technique that, instead of a blatant pulse train, it appears you are putting out noise and it's not as obvious you're there," says Marc Couture, senior product manager, digital signaling at Curtiss-Wright Defense Solutions in Ashburn, Va.
Passive radar and sonar systems, however, do not generate their own signals at all. They capitalize on existing signals in the environment, and the passive radar and sonar receivers essentially just listen for those signals and use power signal processing to sort these signals into useful information.
"With passive techniques, you rely on the environment to light up objects," Couture says. "In 2016, there are a lot of things that light up everything around you in terms of radio emissions. With passive radar, radio waves bouncing off each other from a host of other sources can give you a picture without sending out your own pulse. That also is true with sonar - if you have a sensitive enough system, you can pick up objects without sending out a ping."
"Both require extremely sensitive sensors, certainly more than just one single-point sensor, and a lot more processing," Couture explains. "If you think of radar, you put out a pulse train and expect it to come back, modified by whatever it bounces off. With passive, you're using reflections off clutter and buildings to pull out a target in the sky, so there is a lot more work to correlate all those different wavefronts."
One way to understand that is to look at the downfall of H.G. Wells's Invisible Man character, who could be seen - and tracked - when his invisible body caused what appeared to be a bubble in rain or fog.
In the real world, of course, stealth platforms are not invisible to the naked eye, but appear as something else - usually much smaller - on radar or sonar. There have been two primary ways to do that: special shaping, such as used on the F-117 Nighthawk, the world's first stealth aircraft - or special materials that absorb rather than reflect radar signals, as used on the second stealth aircraft, the F-22 Raptor.
Both methods also have been used on surface and subsurface naval vessels, such as the Navy's new Zumwalt-class guided missile destroyers and Virginia-class attack submarines. More recently it's been used in designs for next-generation unmanned aerial vehicles (UAVs).
These platforms represent the U.S. military's generational lead in stealth over the rest of the world and proved vital to air superiority in the first and second Gulf wars. But a future conflict with a technologically near-peer adversary - such as China, Russia, or Iran - or nations they support could see that advantage melt way. Not because those nations are developing their own stealth platforms - which they have been working on for years - but because they are concentrating on far less expensive and easily deployed counter-measures.
"Faced with the prospect of aerial stealth proliferation, states in the 21st Century are looking for anti-stealth defense options. One such alternative, passive radar, appears a cost-effective counter to stealth," according to an October 2009 National Defense University (NDU) report, "Radar vs. Stealth: Passive Radar and the Future of U.S. Military Power," that is even more pertinent today.