PAF Strategy for using AWACS

[All-Green]

Yer welcome...I used that example only when my class were all men and when things were less 'complicated'. No man failed to grasp the basic concepts.

...very creative Gambit. You should write a book and for women in your readership you may substitute the playboy model with Gerald Butler or equivalent.

Well i must say Gambit your class must have been really attentive with all sorts of ISAR and SAR imaging in their heads...

Thanks for the detailed lecture, very useful explanation

On a serious note the more resolution we want the higher the frequencies we have to use which suffer much more from free space loss.
Practically a lot more transmission power is needed to cover the same range and obtain this additional information as opposed to using larger wavelength radar.
Doubling the frequency introduces an additional 6dB propagation loss which is a tremendous pathloss to compensate for especially considering the high power that radars operate at to achieve a significant range.

The power implication dictates that X-Band Radar would be best suited for tracking and distinguishing purpose via narrower beams and the wide beam coverage be provided by more conventional larger wavelength radars which direct the XBR coverage towards the acquired target.

 

[saurabh]

@gambit

X band radars operate on a wavelength of 2.5-4 cm and a frequency of 8-12 GHz. Because of the smaller wavelength, the X band radar is more sensitive and can detect smaller particles. These radars are used for studies on cloud development because they can detect the tiny water particles and also used to detect light precipitation such as snow.

Bit offtopic, but how do the x band detects awter particles, which are in general smaller than 2.5 cm?

 

[gambit]

@gambit


Bit offtopic, but how do the x band detects awter particles, which are in general smaller than 2.5 cm?

Actually...It is the K band that is most adversely affected by water, which is the most problematic natural substance for radar detection.

Radar Altimetry Tutorial - 3.3.1. Future altimetry techniques: Ka-band altimeter

The one major drawback of Ka-band is that attenuation due to water or water vapour in the troposphere is high. Rain cells --which are often dense and frequent in the Tropics-- will remain a constraining factor, since the radar wave can be attenuated by 2 dB in heavy rain. Typically, if the rain rate is higher than 1.5 mm/h, the radar echoes will be unusable (whereas at Ku-band, echoes are hardly affected at rain rates less than 3 mm/h). However, impact studies carried out on the basis of seven years of TMR data from Topex/Poseidon show that rain rates of over 1.5 mm/h only occur globally 10 per cent of the time. A Ka-band altimeter would therefore still be able to acquire measurements 90% of the time. If the satellite is on a sun-synchronous orbit, rain frequency will also have to be factored in (it rains most often in the Tropics between 6:00 and 12:00 a.m. and 6:00 and 12:00 p.m.). Conversely, this 1.5-mm/h threshold will also be likely to lead to more accurate mapping of rain cells over the ocean --one of the major remaining unknown factors in the global water budget-- and yield more reliable climatology data.

You must understand that raindrops do not fall with constant mass. See below...

EARTH Magazine: Raindrop study splashes old assumptions

The researchers discovered that when a large, or “parent,” raindrop fragments into multiple smaller, or “daughter,” raindrops as it falls, the daughters fall at their parent’s terminal velocity until they relax to their own slower terminal velocity. In fact, these smaller raindrops can move 10 times faster than expected,...

So when it comes to meteorological research, the K band is to be avoided except for specialized circumstances and because of how raindrops behave it is better to use freqs whose wavelengths are larger than the raindrops. In the case of the X band, whose wavelength are close to some raindrop sizes, and of the longer wavelengths, it can be difficult to distinguish individual drops, but then again, it is of specialized studies that we need to study individual drops anyway. When it comes to radar for weather research, it is more useful to detect raindrops as a 'volume' rather than as individuals. Each raindrop reflects a certain amount of the EM signal and these echoes together give us that colorful mass on the local news weather segment.

 

[saurabh]

Actually...It is the K band that is most adversely affected by water, which is the most problematic natural substance for radar detection.

Radar Altimetry Tutorial - 3.3.1. Future altimetry techniques: Ka-band altimeter

You must understand that raindrops do not fall with constant mass. See below...

EARTH Magazine: Raindrop study splashes old assumptions

So when it comes to meteorological research, the K band is to be avoided except for specialized circumstances and because of how raindrops behave it is better to use freqs whose wavelengths are larger than the raindrops. In the case of the X band, whose wavelength are close to some raindrop sizes, and of the longer wavelengths, it can be difficult to distinguish individual drops, but then again, it is of specialized studies that we need to study individual drops anyway. When it comes to radar for weather research, it is more useful to detect raindrops as a 'volume' rather than as individuals. Each raindrop reflects a certain amount of the EM signal and these echoes together give us that colorful mass on the local news weather segment.

I was talking about individual raindrops only.
Raindrops that we see at ground level are <~2.5 cm
How can a radar detect individual drops with size smaller than wavelength (x-band ~ 2.5-4 cm)? Or do the raindrops have larger size in the upper atm? You are saying its difficult, while my understanding says not possible.

 

[gambit]

I was talking about individual raindrops only.
Raindrops that we see at ground level are <~2.5 cm
How can a radar detect individual drops with size smaller than wavelength (x-band ~ 2.5-4 cm)? Or do the raindrops have larger size in the upper atm? You are saying its difficult, while my understanding says not possible.

I will attempt to clarify your question:

"How is radar detection possible when the object's physical dimensions is overall smaller than the signal's wavelength?"

Easy...

Shortwave - Wikipedia, the free encyclopedia

Shortwave radio refers to radio broadcasts on a portion of the radio spectrum in the frequency range of 3,000&#8211;30,000 kHz (3&#8211;30 MHz). Because smaller wavelengths correspond to higher frequencies (given the inverse relationship between frequency and wavelength), shortwave radio received its name because its wavelengths are shorter than the longer wavelengths used in early radio communications. A longer wavelength example is the medium wave AM broadcast band: 1 MHz = 300 meters. An example of shortwave would be 10 MHz which is 30 meters. HF (high frequency) radio is an alternative name for shortwave radio.

The only difference between a communication signal and a radar detection signal is the content which of course the radar signal have none. Radar detection was somewhat 'invented' when it was noticed that communication signals became distorted, echoed or even lost when there are many structures around. Eventually the focus was on the 'echo' part when it was called 'ghosting', then the content of the signal was eliminated and the result was the exploitation of a body's reflectivity to create 'radar detection'.

Meters length signals have been used to detect many smaller objects such as an aircraft or even a human body...

Chain Home - Wikipedia, the free encyclopedia

The Chain Home stations were designed to operate at 20-50 MHz, the "boundary area" between high frequency and VHF bands at 30 MHz, although typical operations were at 20-30 MHz (the upper end of the HF band), or about a 12 metre wavelength.

Chain Home and Chain Home Low were the secrets of the famous WW II Battle of Britain. Note the frequency and wavelength.

Heinkel He 111 - Wikipedia, the free encyclopedia

# Length: 16.4 m (53 ft 9&#189; in)
# Wingspan: 22.60 m (74 ft 2 in)
# Height: 4.00 m (13 ft 1&#189; in)

Now as you can see, the intended target at that time was a symmetrical but irregular body. There are parts of the aircraft where it is less than, quite equal to, and longer than the transmitting wavelength. The He 111 was very detectable from any angle, even from the smallest aspect angle -- front.

1- When the target is smaller than the transmitting wavelength, the target's RCS is said to be in the Rayleigh region. The body's RCS is less than its actual physical dimensions.

John Strutt, 3rd Baron Rayleigh - Wikipedia, the free encyclopedia

John William Strutt, 3rd Baron Rayleigh OM (12 November 1842 &#8211; 30 June 1919) was an English physicist who, with William Ramsay, discovered the element argon, an achievement for which he earned the Nobel Prize for Physics in 1904. He also discovered the phenomenon now called Rayleigh scattering, explaining why the sky is blue, and predicted the existence of the surface waves now known as Rayleigh waves.

2- When the body is within about &#37;10 in dimensions of the transmitting signal, it is said to be in the resonance region. Its RCS tends to be larger than its physical dimensions. If the body is irregular, like an aircraft, then the RCS signal would fluctuate.

3- When the body is larger than the transmitting signal, it is said to be in the optical region. Its RCS is much more closer to its true physical dimensions. This is why the X band is so popular from air traffic controls to missile seekers. And why US 'stealth' aircrafts are designed with the X band in mind.

So...Can the X band detect something smaller than its wavelength, like a raindrop? Yes it can and precisely because a raindrop's physical dimensions is not constant throughout its fall, the raindrop's RCS will wildly fluctuate throughout its journey.

 

[sonicboom]

Sorry for going off topic but its necessary for you to understand.

Air enthusiast and Air strategists maintain that air power is intrinsically offensive in nature and its primary mission should be the destruction of the enemy through aerial bombardment. Air power in the defensive role is acknowledged only to the extent that it is necessary to blunt the enemy&#8217;s air assaults before launching ones own. Logically then, strike missions should occupy the highest priority. Yet in reality this is not true. Air combat missions involving duels between opposing aircraft attempting to shoot each other down in an effort to win control of the air has traditionally occupied the top slot in the various roles that air power is expected to perform. To perform any of the assigned roles of air power, winning control of the air is an essential pre-requisite. While enemy air power can be blunted in many different ways, dedicated air superiority fighters remains the most effective defence against enemy air assaults. When air power of a nation is to operate against an adversary that also possesses air power assets, the first priority of air missions has to be neutralization, or at the least, sufficient degradation of the opponent&#8217;s air power capability.
An offensive counter-air operation that focuses attacks on air infrastructure including aircraft inventory becomes imperative. The other side will attempt to counter it though their air defence network in which air superiority fighters form a key element. Air combat then is inevitable and it plays a vital role in determining if the offensive side has managed to establish control of the air or the defenders have successfully denied it to the aggressors. For both the contestants air combat becomes a central cohesive source of support and stability.

For complete article, please refer to the link:

Excellence in Air Combat: PAF&#8217;s Forte

Air Commodore (Retd) JAMAL HUSSAIN discusses PAF&#8217;s strong point.

 

[saurabh]

@gambit

Can you elaborate resonance region?
Actually I am interested in this part

When the target is smaller than the transmitting wavelength, the target's RCS is said to be in the Rayleigh region. The body's RCS is less than its actual physical dimensions.

2- When the body is within about %10 in dimensions of the transmitting signal, it is said to be in the resonance region. Its RCS tends to be larger than its physical dimensions. If the body is irregular, like an aircraft, then the RCS signal would fluctuate.

Point 1 says that when target is smaller than wavelength, its RCS is smaller than physical dimension, but point 2 says that when target is 1/10 of the wavelength, RCS is larger than physical dimension.

Both points seem contradictory, unless there is irregularity near the 1/10 region.

surface waves now known as Rayleigh waves.

Also, can you give details on this?

I have worked on increasing absorption by decreasing of target size (though in different region).
I am interested to know if absorption can be decreased.

PS: If I am ruining the thread, plz PM me.

 

[gambit]

@gambit

Can you elaborate resonance region?
Actually I am interested in this part

When the target is smaller than the transmitting wavelength, the target's RCS is said to be in the Rayleigh region. The body's RCS is less than its actual physical dimensions.

2- When the body is within about &#37;10 in dimensions of the transmitting signal, it is said to be in the resonance region. Its RCS tends to be larger than its physical dimensions. If the body is irregular, like an aircraft, then the RCS signal would fluctuate.

Point 1 says that when target is smaller than wavelength, its RCS is smaller than physical dimension, but point 2 says that when target is 1/10 of the wavelength, RCS is larger than physical dimension.

Both points seem contradictory, unless there is irregularity near the 1/10 region.

Not contradictory at all and this is where you misunderstood me about that 'target is 1/10 of the wavelength' bit. I am saying that target 'electrical path' and the impinging wavelength is roughly within %10 of each other, not that the target, be it a sphere or a raindrop or an insect, is one-tenth in size compared to the wavelength.

First...Keep in mind that radar detection is essentially about perception based upon target characteristics. The advantage that radar detection have over other methods of target detection is that the user control the medium from which the target must respond, in other words, every target reflects some amount of energy. For infrared detection, the user or seeker, does not transmit an 'IR wave' at the target and read the return. The seeker must rely upon the convenience of the target to emit some IR energy.

Second...The perception formulation is no different than how you would formulate a perception of a new acquaintance, like in a date, for example. The formulation is based upon what you received in that meeting, from how was the handshake to the smile or other signs in body language. For radar detection, the formulation is mathematically quantifiable.

Third...The three regions: Rayleigh, resonance and optical, are like those body language signs and they come from how a wave behave on a sphere, an 'ideal' object, which is used as a reference in RCS modeling.

Fourth...There is a 'shadow' region in a monostatic configuration. A monostatic system is the most commonly deployed in the world.

radar shadow: Definition from Answers.com

A region shielded from radar illumination by an intervening reflecting or absorbing medium such as a hill.

You will have to excuse my pathetic 'artistic' skills here.

The side that is not illuminated by the seeker is the 'shadow' region as illustrated above.

Fifth...Any reflection that is facing the seeker, up until the 90deg point, is called a 'backscatter' as illustrated above. Any reflection beyond that 90deg point is called a 'forwardscatter'. In a monostatic configuration, only 'backscatter' reflections are useful in RCS calculations.

Upon impacting the surface of the sphere, there is an immediate amount of reflectivity back to transmit direction. This initial reflection is called 'specular' reflection. The rest of the signal then travels on the sphere's surface. At this point the sphere's diameter comes into play regarding wavelength. The sphere's diameter is called the 'electrical path' and does have a length. As the signal travels on the surface in the 'creeping wave' effect, minute amount of the signal's energy reflects off the surface, aka more 'specular' reflections.

If the sphere's diameter is smaller than the impinging wavelength, after the initial 'specular' reflection, there is a 'creeping wave' behavior and most of the signal will complete its travel on the sphere's surface, reform on the shadow region and become a 'forward scatter'. All the seeker radar has are the few 'backscatter specular' reflections to formulate a perception of the target's physical dimension. This is why target RCS in this ratio is less than what the target actually look like -- size wise. The target is said to be in the 'Rayleigh' region. If we have meters wavelength hitting a raindrop or an insect neither would show up at all.

If the sphere's diameter is the same as the impinging wavelength, once the wave finally completed its travels around the sphere and the 'creeping wave' meets on the back side, aka the 'shadow' region, we have lost not as much of the signal to 'forwardscatter' as when the sphere was smaller than the wavelength. The 'resonance' effect is the result of in-phase and out-of-phase interference between the many 'specular' reflections and the 'creeping wave' behavior, hence, target RCS from this 'electrical path' versus wavelength relationship varies a great deal and generally indicate a target RCS that is larger than its physical dimension. This in-phase and out-of-phase interference has two results: constructive and destructive, and it does exist in the above Rayleigh region example as well, except that because the above example has the sphere's diameter smaller than the impinging wavelength, the 'creeping wave' interference is usually destructive. As sphere diameter increases, then we approaches the resonance region.

If the sphere is large enough, in other words the 'electrical path' is greater than wavelength, then eventually the entire signal never complete its travel around the sphere before all of its energy is spent thru these many 'specular' reflections. So if the sphere is larger than wavelength, its RCS will appear, or more precisely 'perceived', to be cumulative of these 'specular' reflections and is more approximate, never precise, of the sphere's physical diameter. Remember, the ideal situation is to have %100 of the signal's energy back to the seeker, so we want to gather as much of the signal's many 'specular' reflections as possible before any part of the 'creeping wave' travels to the 'shadow' region.

The sphere is used precisely because it is an ideal object and with other simple objects like the cylinder or a plane we can create references upon which to 'decompose' complex bodies and perform minor RCS calculations upon them. The trick later is on how to recomposite all these discrete simple RCS results into a larger complex one and hopefully it will yield us an accurate overall RCS.

Also, can you give details on this?

Rayleigh scattering? This should be in your physics class.

Rayleigh wave - Wikipedia, the free encyclopedia

I have worked on increasing absorption by decreasing of target size (though in different region).
I am interested to know if absorption can be decreased.

Not exactly certain what you are talking about here.

 

[saurabh]

Deleted...

 

[saurabh]

@gambit
Thanks, was very, very informative (Especially since it drow me to studies, which I rarely do!!).

Rayleigh scattering? This should be in your physics class.

Rayleigh wave - Wikipedia, the free encyclopedia

I had studied Rayleigh scattering and not Rayleigh waves/surface waves, so got confused. Now clear, thanx again!!

Not exactly certain what you are talking about here.

Actually I had done experiment on ultra high power laser and matter interaction. Used nanoparticles to increase the absorption of waves.

PS: You have praiseworthy "pathetic artistic skills"

 

[pakpower]

damn another fan boy coming straight from Bharat Rakshak & has just watched a sunny deol movie!

IAF Grounds MIG-27 | Indian Airforce | Accidents | Checks | Engine Faults | Turbine Blades - Oneindia News

Brother you literally take my words and publish them here very sad to see such a fanboy portraying their Bollywood or Hollywood stuff over here.