It's common knowledge that one of the things that makes stealth aircraft so stealthy is that the body is covered in a skin of special material that interferes with radar. Would it be theoretically possible (for a more-or-less reality-based superhero concept) to create a "stealth car" based on the same principle, that won't register on a cop's radar gun?

• I need this! :o Apr 20 '16 at 11:26
• The cops might not be amused, and instead of just a speeding ticket you may find yourself with something more serious going on. Remember that stealth aircraft are designed that way in order to avoid detection for as long as possible; a cop's radar gun is only used long after detection of the vehicle.
– user
Apr 20 '16 at 11:34
• Before worrying about radar, worry about visibility. Cops can tell your rough speed by counting the time between two road landmarks. And if you don't have a license plate, it is going to pull you over anyway. Apr 20 '16 at 16:35
• "It's common knowledge that one of the things that makes stealth aircraft so stealthy is that the body is covered in a skin of special material that interferes with radar." and it's common knowledge that common knowledge is often wrong. The shape is far more important than the skin. Apr 20 '16 at 16:55
– vsz
Apr 20 '16 at 17:54

Therefore, my conclusion: Yes, you can build such a car. My understanding is it would even be legal under FCC regulations, since you're not emitting anything to jam. However, the difficult choices in materials and the design sacrifices you would have to make in other areas (no sharply curved surfaces, shrouding of the exhaust, etc) would not make it worthwhile.

• I might be a lot cheaper to just pay for the speeding tickets... ;) Apr 20 '16 at 12:43
• where did that fourth power figure came from? I understand if it was four times the square of distance (because it has to go and then back). Apr 20 '16 at 16:31
• @Mindwin Let's say the radar emits 128 units of energy. It goes 2 units of distance and falls to 32. That 32 reflects off the car and must travel back 2 units. Now it's down to 8. 128 / 2^4 = 8. Apr 20 '16 at 16:59
• @fgysin It would be cheaper to just not have a number plate. Bruce Wane never paid for Batman's speeding tickets.
– Aron
Apr 21 '16 at 1:11
• @Mindwin The reason the 4th power term shows up is because, when the radar bounces off the car, it doesn't reflect right back to the receiver. It becomes its own antenna, emitting on the frequency the radar emitted on. This goes in all directions, not just right back at the receiver. Thus you have distance^2 losses going out to the car, and then distance^2 losses again coming back. The two losses are multiplied, giving you distance^4. Apr 21 '16 at 6:12

## Let the arms race begin!

Let's start by saying yes, it is theoretically possible but no, it's not practically possible. Being stealthy in the air is very hard to do. Being stealthy on the ground against current radar/lidar speed guns is tricky at best.

## Passive Measures

If one designed a car to bounce radar and IR energy somewhere other than back towards the radar gun, then it would be difficult or impossible for the radar gun to get a good enough measurement. If this technique worked, the radar gun would think it is pointing at empty sky.

Passive measures will deal primarily with the shape of the car. Highly angular surfaces that reflect the radar away from the gun should be effective. Basically, you'll get a car that looks like the USAF F-117.

If you have the compute power, you can work out radar sneaky curved surfaces which will get you airplanes that look like the USAF B-2.

You'll need a specially shaped car, with radar/IR absorbent paint. Also, ensure that all of the bits on your car will contribute to sneakiness. Having a giant cooling fan hidden in your front grill could make you stand out like a lighthouse, even if the rest of the car is very sneaky. Side view mirrors could also ruin the sneakiness. You'll have to be very careful with the shape and reflectivity of all car parts....like the shape of your wheel rims (if radar goes through rubber).

The plus side to passive measures is that it's not illegal to make a weird shaped car. If you go with curved surfaces, no one will probably even notice. If you go angular surfaces (a la F-117), you'll have a very distinctive car....that may look something like this: ...except without two giant radar reflectors on the front in the form of wheel rims or the extra fiddly bits to form nice big radar reflectors. Remember that the corner reflector is one of the most efficient radar reflectors there is, so avoid that shape in your car if at all possible.

In short, yes, you can make a passively radar/lidar sneaky car, it's just crazy hard and very expensive.

## Active Measures

In the Vietnam war, with the advent of surface-to-air missiles, an arms race began between radar operators and the aircraft flying against them. The aircraft would get a new jammer that would decrease the effectiveness of the radar, so the next generation radar would hone in on those jamming signals. Then the jammers would get a little bit more clever in how they jam to defeat the radar's new detection ability....you can see where this is going. The race continues to this day.

So, ignoring that active radar jamming is illegal, and laser jamming sometimes illegal, your active jamming measures must compete against the radar/lidar guns themselves. If lidar guns develop that can detect active jamming attempts, your jammer will need to account for that. If they send out lidar pulses in carefully coded bursts to ensure that the laser light coming back came to the gun came from that gun, you'll need to account for that too. Conceivably, radar/lidar guns could start to use quantum cryptography on the light they send out...let's hope it doesn't get to that point.

## Complicating Factors

If this car is just avoiding speeding tickets, it's likely not worth it. Let's assume a perfect stealth car, invisible to lidar and radar. When you blow past a police officer, they will see you go past. Even if they don't know exactly how fast you were going, they will know that you were going faster than you should have which is enough for him to give chase. Stealth airplanes are designed to work beyond visual range where radar is the only way to detect an airplane. A stealth car inherently works within visual range so mitigating radar and lidar doesn't make as much sense.

## How much is it worth to you?

What kind of threat are you attempting to counter with this car? You could easily spend many millions of dollars in passive and active countermeasures. If this is just to avoid getting a speeding ticket then your cost-benefit ratio is completely bonkers. If this is for a super-secret billion dollar delivery then sure, it's worth it.

• So it's about the geometry of the vehicle? I'd always heard it was about the "radar-absorbing materials" that the stealth aircraft were covered in. Apr 20 '16 at 15:11
• @MasonWheeler Oh yeah. The geometry has a huge impact on the radar observability of a ship/plane. If you go looking, you'll see that the SR-71 is somewhat stealthy even though no radar absorbant materials were used in the skin (and likely couldn't be used.) All stealth in that airframe is just from the shape of the plane, not the skin materials. Apr 20 '16 at 15:14
• @Green Excuse me, I wasn't criticizing its inclusion but adding to it. I think it's a great part of the answer for exactly the reasons I gave: shape (plus a lot of math), not materials or paint, is the primary factor in stealth. And it comes at the expense of aerodynamics. Apr 20 '16 at 17:36
• If this is for a super-secret billion dollar delivery, being invisible to radar is just a very dangerous way to draw attention. It's safer to accept the tickets, or even - gasp - obey traffic laws for a while. Ya know, keep a low profile. Apr 20 '16 at 18:45
• @EmilioMBumachar One law at a time. Only ever violate one law at a time. It's really that simple. If you are transporting contraband, don't draw attention to yourself by driving at twice the maximum allowed speed past a police officer. If you have to drive at twice the maximum allowed speed past a police officer, at least don't transport contraband.
– user
Apr 20 '16 at 21:30

In short attempting to do that would be rather impractical.

The stealth aircraft reduce their radar cross section primarily in two ways: shape that reflects the radar waves away from their origin and radar absorbent materials. Those measures aren't perfect but they reduce the target's visibility to radars, which makes it a lot harder to acquire and maintain lock on it, which is primarily useful for avoiding stationary SAM sites. However if the airplane passes too close or is poorly oriented it can still be locked on and shot down. Which is why it is not a very practical solution for a car, as the car will be radared from very close range where even minor imperfection can give sufficient return, and the cop radar doesn't need to maintain lock for 30 seconds to tell speed.

Conclusion: you'd be much better off jamming or spoofing it (which is actually not that difficult in real life), rather then bother with stealth.

Since you can be tracked by visual means (as mentioned in other answers), you really need to think about a different means of avoiding detection: metamaterials.

Metamaterials are engineered substances which can refract light or other energy in directions that the engineer desires, rather than the direction that happens naturally (much like a pencil in a glass of water):

Since refraction depends on the wavelength of the incoming energy, metamaterials designed to refract light will be different from metamaterials designed to refract radar. Metamaterials can be designed to cloak submarines from sonar, and even theoretically refract shockwaves from earthquakes around buildings (on that scale they would look like an irregularly spaced group of pilings driven into the ground around a building).

With proper design, metamaterials could refract light around an object in such a fashion that the observer isn't even aware of the object, there will be no "hole" in their line of sight.

The downside of metamaterials is that they are tightly tailored to the wavelength of the energy being refracted. A sonar cloaked submarine will still be visible to the naked eye and radar on the surface, unless there are several layers of metamaterials overlain on each other. How they would interact is an interesting question, and to my knowledge, no one has done research on this to date. As well, most metamaterials utilize a technique called an "optical lattice", so the tiny spacing in the lattice are vulnerable to damage or being filled with dirt. This means unless there is very careful maintenance and driving, the benefits of having a metamaterial coating will be negated quickly

Optical lattice

The irony is in order to best preserve your metamaterial coating you should drive according to the rules of the road, avoid speeding or otherwise putting additional stressors on your car that could damage or degrade the coating.....

• The downside of metamaterials is that they are tightly tailored to the wavelength of the energy being refracted. I would actually call that an advantage. Driving a car that other drivers can't see would be a very bad idea. (Even if you can avoid them; you're not always in a situation where they can avoid you; you'd get rear-ended at the first stop sign or red light you came to, for example.) Apr 20 '16 at 20:36

I remember reading an article about this in Car & Driver about 1990. They said the best production car for this was a dark blue Ford Probe with no front license plate and the headlights down. They sighted minimal vertical surfaces on the front and a dark color.

Now that was for laser detection because that was the new thing. And the car was not invisible, but the detection range was reduced to 400' instead of a standard 800', enough time to visibly notice the officer.

The project car of a Camaro website used radar absorbent material (RAM) to make it stealth. Metal fenders and hood were replaced with fiberglass units with RAM beneath it. Lots of details are covered: http://www.camarotech.com/rcs.html

It is all based on passive absorption of microwaves.