Stealth Durf on The Durf Blog by Kyle Durfee

Primer on Radar Stealth

By Ronnie Macdonald from Chelmsford, United Kingdom (Boeing F-18 Super Hornet 3) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

F/A-18 Super Hornet

In December 2016, Trump started agitating for the possibility of getting upgraded F/A-18 Super Hornets instead of fulfilling a contract for F-35 Lightning II’s. The F-35 program had long been beset by cost overruns and delays (see my post on “Current Controversies“) and Trump wanted to find a better deal.

Super Hornets might perform well enough for “80%” of what the U.S. needs. They’re allegedly cheaper, they perform well, and they could be upgraded to do much of what the F-35 program calls for.

Unfortunately, a large part of that “20%” that Super Hornets cannot do involves its stealth abilities against radar. Boeing cannot simply upgrade its Super Hornets with stealth capabilities – engineers must bake stealth into the design. Which brings us to the interesting topic of radars and radar stealth.

How Radar’s Work

By The original uploader was Averse at German Wikipedia (Transferred from de.wikipedia to Commons.) [CC BY-SA 2.0 de (http://creativecommons.org/licenses/by-sa/2.0/de/deed.en)], via Wikimedia CommonsRadars emit a signal, and listen for how long it takes for the signal to bounce back – like listening for an echo. Bats use this same principle when they do “echo-location“. Bats will make a noise, and listen for how long it takes for their noise to echo back. The longer it takes, the farther the object.

Instead of using sound waves, radars use electromagnetic waves. Electromagnetic waves are what your eyes detect to see things – it’s just “light” energy. The difference between visible light, and radar waves, and even microwaves, x-rays, infrared and other waves, is their frequency.

Frequency

Electromagnetic waves oscillate similar to ocean waves. Ocean waves seem to go up and down, with the high part coming in at regular intervals. If 10 waves come to shore each minute, we might say it has a frequency of 10 per minute. We can measure light oscillations as well, but we’ll usually measure their oscillation in seconds rather than minutes.

Radar waves oscillate relatively infrequently compared to other electromagnetic waves. That is, for our eyes to see light, the waves need to come in at a higher frequency than what a radar can “see”. Where X-rays and Gamma rays have higher frequencies than visible light, microwaves and radar waves have lower frequencies.

Wavelength

The wavelength of the wave directly relates to the frequency. Specifically, the higher the frequency, the shorter the wavelength. The reason for this has to do with the speed of the wave.

Sine Waves of Different Frequencies

Waves of different frequencies & wavelength

It’s easy to think of high frequency waves as moving faster than low frequency waves, but that’s not actually what happens. Electromagnetic radiation travels at the same speed (the speed of light) regardless of its frequency. How can you have waves that move at the same speed but with different frequencies? By having different wavelengths.

You can think of high frequency waves as being scrunched together, while low frequency waves are stretched apart. You can imagine this with ocean waves – you might see 10 waves come a minute, or 100, but each of those individual crests might have traveled across the ocean at 1mph. More came in when there was less space between each crest.

Although radars are often described by the frequency of the waves they use rather than the wavelength, the wavelength of the electromagnetic waves plays a major role in modern stealth issues. We’ll address that later.

Troubles with listening

When a bat listens for an echo, it needs to listen for its own “voice” and ignore other noise around it. In addition to distinguishing from rustling leaves and other animals, bats will need to distinguish from other bats.

Radars need to do the same thing. Electromagnetic radiation of all sorts bounces around our atmosphere, similar to forest or cave noise a bat deals with. Radars also need to account for signals sent out by other radars.

Radars can cut through noise in a few ways. They can send out specific patterns of pulses and listen for that pattern to return. They can also switch between frequencies and listen for signals to come back on the specific pattern of frequencies used.

Radars also have to deal with ways in which the signal gets altered in transmission. Just as how your echo doesn’t sound exactly like you, radar signals can experience changes as they bounce off of objects or travel through the atmosphere. Radars have to do all sorts of calculations and signal management to recognize its own “voice” when it comes back.

Radar Frequencies

Radars don’t all operate on the exact same frequency. Most radars operate in bands sometimes called the “Super High Frequency” or even “Extremely High Frequency.” In spite of their names, these bands still oscillate at a lower frequency than visible light.

These bands allow for air traffic control, weather pattern mapping, and even missile guidance. Stealth aircraft gain their advantage by hiding from radars on this frequency band.

Some radars make use of the “Ultra High Frequency” (UHF)or “Very High Frequency”  (VHF) bands. The waves in these bands are even less frequent than the bands used by other radars. Since they have a lower frequency, they also have a longer wavelength.

A special type of radar referred to as “Over the Horizon” radars uses an even lower frequency. One way these radars work is by using the “High Frequency” band. At these frequencies, the wave will bounce off of the ionosphere. These radars make use of this property to perform a sort of “bank-shot” with the radar wave. The radar will emit a signal that bounces off of the ionosphere to reach a part of the earth too far to reach directly, which will then bounce off of an object, hit the ionosphere again, and return to the radar receiver.

Why do we care about the frequency that the radar uses? Let’s find out by finally looking at how stealth beats radars.

How To Beat Radars

Objects can “beat” radars in two ways: being “quiet”, and being “loud.” Stealth technologies help objects stay quiet. Electronic Attack (EA) technologies make the environment loud.

How to stay “quiet”

Quiet shapes

“Quiet shapes” help stealth aircraft avoid radar detection. Shapes help by either scattering radar energy away from the radar receiver, or by preventing resonance.

A U.S. Air Force F-117A 'Nighthawk' Stealth Fighter aircraft flies over Nellis Air Force Base, Nev., during the joint service experimentation process dubbed Millennium Challenge 2002. Sponsored by the US Joint Forces Command, the Millenniun Challenge 2002 experiment explores how Effects Based Operations can provide an integrated joint context for conducting rapid, decisive operations. (U.S. Air Force photo by Staff Sgt. Aaron Allmon II) (Released)

F-117 Nighthawk

Sharp angles and bumps reflect radar energy well. As a counter measure, the F-117 used odd-angled panels, designed to bounce radar energy in directions other than toward the receiver. Engineers can now design smoother shapes that still scatter radar energy elsewhere without requiring jagged panels.

Scattering the waves away from the receiver works well for radars operating in higher frequencies, but radars with lower frequencies (and longer wavelengths) have a trick that can overcome this measure.

Their trick is resonance. Resonance properties depend on the wavelength of the wave, and the size of the reflecting material. Objects that are less than ~8x the size of the wavelength will “resonate” and scatter energy in all directions. Radar receivers can pick up on this energy.

Radars in regular frequency bands have very wavelengths measured in centimeters, or even nanometers. This gives the radar a high degree of precision, but means that aircraft won’t exhibit any resonance. They must rely on the energy being reflected back.

Radars in the VHF or UHF band have wavelengths up to a meter and a half. This means that any part of the plane less than ~8 meters long will give off some amount of resonance, and a radar might pick up on it.

FILE PHOTO -- The B-2 Spirit is a multi-role bomber capable of delivering both conventional and nuclear munitions. A dramatic leap forward in technology, the bomber represents a major milestone in the U.S. bomber modernization program. The B-2 brings massive firepower to bear, in a short time, anywhere on the globe through previously impenetrable defenses. (U.S. Air Force photo)

B-2 Spirit

This is partially why the B-2 Spirit stealth bomber (and coming B-21 Raider) don’t have tail flaps. Both their angle and size make tail flaps very “loud” shapes, akin to reflect radar energy back, and readily resonating.

Fortunately for stealth aircraft, VHF and UHF radars lack the clarity and accuracy of other wavelengths. Sometimes, VHF and UHF radars cannot even provide the detected object’s location within several miles. This isn’t accurate enough to send another aircraft to intercept you, much less guide a missile to the target.

Radar technology continues to advance, however, and in several years, advanced computing analytics and other measures may make VHF and UHF band radars viable for weapons targeting. This will of course limit the effectiveness of stealth vehicles, though it will still be advantageous to be less visible, even if you’re not completely invisible.

Quiet Skin

“Quiet skin” also helps prevent radar detection. Engineers typically design quiet skin by using Radar Absorbing Materials (RAM).

RAM work a lot like black objects in visible light. A black car gets much hotter in the sun than a white car because the color black absorbs much of the light and turns it into heat. RAM similarly absorbs radar energy and turns it into heat instead of reflecting it out.

RAM isn’t perfect, and it doesn’t work equally well against all frequencies. Just as how green objects mostly reflect green light and absorb the other colors, RAM will absorb some frequencies while still reflecting parts of others. Engineers decide what wavelengths to focus on when they design the material.

Quiet Eyes

Stealth craft have radars of their own, acting as their “eyes”. Unsurprisingly, if reflecting a radar signal can give you away, sending your own signals out might get you noticed as well. Besides simply turning your own radar off, there are a few ways to keep your eyes “quiet”.

First, radars can switch between frequencies to avoid detection. If an enemy radar picks up an unusually strong signal on any given frequency, it can suggest the presence of an enemy radar. However, one strong ping cannot be distinguished from other electromagnetic noise, and does not provide enough information to track position and velocity with any usefulness. If your radar continues to bombard an enemy receiver with that same wavelength, they can simply tune in and find you. If instead your radar switches frequencies frequently, the enemy must continue to switch its own frequencies to find you.

Second, and related, you can use a wider range of frequencies than normal. If your enemy does not expect to find your signal in a certain frequency band, they’ll miss you entirely.

Third, radars can simply emit weaker signals. The weaker the emission, the more the signal looks like background noise. This limits how far the radar can see, but operators don’t always need to see as far as full power will take them.

Finally, radars can sweep in less predictable directions (while limiting the amount of signal that “leaks” in different directions). If your radar just circles around in a regular interval, your enemy will have a better chance of recognizing your pulse as something other than noise. Sending your radar signals in different directions randomly can help avoid revealing yourself through regularity.

Other measures exist to keep your shapes, skin, and eyes “quiet,” but require too much physics for a primer.

How to be “Loud”

While stealth vehicles stay quiet, Electronic Attack (EA) can increase surrounding noise. It’s like being a ninja in a car factory – with so much ambient noise, he can probably knock over a toolbox without anyone thinking the noise came from anything besides the normal machinery.

EA largely involves radar jamming, and often comes from vehicles other than the stealth. Jamming can be done in many different ways, from focusing a high amount of energy on a single frequency, to sweeping through many frequencies, to “shouting” at lots of frequencies at once.

Since jamming involves sending strong signals, it risks giving away the position of the jammer. Some measures can help prevent this, such as timing the pulse with a certain position of the radar receiver, making it look like the jammer is somewhere else. Jammers can also try to detect the frequency of the radar and send back similar signals at different time intervals or manipulated in certain ways to make it look like you are not only in a different place, but traveling at a different speed than reality.

There’s also good ol’ fashioned blowing-up your enemy’s radar, often using Anti-Radiation Missiles (ARMs) High-speed Anti-Radiation Missiles (HARMs), or Air Launched Radiation Missiles (ALARMs)

So…Hornets or Lightening?

U.S. Air Force F-35A Lightning II Joint Strike Fighter pilots from the 58th Fighter Squadron, 33rd Fighter Wing, Eglin AFB, Fla., navigate their aircraft toward an Air Force Reserve KC-135 Stratotanker from the 336th Air Refueling Squadron, March ARB, Calif., May 16, 2013, off the coast of northwest Florida. The 33rd Fighter Wing is a joint graduate flying and maintenance training wing that trains Air Force, Marine, Navy and international partner operators and maintainers of the F-35 Lightning II. (U.S. Air Force photo by Master Sgt. John R. Nimmo, Sr./RELEASED)

F-35A Lightning II

The F-35 Lightning II costs a lot of money, but F/A-18 Super Hornets cannot replicate its stealth features. The shapes on the F/A-18 can only change so much to become quieter, and RAM only works well if the shapes are in proper alignment. Hornets might get quieter eyes and have good EA coverage, but physics dictates the principles of radar stealth, and physics does not approve of the foundational F/A-18 design.

Stealth isn’t everything though, especially as counter-stealth measures improve. If the U.S. can get more F/A-18’s for the same price as an F-35, there may reach a point where the quantity outweighs the quality of the F-35.

Time will tell, but I don’t think the F-35, or an emphasis on stealth technology, is going away.

Keep seeking truth.

 

 

You may also be interested in:

US Military Aircraft: Current Controversies

Primer on Modern U.S. Military Aircraft

Why Politics Frustrates you: The Conflict of Visions

Photo attributions:

 

F/A-18: By Ronnie Macdonald from Chelmsford, United Kingdom (Boeing F-18 Super Hornet 3) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

Radar Gif: By The original uploader was Averse at German Wikipedia (Transferred from de.wikipedia to Commons.) [CC BY-SA 2.0 de (http://creativecommons.org/licenses/by-sa/2.0/de/deed.en)], via Wikimedia Commons

Waves of different frequencies: By No machine-readable author provided. LucasVB assumed (based on copyright claims). – No machine-readable source provided. Own work assumed (based on copyright claims)., Public Domain, https://commons.wikimedia.org/w/index.php?curid=1536518

F-117: By Staff Sgt. Aaron Allmon II – http://www.defenselink.mil/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3770855

B-2: By USGov Military-Air Force – USGov Military-Air Force, Public Domain, https://commons.wikimedia.org/w/index.php?curid=15743

F-35: By MSgt John Nimmo Sr. [Public domain], via Wikimedia Commons

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