Application of Wave Technology in Military Aspect

“Hide and Seek” in the World of Wave

Student name: Choi Ho Lam

Thank you for giving this valuable chance for me to investigate on this topic. As this topic involves military technology, it gives a deeper understanding of wave principle and usage in daily life. However, in normal settings, such kind of topic is not included, as it can be quite controversial. Fortunately, in this CCST course, I am allowed to choose this topic and enrich my engineering knowledge to a deeper extend.

National geographic

After watching the show “Hilter’s Stealth Fighter”, it gives me a clearer direction on starting the research on this topic. It enriches my knowledge on the use of RAM in relation with RF wave properties.

Teaching assistants of the course

The teaching assistants in the course are helpful. Their opinions help me to narrow my direction in investigating to specific topics like radar and specific technology related to radar. Without their help, I would still wonder in the wild world of military and aimlessly searching for a suitable topic.

2. Introduction

Radar detection and stealth technology are like a game of “hide and seek” with the application of wave technology. They share the opposite aims, either finding one or avoid from being found. In the world of military, this is a game that changes the fate of the world and losing is not an option.

During the World War II, the deployment of radar in British had changed the tide of battle in the English Channel. Fighting against 2550 German fighters and bombers, the Royal Air Force (RAF) was able to attack strategically and stopped German invasion with just 1963 planes. After this battle, British served as the last stand among European nations against Nazi Germany.

With such important history, the military value for radar as well as its counterpart, stealth technology were deeply investigated. Nazi Germany had suffered from a direct defeat from this battle. Since then, Nazi Germany carried out various researches on stealth aircrafts. Just one month near the end of WWII, the first prototype stealth bomber, Horten Ho 229 was developed. Recent research from National Geographic show that such plane decreases the detection range of British radar by 37%, which is already enough for wiping out the entire RAF. It was so fortunate that the plane wasn’t in mass production or the course of history would have been change completely.

This paper aims at studying the application of wave technology in the use of radar, as well as the principles in some of the counter radar measures. Since it is a CCST coursework, some of the complicated mathematical procedures and modern method of radar jamming would be omitted. Nevertheless, by studying such technology, we can establish a better understanding of different wave phenomena and know how they change our life.

3. Content

3.1 Radar

Radar stands for Radio Detecting and Ranging. In general, radar detects a far object by sending a pulse of EM wave and measures the time and frequency for the reflected pulse. By analyzing these factors, the speed and the distance of the far object can be found.

In a macroscopic view, radar consists of 3 basic parts, a transmitter, a duplexer and a receiver.

The transmitter emits a short-duration high-power radio frequency (RF) pulse. The pulse will travel through the transmitter antenna and emit outwards. The antenna can give appreciate space for efficient distribution of RF pulse.

When the RF pulse hit an obstacle, the RF pulse is reflected back to the receiver. The receiver antenna will receive the signal and analysis the time of travel as well as the frequency of reflected pulse.

Since the receiver antenna is very sensitive as it is assumed to be receiving the small pulse reflected back from far distance, a duplexer is installed to switch the between two antenna. This can ensure only one antenna is in use and prevent receiver antenna receiving signal directly from the transmitter antenna, which can potentially damage the receiver antenna.

In a microscopic view, an alternating current (AC) is supplied to the antenna. It then produces a perpendicular alternating magnetic field around the antenna.

By Lenz’s Law, for a closed loop carrying a current, it would produce a magnetic field perpendicular to current around the loop. In reverse, for a closed loop experience a change in magnetic field, a current perpendicular to magnetic field is generated to oppose the change.

The combination of alternating current and magnetic field produces the electromagnetic wave that used for detection. The frequency of the EM wave can be changed by modifying the AC frequency. The frequency of the electromagnetic wave is usually set to between 3 kHz to 300 GHz at radio frequency (RF). It varies depends on the usage. With a higher frequency, using relation with speed of light (C) and wavelength (λ) C=Fλ, the wavelength of RF wave is shorten. This decreases the electrical length of the antenna to and makes it easier to build.

In military use, the RF pulse would set to lower frequency portion in the RF spectrum, usually between 7 to 11.2 GHz, which is called the X band. Being in RF spectrum, the high frequency radar pulse can create a radar cross-section of aircraft with a higher resolution, which helps identify aircraft more accurately. In more advance use, a much lower frequency is used to reveal stealth aircraft since it can give a much more sensitive signal. With a low frequency, the wavelength of radar pulse is comparable to size of stealth fighter and creates a scattering in resonance region instead of optical region, turning it more visible on radar. However, the frequency of the radar pulse cannot be lower than 800 MHz, otherwise the EM pulse would have insufficient penetrating power and easily be absorbed by raindrops. Other than that, the resolution of low frequency radar is relatively poor, and gives a poor display. It is only effective against stealth aircraft of length comparable to wavelength.

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When the radar pulse is bound back to the radar, the antenna experience an alternating change in magnetic field. This will produce an alternating electrical field and thus an AC current to flow in circuit. Since the returning signal is weak, it would pass through an amplifying circuit to increase the amplitude.

In analyzing the signal, the frequency and time of travel would be taken to find the speed and velocity of detection target. When the radar pulse reaches an object, it is reflected and the frequency is shifted by Doppler Effect.

Since the speed of RF wave is always the speed of light (C), the distance (S) can be express as a time (t) function,

S = Ct/2.

It is divided by two since the radar pulse travel through the same distance twice.

To calculate speed of moving object, we can find out the change in frequency (Doppler frequency) of the returning wave. If the far object is moving, the object would create a Doppler shift to the radar pulse. This Shifted frequency () can be expressed by original frequency (), the moving speed (V) and speed of light (C).

,

which gives us the Doppler frequency () .

For convenience, one assume, such that, and simply the equation as

.

3.2 Radar jamming and deception

Radar jamming and deception usually refer to the intention of saturating the radar with noise and false information, lowering the detection range of radar as well as the accuracy of the radar. There mainly two types of jamming, either mechanical jamming or electronic jamming.

Mechanical jamming

Mechanical jamming usually refer to use of physically existed thing to reflect and re-reflect RF signal back to radar, causing the radar to be saturated with false return signal.

The commonly used mechanical jamming methods that adopt wave principles are chaff and corner reflectors.

Chaff is bundles of reflective aluminum strip of various lengths. The aircraft drops these strips in air. When radar pulse reaches these strips, the radar pulse is reflected and scattered in all direction, creating huge return signal to the receiver. This could deceive the radar as “seeing” huge plane and deplete enemy resources like ammunition on shooting down the false target.

Corner reflector reflects incoming radar pulse at other angle. Corner reflector is composed of either two or three reflective planes, perpendicular to each other. This configuration allows the incoming radar ray fully reflected back to the source. Yet this also increases special occupation of corner reflector, making it harder to carry by aircraft. The tails of the stealth fighter are designed to be acute angle, such that it avoids forming corner reflector, allowing radar to identify the airplane.

Electronic jamming

Electronic jamming refers to the active emission of RF pulse to the radar, saturating the radar with false return and noises. It includes spot jamming, sweep jamming, barrage jamming, cover pulse jamming and digital radio frequency memory jamming.

Spot jamming, sweep jamming, barrage jamming uses the same principle, occupying the radar with high energy pulse and causing destructive interference of the RF wave.

Spot jamming refers to jamming one specific frequency with high energy signal.

Sweep jamming refers to jamming one specific frequency with high energy signal when the radar is switching from one frequency to another.

Barrage jamming refers to jamming all specific frequency with high energy signal, however since it cover full range of frequency, each frequency are partially jammed.

Cover pulse jamming refers to false return signal as noise. As radar receives certain amount of ambient noise, for example, like scanning through eagle and other types of bird, so it could be used as disguise for fighter to hide from radar. With pulse jamming system installed, the aircraft analysis the incoming radar signal and find suitable frequency for false noise return.

Digital radio frequency memory is a repeater technique that analysis the incoming radar pulse and changes the returning frequency as well as time of return. The signal would be delay and diminish. This gives a false sign of far distance between aircraft and radar. Moreover, the frequency would also be shifted and produce a false Doppler shift on radar signal. This returns with false information about the velocity of aircraft.

3.3 Use of RAM

Besides of active jamming and deceiving radar, the military develop RAM to avoid radar detection from the enemy.

RAM stands for the Radar-Absorbance Material. According to the detecting radar frequency, different types of RAM are used to build stealth aircrafts.

Iron ball paint absorber

Iron ball paint is one of the mostly used RAM. It contains tiny spheres coated with carbonyl iron and ferrite. As radar waves induce molecular oscillation in this paint, the energy would dissipate as heat. This can absorb the radar and reduce the reflection of radar wave.

Re-entrant triangles

The structures of re-entrant triangles in between the surface foam absorber layer help reduce reflection of radar ray back to radar transceiver. Inside the foam absorber layer, the foam was cut into tiny square pyramid. This structure reflect incoming wave several times and reduce the energy of the radar wave.

In practice, the width and height would change according to the incoming radar wave. The main purpose of adjusting height and width ratio is to achieve maximum internal reflection within the plane surface as possible.

Jaumann absorber

The wave principle deployed by Jaumann absorber is destructive interference. Gerneally, Jaumann absorber consist of two equally spaced layer and a ground metal plate. It cancels out the incoming wave by producing destructive interference.

When a radar signal strikes on to the surface of Jaumann absorber, the wave split in to two waves, one is reflected at glossy surface of the Jaumann absorber, while the other enters the layer. The ray will then reflect back by the metal ground plate. The two layers having a total spacing of λ/4 will allow the wave reflected back to surface at anti-phrase compare to incoming ray, where the ray have travelled a total of λ/2 distance.

Conclusion

Radar principle make use of various wave property in tracing the enemy. In engineering, there is a sub-discipline for radar analysis, for a more precise interpretation of radar signal. This paper includes only few example and it was just a corner of iceberg. However, in just a brief analysis, we can see advancement of technology as people know more about wave and using this knowledge to build stealth aircraft. This game of “hide and seek” can be devastating to world as it brings a much higher casualties in war, however this game has also proven to be saving life war and pushing the advancement of technology.

Would this game be beneficial to human? It would be an unanswered question waiting for the people to decide.

Reference

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