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Stealth aircraft and radar

by michaellross
Tags: aircraft, radar, stealth
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michaellross
#1
Oct16-03, 04:26 PM
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I am trying to gain an understanding of how Stealth technology defeats conventional radar, and wondering about strategies to get around it. In addition to explaining the basics of the Stealth technology, can someone explain why the planes are not simply detected by infrared sensitive devices? Thanks.
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russ_watters
#2
Oct16-03, 04:36 PM
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With regard to radar, there are two components to stealth - absorption and deflection. Through the shape of the object, it is possible to deflect the radar waves away from the source. And with special materials (iron ferrites generally) it is possible to absorb a large portion of the radar.

For infrared, certainly it is another possible detection mode. Stealth planes also generally employ infrared suppression by mixing outside air with their exhaust streams. They also fly (mostly) subsonic to keep the skin temperature low. Infrared is also not good for long range detection anyway - all you really have to worry about is defeating shoulder fired missiles.
turin
#3
Oct16-03, 08:51 PM
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I heard that there is a coating on the aircraft that has a smooth gradient in its intrinsic impedance from 377 ohms (the intrinsic impedance of the air) on the surface to very low at the boundary of the metal. This is supposed to reduce the reflection of radar.

Fuego
#4
Oct17-03, 12:42 PM
P: 43
Stealth aircraft and radar

radar cross section (RCS) of an aircraft is determined by two major factors: the shape of the aircraft, and the electromagnetic properties of the aircraft materials. stealth aircraft are coated in radar absorbing material. i don't really know exactly how this works but it greatly reduces the intensity of radar reflections.

aircraft shaping is effective over a wide range of radar frequencies but only a limited range of aspect angles to the radar. the aircraft can be shaped to ensure most radar waves will be scattered and not reflected back to the transmitter. leading edges and trailing edges of wings, control surfaces, inlet doors/gaps etc can all be aligned to ensure that any radar waves that are reflected, are reflected in a one or two precise directions. this means that if the aircraft is oriented in precisely the right direction to the radar, the radar will detect it easily, but in all other directions, the radar will not detect it.

there are two methods used to limit RCS through the aircraft's shape. the first is known as faceting. in this case, the aircraft's outer surface is made almost entirely of flat surfaces (e.g. the F-117, the most well known stealth aircraft). faceting limits the number of normal reflections back to the radar.

in the second method, the aircraft's outer surface is smooth, and has a continuously varying curvature (e.g. the B-2 bomber). this essentially achieves the same affect as faceting, but requires much greater computational power to predict the correct curvatures.

another major contributor to RCS is the compressor blades of the engine. radar waves hitting compressor blades head on produce a very strong reflection. in stealth aircraft, the front of the engine is hidden from the view of radar by using a slightly s-shaped air intake.

there aren't many strategies against stealth technology. more powerful radars are one option, but the increase in power would have to be massive. it is believed that the latest russian SAM systems employing multiple high-powered radars can get significant reflections off F-117s at close ranges. the F-117 is, after all, 30+ year old technology.

IR signatures can be supressed with special engine nozzles that disperse the heat and materials that do not radiate strongly. IR signatures can only be detected at relatively short ranges. the idea of a stealth aircraft is that it will be able to destroy any threats from long range without being detected, thus keeping out of range of IR detection. if a stealth aircraft does enter IR detection range, it comes under great threat. modern IR guided missiles will still lock easily onto the jet pipes of stealth planes.
Pandemonium
#5
Oct18-03, 07:52 AM
P: 29
Is that why when stealth bombers start dropping bombs, it becomes easily detectable by radar?
Fuego
#6
Oct18-03, 09:05 AM
P: 43
all true 'stealth aircraft' (F-117, B-2, F-22, F-35) lose a lot of their stealthiness when they deploy munitions. All those aircraft carry their bombs and missiles internally, and weapon bay doors have to be opened to release them. when the doors open, the stealthy shape of the aircraft is essentially ruined; radars get relatively huge reflections from opened bay doors.

However, most of these aircraft are able to deploy their munitions from very long ranges to the target using stand off weapons (e.g. cruise missiles). the radar signature of a weapon bay door is pretty damn small from 100+ miles away.

of course, once the weapon is deployed, the doors shut and the aircraft becomes stealthy again. a radar signature is only useful for target aquisition if it lasts for a considerable period of time.
Andre
#7
Oct18-03, 10:08 AM
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IR detection is usually exagarated. IR is heavily dependant on atmospheric conditions. It behaves much more like light than like Radar radiation. Therefore the range is limited and clouds and haze degrade it severly. Most IR guided missiles are short range because of that problem. Air to Ground IR missiles don't reach very high and if an air defence fighter gets close to stealth aircraft, IR detection is not your main problem.

Radar stealth is also frequency dependent. Very low frequency radars, (the giant ones) are less distracted by stealth but they have several other problems.

Another trick is parasite radiation. If you know the overal electromagnic radiation pattern in a certain area, you could detect changes in them when pieces of metal disrupt that pattern. Works just about the same as the security ports at the airfield check in. However, continious changing electromagnic jamming should take care of such a trick.
Fuego
#8
Oct18-03, 10:20 AM
P: 43
Originally posted by Andre
IR detection is usually exagarated. IR is heavily dependant on atmospheric conditions. It behaves much more like light than like Radar radiation. Therefore the range is limited and clouds and haze degrade it severly. Most IR guided missiles are short range because of that problem. Air to Ground IR missiles don't reach very high and if an air defence fighter gets close to stealth aircraft, IR detection is not your main problem.
modern IR systems are a lot less dependant on atmospheric conditions. also, modern IR missiles can have pretty good ranges (10 miles head on for a russion archer missile).

IR sensors are used in air to surface missiles (e.g. the D-model maverick) for the precise purpose that they can see through haze and dust clouds.

i would say that if an air defence fighter gets close enough to a stealth aircraft, IR detection is the main problem. radar guided missiles probably won't be able to get a lock on a stealth aircraft, while IR missiles will.
Andre
#9
Oct18-03, 01:28 PM
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Hey, we're talking detection, not tracking and locking. Detection is something you want to do at 100 miles range. If you did not detect anything at 10 miles (Archers range, you're history)
Fuego
#10
Oct18-03, 03:37 PM
P: 43
Originally posted by Andre
Hey, we're talking detection, not tracking and locking. Detection is something you want to do at 100 miles range. If you did not detect anything at 10 miles (Archers range, you're history)
ah, right. i thought you were talking about an air defense fighter getting into close range with a detected stealth aircraft. if the stealth aircraft isn't detected in the first place then you're certainly right.
Andre
#11
Oct19-03, 09:41 AM
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Right,

There is much theory building up about Air power. Stealth is important in more than one aspect. Vulnarability is one, but keeping the surprise is the more important one, as well as freedom of operations and freedom of planning the air campaign just the optimal way without having to deal with opposing forces.
S = k log w
#12
Nov17-03, 04:18 PM
P: 65
Would a stealth plane lower it's detection vulnability by deployment of (projectile weaponry) from the top of the plane (with respect to ground) so that weapons doors are less likely to be detected? Also would the radar profile of a projectile weapon be less if that weapon was fired, for example, a missle, 'up' in relation to the ground bsed radar antenna? Could, in theory, a radar signal be not absorbed, but rather dispersed, and a transmitter on the plane send a signal to the ground that would be shaped/pulsed to make the ground based radar appear as if the stealth plane were not there?
Soveraign
#13
Nov17-03, 05:17 PM
P: 54
Originally posted by S = k log w
Would a stealth plane lower it's detection vulnability by deployment of (projectile weaponry) from the top of the plane (with respect to ground) so that weapons doors are less likely to be detected? Also would the radar profile of a projectile weapon be less if that weapon was fired, for example, a missle, 'up' in relation to the ground bsed radar antenna? Could, in theory, a radar signal be not absorbed, but rather dispersed, and a transmitter on the plane send a signal to the ground that would be shaped/pulsed to make the ground based radar appear as if the stealth plane were not there?
The idea of launching something from the top seems to be logically harder to see from below, but who knows what sort of engineering nightmare that would be.

WRT the pulse thing: You are correct that an aircraft can send fake signals to a radar system to confuse the tracking. But with most counter-measures, there has been a counter-counter-measure figured out. In this case, things like frequency hopping are effective.
Tyro
#14
Nov17-03, 10:19 PM
P: 106
About defeating stealth technology:
As Russ_Watters pointed out, most contemporary stealth technology relies on absorption or deflection of oncoming EM radiation. Once countermeasure that was (quite unexpectedly) discovered was to use this very property to detect stealth aircraft.

A simple example: Say you have a source A and a receiver B. If a stealth aircraft flies in between them, the reception at B is temporarily cut off. But it isn't supposed to be (if all disturbances are accounted for). So you conclude that there is something there, and at times of war (especially if you're facing the Americans ) the safe bet is that it is a stealth aircraft there.

Passive IR sensors for limited detection and/or tracking has already been discussed.

Another possibility is using roving detectors to continually search for stealth aircraft. Basically, stealth aircraft are not equally stealthy in all directions. The flight path of stealth aircraft often has to be meticulously planned to ensure that the radar cross section to known radar stations is always minimised. One option to this effect is AWACS-type planes. Another would be Unmanned Aerial Vehicles (UAV's) which loiter over an area with a random flight path and continously search for targets. UAV's are already routinely used for reconnaisance missions, although of a typically different sort.

Another possibility is to use focused beams to detect stealth aircraft, effectively 'boosting' the power of the radar by simply increasing its focus. Basically, you can increase the W/area by increasing the focus. A poor man's equivalent of a powerful radar. While their obvious disadvantage would be a tiny sweep area, most stealth aircraft are slow and unmanoeuverable. Rapidly sweeping the skies and/or pulsing the beam may solve this shortcoming.

Human eye, Mark I. Stealth aircraft may be (at times) invisible to radar, but they can still be spotted. That is why stealth missions are restricted to the night, when the black coat blends in with the sky. For various reasons, a white or sky-blue painted aircraft is still partially visible to observers during the day. Work is currently underway in developing actual "shiny" aircraft (complete with light kits!) to blend into the daylight sky better. Successful implementation on stealth aircraft will enable them to fly both day and night operations.

Originally posted by S = k log w
Would a stealth plane lower it's detection vulnability by deployment of (projectile weaponry) from the top of the plane (with respect to ground) so that weapons doors are less likely to be detected? Also would the radar profile of a projectile weapon be less if that weapon was fired, for example, a missle, 'up' in relation to the ground bsed radar antenna? Could, in theory, a radar signal be not absorbed, but rather dispersed, and a transmitter on the plane send a signal to the ground that would be shaped/pulsed to make the ground based radar appear as if the stealth plane were not there?
The reason why planes tend to drop-deploy their weapons is because, like all things, sometimes mistakes happen. Missiles on aircraft pylons, for example, typically have separator charges which go off to separate the weapon from the plane before the rocket motor engages. If you get poor separation (perhaps because of a dud charge) it is not such a big deal. But if you mount it over the top of the aircraft, the last thing you want is (literally) a live weapon going *clunk* on your head!

Basically, a radar will say "nothing's there" if it gets no return signal, or a known return signal (if there are known obstructions in the area). But to address the question, such a setup will require very good sensors, a very fast computer and accurate transmitters. Is it really worth it?
NateTG
#15
Nov17-03, 11:59 PM
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One potentially effective tracking method is to set up a network of low frequency microphones over a large reason, and then listen for the airplane.

For various reasons, stealth planes fly low & relatively slowly. This means that they would be relatively vulnerable to that type of detection. In addition, it's passive, so it's hard to identify.

Depending on the speeds and altitudes involved, you can probably pin the plane down within a quarter mile or so which is probably small enough to put up a flare (gotta love WWI tech.)

FYI it's possible to track and identify helicopters using conventional seismometers.

Regarding weapons: If you're feeling insane, you could also tow the weapons behind the plane.
Tyro
#16
Nov18-03, 06:53 AM
P: 106
Originally posted by NateTG
One potentially effective tracking method is to set up a network of low frequency microphones over a large reason, and then listen for the airplane.

For various reasons, stealth planes fly low & relatively slowly. This means that they would be relatively vulnerable to that type of detection. In addition, it's passive, so it's hard to identify.

Depending on the speeds and altitudes involved, you can probably pin the plane down within a quarter mile or so which is probably small enough to put up a flare (gotta love WWI tech.)

FYI it's possible to track and identify helicopters using conventional seismometers.

Regarding weapons: If you're feeling insane, you could also tow the weapons behind the plane.
Interesting proposals. But newer generation stealth planes like the F-22 can fly at Mach 1.8 - faster than the speed of sound. As a point of interest, the Eurofighter is a Mach 2.0 capable fighter; the JSF Mach 1.8, Mig-31 Mach 2.3, Mig-25 (B-variant) Mach 3.2. As such, acoustic detection methods will have rather delayed latencies. The stealth aircraft can also compensate by using longer range weapons or gliding (not that they will probably be very good at it, but it is an option) when they are very close to the target.

Towing weapons will have an aerodynamic penalty, and they can still be detected (this is why internal bays are used in stealth aircraft). You also don't want them bumping into each other if 2 or more weapons are on different cables!

Of course, there is rarely a single 'best' solution and a wide repertoire is often the best approach.

You don't have to get a strong detected signal to attack a stealth aircraft. You could do it Baghdad-style (just fire like crazy into the sky ) or perhaps more effectively, if you know there is a stealth aircraft in the vicinity, fire an IR missile 'dumb' at the general vicinity and hope it gets a lock.
NateTG
#17
Nov18-03, 12:28 PM
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The idea was to have a large array of the microphones over a large area, and network them. It takes a bit of technology to make it all work, but it should be reasonably economical.

A crude calculation gives the location of the plane to within Av/m where A is the altitude of the plane, v is the velocity of the plane, and m is the speed of sound.

So,against an opponent at 50,000 ft, and mach 1.8 it's pretty useless. On the other hand, a B2 plane traveling at 200ft and mach 0.7 using ground following radar would be pinpointed to within less than 50 yards. (Provided there was a sufficiently dense microphone array.)

With a large (e.g. nationwide) array of microphones networked together, it should be possible to identify the heading and altitude of planes as well.

Naturally, this would require quite a bit of compute power, but the primary costs would probably be networking/communication.
russ_watters
#18
Nov18-03, 02:04 PM
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Originally posted by NateTG
For various reasons, stealth planes fly low & relatively slowly. This means that they would be relatively vulnerable to that type of detection.
Depends on what you mean by low and slow. We aren't talking A-10's at 5,000 ft and 300 kts. The F-117 has a top speed of 600 kts or so and ceiling of 60,000 feet. Its capable of fairly typical bomber flight profiles.
But newer generation stealth planes like the F-22 can fly at Mach 1.8 - faster than the speed of sound. As a point of interest, the Eurofighter is a Mach 2.0 capable fighter; the JSF Mach 1.8, Mig-31 Mach 2.3, Mig-25 (B-variant) Mach 3.2. As such, acoustic detection methods will have rather delayed latencies.
Sound isn't that much of an issue. Even a subsonic plane can't be heard until it has already flown over if it is high enough and fast enough. But supersonic flight poses a pretty major problem for stealth: heat. It makes infrared detection much easier.
You don't have to get a strong detected signal to attack a stealth aircraft. You could do it Baghdad-style (just fire like crazy into the sky ) or perhaps more effectively, if you know there is a stealth aircraft in the vicinity, fire an IR missile 'dumb' at the general vicinity and hope it gets a lock.
While true, thats actually counterproductive. Tracers point both up and down, and in Bagdhad in 1991, no stealth fighter was ever hit afaik.


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