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[P]
Introduction to Airborne Radar

By StormShadow in Technology
Fri Jul 04, 2003 at 10:08:41 AM EST
Tags: Science (all tags)
Science

This article will be the first in several that will discuss airborne radar technology. Most of the material will be applicable to all types of radar but for our purposes I will concentrate on airborne radar systems. Given the limitations of HTML and my desire to explain the concepts to individuals who may not have a physics or engineering background (i.e. reach the widest possible audience), I will attempt as much as possible to avoid the use of any but the most trivial equations. I already have in mind what future articles will discuss but I would be open to suggests or requests.

These articles will discuss the technical aspects of radars but I may, if there is interest, discuss the history of radar. One obvious question is why does anyone want to know how radar functions? Curiousity would be enough for me but, if this is not enough, consider how widespread and important is the use of radar technology in modern society. It is used by the cop on the corner who catches you speeding to the proposed Ballistic Missile Defense (BMD) program currently being researched.


A SIMPLE EXPLANATION OF THE THEORY

Radar (radio detection and ranging) functions by exploiting a very simple concept: An electromagnetic wave traversing a medium will be partially or completely reflected when it encounters another medium of vastly different dielectric constant. What does this mean in everyday english? Think of a typical flashlight and its use in a dark, empty room. If you are inside a room with clear air, you do not see the beam of light until it intersects one of the walls. Why do you suddenly see the light when it hits the wall but not as it crosses the air? Because visible light lies in a particular frequency band to which air is relatively transparent but the wall is not (purists may object by stating the beam cannot be seen because clear air has few scatters in that frequency band but this is not the only reason). If this seems strange to you, then consider the fact that X-rays are used to take pictures of your bones because skin, organs and fluids are transparent to X-rays but not bone which can scatter or reflect the X-rays.

In a similar fashion, radiation of the frequencies typically used by radar systems will traverse air with little or no reflection but will be reflected strongly by metals, the ground and, albeit weakly, even water droplets present in the atmosphere. These reflections propagate back to the radar system which then interprets them to form the images we are typically familiar with. Many radar systems are in fact very similar to a flashlight that operates at a different frequency. A relatively narrow beam of light (i.e. typically near the microwave band) is emitted from the radar antenna and reflections are used to illuminate the target just as your eye can see the wall because of visible reflection from a normal flashlight. The typical wavelenth of visible light (i.e. what you see with your own eyes) is approximately 500 nanometers while the frequencies typically used in radar systems are in the 10 centimeter range. This means a radar image will never be as sharp as a convention image (e.g. a photograph) because it cannot resolve two points closer than 10 centimeters. In the absolute grossest sense, this is the theory behind radar systems.

INTRODUCTION TO A GENERIC PULSED RADAR

In this first article, we will discuss the one of the simplest and earliest radar systems: a generic 'pulsed' radar. A radar system is typically composed of a few major components. The first is a device designed to convert electrical energy into electromagnetic radiation of the desired frequency. The earliest radars used a magnetron to convert DC current into high energy microwave radiation. This is the same component found in microwave ovens. The high energy radiation is then guided to the radiating element of the system (e.g. the antenna) by a waveguide that confines the radiation to a set path through the system. Once the radiation arrives at the antenna, it is emitted by a central feed onto a parabolic dish that forms a tight radar (i.e. radiation) beam. It is not required that a radar system use the same antenna both for transmission and reception but this is the typical situation and what I will assume in most my articles (when this is not true I will explicitly say so).

Since the same element is typically utilized both for transmission (TX) and reception (RX) so that care must be taken to prevent energy from leaking into the RX components during transmission as this would damage the delicate RX components. This can be accomplished by alternating between transmission and reception whereby the high energy radiation is blocked from entering the RX components during TX but the returning reflected radiation is allowed to enter the RX components (and obviously blocked from leaking into the TX system). The radar, therefore, has its own operating frequency separate from the transmission frequency (i.e. the TX/RX switching rate). The rate of TX/RX switching is called the pulse repetition frequency (PRF) of the radar system. When in TX mode, the radar system emits a set of pulses of a given frequency whose echoes are subsequently received when in RX mode. The echoes are then processed in order to form an image.

In the simplest sense, a radar image is formed by calculating the range (i.e. the distance) the received energy came from. Since the speed of light 'c' in any given medium is a constant, this can easily be done by multiplying the speed of light by half the round-trip time of the pulses. This is called 'pulse delay ranging.' The azimuth of the target (i.e. the angle the target lies at relative to the radar system) is determined by the angle the antenna is facing. A typical radar beam is approximately 3-4 degrees wide and this give you an angular range where the target may lie (e.g. a target that appears to be at 21 degrees actually lies somewhere between 20 and 23 degrees). At large ranges, even a small angle error of 3 degrees can translate to hundreds or more meters of cross-range error (i.e. cross-range is defined perpendicular to the range). This can be corrected by a number of techniques that will be discussed in a future article.

A PREVIEW OF DIFFICULTIES TO DISCUSS

A number of difficulties have been glossed over in my very brief introduction to radar. One important difficulty arises from the competing demands of range resolution and sensitivity. For a radar system to increase how far it can see (i.e. its range) and how small an object it can see at a given range, it must increase its average emitted power. For a given frequency, the average emitted power can be increased by transmitting for a longer period (i.e. increasing the length of the TX pulse) but this will make it impossible to resolve closely separated targets because their echoes will overlap. Therefore, you are generally limited to pulse lengths of no more than 1 microseconds for adequate resolution. A number of techniques are used to solve this dilemma.

Typically, the received pulses are compressed to increase the received energy. One method used to compress the echoes is to encode the pulses by modulating the phase of the TX pulses. An example of this would be to flip every third pulse "upside-down" and observing the phase of the received echoes. Another method of compression called 'chirp' linearly increases the frequency of transmission as a function of time. Since the energy radiated is proportional to the frequency of emssion, this increases the total power emitted for any given length of time. The echoes are then passed though a filter that introduces a delay which decreases as a function of frequency (i.e. the lowest frequencies are delayed the most and the highest frequencies the least). This compresses the echoes into a narrow pulse and improves the resolution of the radar system. For radar systems that operate at too high a PRF, pulse delay ranging will not work well. Such CW (continuous wave) radars must rely on alternate techniques. One typical method is frequency modulation. The emitted energy has its frequency modulated and range is determined by timing the delay in the frequency modulation of the receieved echoes.

FUTURE DISCUSSION

More advanced radars can determine the relative velocity of a target by observing the tiny shifts in frequency of the echo due to doppler shifting. These radars are typically known as 'pulsed-doppler' radars and they are not simple to build as they impose a number of additional constraints on the radar system. In the next article, I will introduce a generic pulsed-doppler radar and illustrate the main differences between it and its simpler cousin. Future articles will return to discuss how angular resolution is improved (i.e. getting better than 3-4 degrees), automatic radar tracking is done, synthetic aperture radar (SAR) and discuss the special requirements of steath radar systems (i.e. passive and active ESA).

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Introduction to Airborne Radar | 81 comments (48 topical, 33 editorial, 0 hidden)
Questions (3.50 / 2) (#2)
by StormShadow on Wed Jul 02, 2003 at 09:16:26 PM EST

1. Is it too short? I can easily increase the length by adding details but at the risk of overexplanation. The use of diagrams would be very nice but since image posting is not allowed, I would be forced to describe every component and that may be tedious for the reader.

 2. Any parts that should be expanded with additional details? Please note I intend to take certain sections and expland them into stand-alone articles (entire books have been written on just how to improve the range resolution through many techniques) so I thought it best ot keep this article brief and as a very high overview.


-----------------
oderint dum metuant - Cicero
We aren't killing enough of our [America's] enemies. Re-elect Bush in 2004 - Me
12/2003: This account is now closed. Password scrambled. Its been a pleasure.


Well, yes (4.50 / 2) (#6)
by epepke on Wed Jul 02, 2003 at 09:58:44 PM EST

Well, yes, I thought it was a bit short. You don't even spell out the acronym (radio detection and ranging.

Also, a lot of the concepts that you try to explain radar in terms of seem odd to me. Consider this:

An electromagnetic wave traversing a medium will be partially or completely reflected when it encounters another medium of vastly different dielectric constant.

This seems a bit backward to me, as if you expect your audience already to be familiar with dielectric constants but know little about engineering or physics.

You might expand the audience by mentioning some of the history of radar, such as the fact that there was a primitive expeimental radar installation at Pearl Harbor that detected the Japanese planes but was dismissed as a glitch, the way that the development of the technology in World War II speeded up the development of affordable television sets later, etc. Also means of defeating or confusing radar, like throwing aluminum scrap out of airplanes, 45 degree angles on stealth planes, vibrating plates to confuse Doppler radar, etc. Also why there is a Doppler shift in the first place and how it's different from the more familiar Doppler shift with sound.


The truth may be out there, but lies are inside your head.--Terry Pratchett


[ Parent ]
Good comments (none / 0) (#14)
by StormShadow on Wed Jul 02, 2003 at 10:28:44 PM EST

<i>Also means of defeating or confusing radar, like throwing aluminum scrap out of airplanes, 45 degree angles on stealth planes, vibrating plates to confuse Doppler radar, etc. Also why there is a Doppler shift in the first place and how it's different from the more familiar Doppler shift with sound.</i>
<br><br>
Some of these concepts I am planning to cover in future articles as I mention at the end. Especially the radar requirements of steath airplanes.
<br><br>
<i>This seems a bit backward to me, as if you expect your audience already to be familiar with dielectric constants but know little about engineering or physics. </i>
<br><br>
You are correct. This is my first cut and I explained it as I would to second year physics students. I will put in explanations and links to sites that can expand on the physics.
<br><br>
About the acronym, you are correct I should have put that in the article. I'll update it later tonight if I get a chance.


-----------------
oderint dum metuant - Cicero
We aren't killing enough of our [America's] enemies. Re-elect Bush in 2004 - Me
12/2003: This account is now closed. Password scrambled. Its been a pleasure.


[ Parent ]
radar jamming (none / 0) (#79)
by garlic on Mon Jul 21, 2003 at 05:41:49 PM EST

I work on radar ECM, so I'd be happy to help explaining counter measures.
This article was too short I think, but pretty good. I look forward to the next part.

HUSI challenge: post 4 troll diaries on husi without being outed as a Kuron, or having the diaries deleted or moved by admins.
[ Parent ]

It was prematurely posted (none / 0) (#80)
by StormShadow on Tue Jul 22, 2003 at 01:46:02 PM EST

I left in the queue for 1 day and the next morning I discovered it'd been voted up. I can only guess some cabal wanted to be funny and vote down my article over night. I'll post version 1.5 in a few days -- I've been promising to for a while but I've been busy.


-----------------
oderint dum metuant - Cicero
We aren't killing enough of our [America's] enemies. Re-elect Bush in 2004 - Me
12/2003: This account is now closed. Password scrambled. Its been a pleasure.


[ Parent ]
diagrams (5.00 / 1) (#10)
by illuzion on Wed Jul 02, 2003 at 10:20:37 PM EST

If you have access to another server - a website somewhere - you could post images there, and simply link to them from the article.

I thought it was clear enough anyway, even without images, although I suppose some of the more complicated future articles might not be.

One suggestion though: in order to make it accessible to everyone, you might need to explain what some of the terms mean. For example, you referred to 'dielectric constant'. I rather doubt that many people know what this means - I don't :) I can guess, but that's still not the same as having been given a good explanation. Also, in that same paragraph you use words like 'medium', 'traversing', etc. That sentence, although what it's saying is really simple, just sounds pretentious. I don't recommend changing the vocabulary you use since they're important technical terms, but perhaps rewriting that sentence to sound friendlier (or at least introducing it first) might be good. Perhaps you could put it in italics to indicate it's a definition rather than normal body text, and to make sure people realise it's different to how the rest of the normal body text will sound or is written. It's just it's the first part of the body of the article and you want all the early bits to be really readable.

You might also want to explain how a 3-4 degree wide transmission beam can return somehow a 23-degree wide view... or something... I didn't quite understand exactly what you were getting at in that bit, it's not clear.

You might also want to divide it into sections with headers - for example, the introduction how all radar systems work, then the bit about that one simple kind of radar, then the problems it has, and then the conclusion / preview of what's going to be in the next article or two. When an article's split up with headers like that, it makes it seems a lot more organised and easier to read.

It's a good article, and very interesting - I learnt a fair bit from it. Thankyou for posting. I'll vote it up if I'm around when it's in the voting queue.

[ Parent ]

This is my first cut... (none / 0) (#16)
by StormShadow on Wed Jul 02, 2003 at 10:33:34 PM EST

...so I wanted to through it out there. You are correct, I will explain or change some of the terms and I will probably add appropriate links for more indepth explanations.

You might also want to explain how a 3-4 degree wide transmission beam can return somehow a 23-degree wide view... or something... I didn't quite understand exactly what you were getting at in that bit, it's not clear.

I am sorry that this is not clear. What I mean is the following: Say your radar detects a target at 21 degrees than what this really means is that the target lies somewhere between 20 and 23 degrees because the angular resolution is 3 degrees or so (by a 3 degree resolution I mean it cannot tell apart two points that are separated by less than 3 degrees). Is this clearer?


-----------------
oderint dum metuant - Cicero
We aren't killing enough of our [America's] enemies. Re-elect Bush in 2004 - Me
12/2003: This account is now closed. Password scrambled. Its been a pleasure.


[ Parent ]
ahhh, yes, makes sense now. (none / 0) (#30)
by illuzion on Thu Jul 03, 2003 at 03:18:28 AM EST

Could you explain it like that in the article itself?

Oh, and I still think headings (thus clearly-defined sections) are a good idea, it's too confused as it stands now.

[ Parent ]

try to use more metaphors (none / 0) (#51)
by tranx on Thu Jul 03, 2003 at 05:02:54 PM EST

kind of like nine4mortal did in their comment. You started with the light in the dark room thing, then you abandoned that way...

To me it would make the diff between +1 and +1FP


"World War III is a guerrilla information war, with no division between military and civilian participation." -- Marshall McLuhan
[ Parent ]

Good comment (none / 0) (#63)
by StormShadow on Fri Jul 04, 2003 at 05:43:16 PM EST

I meant to expand this article and use the metaphor where ever possible but it got kicked out of the cue and into voting while I was partying in DC.

Off topic: I learned one thing today. Don't post an article for more than 1 day asking for feedback. I am writing this articles not so much for K5 but for another project I am working on and I had thought of using K5 as a yardstick to measure the readibility and understandability of the explanations. I had hoped to leave the article in the queue for a 2-3 days and use the feedback I got to improve the article and know how to structure the material but that was short circuited. I also meant to add links to overcome the lack of diagrams.


-----------------
oderint dum metuant - Cicero
We aren't killing enough of our [America's] enemies. Re-elect Bush in 2004 - Me
12/2003: This account is now closed. Password scrambled. Its been a pleasure.


[ Parent ]
Diagrams (none / 0) (#57)
by damiam on Thu Jul 03, 2003 at 10:57:03 PM EST

The use of diagrams would be very nice but since image posting is not allowed

Rusty has been known to enable inline images in stories if the authors ask him (and he approves the images, of course). You might give it a try.

[ Parent ]

Query (none / 0) (#72)
by scart on Wed Jul 09, 2003 at 05:11:42 AM EST

When in TX mode, the radar system emits a set of pulses of a given frequency whose echoes are subsequently received when in RX mode.

It's been 5 years since I worked in EW, but I believe this statement is incorrect. When a radar is running at a stable PRF it only emits one pulse during the TX mode, before switching to RX mode. If multiple pulses are emitted, the time between transmission and reception becomes ambigious.

You're description of CW also seems a bit ambigious. I think CW requires a couple of paragraphs to do it justice.

To me, a series on radar would not be complete without a discussion of active defensive measures that aircraft use to avoid and confuse radar. That would include brute force jamming, range gate spoofing, and maybe even a mention of radar homing missiles.

I like this article, and I think if you take heed of the other posters' requests for a more in depht explanation of the technical terminology the series could become a valuable addition to kuro5hin's library.

[ Parent ]
A typical radar works near the microwave band (none / 0) (#73)
by StormShadow on Wed Jul 09, 2003 at 08:05:06 AM EST

which is around a 10cm wavelength. This would be in the GHz (109) frequency band. If a typical PRI (pulse repetition interval or the inverse of the PRF) is around 1 microsecond (10-6), then you see that 103 pulses would be emitted in that TX interval.

Transmitting a single pulse would render the TX frequency meaningless and probably would transmit too little power to detect a target. The compression techniques I mentioned (in 'chirp' compression), require more than 1 pulse to be transmitted in any given TX interval.

I didn't mean this article to be posted as is (I think it got spam'md out of the queue for some reason) so it actually is incomplete. I plan to post version 1.5 in my diary in a few days. I will better explain CW radar and look up your query (i.e. one pulse transmitted during TX?) to make sure that all radars TX more than a single pulse as I claim.


-----------------
oderint dum metuant - Cicero
We aren't killing enough of our [America's] enemies. Re-elect Bush in 2004 - Me
12/2003: This account is now closed. Password scrambled. Its been a pleasure.


[ Parent ]
Terminology (none / 0) (#75)
by scart on Thu Jul 10, 2003 at 06:15:19 AM EST

It looks like we might be used to using differant terminology. I agree that there are more than one rise and fall cycle of the underlying carrier frequency within one pulse from a radar, but it seems as if you are refering to both the rise and fall cycle, as well as the time that the radar is emitting the carrier frequency, as a pulse.

In the work that I did, only the latter definition of pulse was used.

The PRI values you mention are quite high.
The radar signal travels at 300 000 000 meters per second. With a PRI of only 1 microsecond you get an effective unambigious range for the radar of only 150 meters, excluding delays caused by the electronics. The lowest PRIs that I am aware of were closer to 100 microseconds, and that only on short range tracking radars. Some search radars had PRIs of a few dozen milliseconds.

[ Parent ]

True (none / 0) (#76)
by StormShadow on Thu Jul 10, 2003 at 09:21:41 AM EST

I used 1 microsecond as that is the largest value I've heard used for airborne systems. My experience in radar is not hands-on but analytical in nature (i.e. my FFRDC does technical analysis of such systems) so you probably know more than I do. What I've learned I've learned because of the projects I've been involved with. Incidentally, did you mean: The PRI values you mention are quite low.

I see your point about the terminology. To be absolutely clear, a you've heard a 'pulse' refer to the radiation emitted in one TX interval (which contains more than one 2pi cycle of the carrier frequency). This is what I meant but I apologize if I was not clear. I'll fix this for version 1.5 that I will post in my diary soon.


-----------------
oderint dum metuant - Cicero
We aren't killing enough of our [America's] enemies. Re-elect Bush in 2004 - Me
12/2003: This account is now closed. Password scrambled. Its been a pleasure.


[ Parent ]
Yes (none / 0) (#78)
by scart on Fri Jul 11, 2003 at 07:29:37 AM EST

Incidentally, did you mean: The PRI values you mention are quite low.

Yes, that's what I meant. I was juggling PRI and PRF and didn't do a proper job of proofreading when I settled on PRI. Sorry about that.

To be absolutely clear, a you've heard a 'pulse' refer to the radiation emitted in one TX interval

That's correct. All our radar recognition manuals, as well as the military experts that we worked with, used this definition of pulse width.

[ Parent ]
Inroduction to Radar (2.66 / 6) (#15)
by United Fools on Wed Jul 02, 2003 at 10:31:01 PM EST

What is a radar? It is something like your ear.
Science made simple, by United Fools
We are united, we are fools, and we are America!
Quite simple, really. (1.57 / 14) (#25)
by Pink Shirt and Leather Pants Radar on Thu Jul 03, 2003 at 01:44:02 AM EST

Pink, puffy, hawaiian, embroidered, jeaned, rainbow colored, chapped, studded, spandex, or leather will be a really good indication.

Stop being so smug about it. (2.33 / 12) (#26)
by elenchos on Thu Jul 03, 2003 at 02:02:13 AM EST

Bats had radar figured out hundreds, even thousands, of years ago. And they're just dumb rodents. The real question is why it took people so long.

Adequacy.org

Sonar, not radar [n/t] (3.33 / 3) (#27)
by epepke on Thu Jul 03, 2003 at 02:19:42 AM EST


The truth may be out there, but lies are inside your head.--Terry Pratchett


[ Parent ]
Hey! (5.00 / 1) (#31)
by i on Thu Jul 03, 2003 at 03:30:49 AM EST

Your website seems to be down.

and we have a contradicton according to our assumptions and the factor theorem

[ Parent ]
They dont (4.20 / 5) (#35)
by twickham on Thu Jul 03, 2003 at 06:31:40 AM EST

As someone said, bats use sonar... so do Dolphins for that matter.

And I dont think bats have really "figured out sonar"... else bats would be build better sonar units. To answer your briefly final question, the reason it took mankind so long figure out radar was firstly we needed to

1. Invent fire

2. Invent argiculture

3. Invent the city

4. Invent industry

5. Start to understand physics and develop the scientific method

6. Obtain advanced enough matematics to study important industrial problems such as heat flow

7. Apply rigerous anaylsis to the study of electrical and electromagnetic phyics

8. Develop the engineering technologies required to build working radar.

Ive probably missed a few... but you get the gist. Weve come a long way.

[ Parent ]
heh (2.33 / 3) (#40)
by tps12 on Thu Jul 03, 2003 at 08:55:41 AM EST

"Invent fire," eh? The sun must have been pretty chilly before we clever humans came along.

[ Parent ]
Perhaps you will be surprised now (4.00 / 5) (#41)
by Amorsen on Thu Jul 03, 2003 at 09:10:05 AM EST

The Sun is not on fire. (One good reason being that there is very little oxygen there.)

[ Parent ]
of course it isn't (1.00 / 2) (#42)
by tps12 on Thu Jul 03, 2003 at 10:07:29 AM EST

It's made of fire.

[ Parent ]
Depends... (2.50 / 2) (#45)
by twickham on Thu Jul 03, 2003 at 11:11:19 AM EST

...on how you define fire. It you define it as

"The combustion of organic plant matter in the presence of oxygen".

Then the sun isnt made of fire.

Oh and my 1. should have read Invent techniques for causing controlled fire :)

[ Parent ]
unless (1.00 / 1) (#46)
by tps12 on Thu Jul 03, 2003 at 11:44:07 AM EST

You define "organic plant matter" as "hydrogen atoms." You can play all the word games you want, but the fact is that it's common sense that the sun is made of fire.

[ Parent ]
Ahhhhhhhhh (5.00 / 1) (#49)
by twickham on Thu Jul 03, 2003 at 02:01:10 PM EST

Well if its common sense then I stand corrected :)

You could of course define organic plant matter hyrdogen, but you would be wrong. Common uasage of fire usually refers to "oxidation of a combustible substance". The sun is fusing.

Anyways... I usually despise anal retentive arguments like this, so... like... whatever. The sun is made of fire. And so is the fluorescent lighting above me :P

[ Parent ]
Oh, that is insane. (1.00 / 2) (#56)
by elenchos on Thu Jul 03, 2003 at 10:50:54 PM EST

The word "fire" existed since the earth was created in 3,000 BC, long before anyone ever knew a thing about this gas "oxygen" or had the slightest clue about "oxidation". And to this day few humans besides those with heads full of elite college "facts" know oxidation from Oxodol. But any fool knows that fire is hot, and that it burns. The sun is hot, it burns, it is firey, and it is obviously made of fire.

You need to admit that the common usage of the word "fire" is much broader than your high school physics class definition of "oxidation".

Adequacy.org
[ Parent ]

What happens to the sun bears no relation to fires (none / 0) (#68)
by Amorsen on Sun Jul 06, 2003 at 11:49:37 AM EST

The only similarity is that both processes release heat.

[ Parent ]
I know fish don't use radar. (2.33 / 3) (#55)
by elenchos on Thu Jul 03, 2003 at 10:42:27 PM EST

So you are correct about dolphins.

But when was the last time you saw a bat swimming in the water? Charlie don't surf, my friend. They fly through the air, and the one most correct thing we see in this whole article is right in the title: RADAR is airborne, just like bats.

And I can assure you, bats have never developed advanced matematics or studied industrial heat flow, rigerously or not. They never even got past step one, Invent fire. By the way, fire didn't need to be invented. Things were catching fire long before we "invented" fire.

I suppose that is our problem. We spent so much of our time "inventing" things that already existed in nature that it took us centuries to do what even a dumb bat could do without even trying. And bats are dumb; they are statistically the the stupidest mammal there is after the wallaby.

Adequacy.org
[ Parent ]

sonar (5.00 / 1) (#58)
by adiffer on Fri Jul 04, 2003 at 12:17:35 AM EST

Sonar uses sound.  Sound travels through air or water or rock, so the airborne concept makes little difference.  Unless bats can radiate electromagnetic radiation like our radio antennae do, though, they can't be using radar.
--BE The Alien!
[ Parent ]
But there is a moth with 'ECM' (none / 0) (#64)
by hughk on Fri Jul 04, 2003 at 06:54:24 PM EST

Well actually both use sound. The bat emits ultrasonic pulses but a particular type of moth seems to be able to make a noise with similar characteristics (don't ask me how, insects don't have voiceboxes). Most noisy insects uses their legs or wings to make a noise which is kind of difficult in flight.

Anyway, this particular moth can do the neat trick of confusing the bats 'radar'. Others pick up the pulses and then take avoidance action. However only one seems to be able to fight it. I tried googling for this research but wasn't able to find the link.

[ Parent ]

Tiger moth, I think. (none / 0) (#67)
by n3uropil on Fri Jul 04, 2003 at 11:00:52 PM EST

I think you are talking about a species of Tiger moth (Cycnia tenera) that fires BACK a series of ultrasonic "clicks".

However, there is little evidence or investigation of whether said clicks actually DO anything to confuse a bat's final approach...

Finally, my years as a biology geek pays off in a Kur05hin post! :)

Sincerely,
n3uropil

[ Parent ]

Do-it-yourself radar (5.00 / 1) (#34)
by Vs on Thu Jul 03, 2003 at 05:46:13 AM EST

"These radars are typically known as 'pulsed-doppler' radars and they are not simple to build as they impose a number of additional constraints on the radar system."

I remember an article in a (non-technical!) newspaper where they built a home-made "radar" (either from a CB radio or a regular radio) which they connected to a computer.It was quite neat and supposedly even able to distinguish between ariplanes and choppers, and completely legal. Do you know anything about this?
--
Where are the immoderate submissions?

Interesting (none / 0) (#38)
by StormShadow on Thu Jul 03, 2003 at 08:00:48 AM EST

I'd not heard of this but I will try to look for information about this. I said they are not simple to build because they required the invention of 3 additional components that didn't come into being until the mid-60s.


-----------------
oderint dum metuant - Cicero
We aren't killing enough of our [America's] enemies. Re-elect Bush in 2004 - Me
12/2003: This account is now closed. Password scrambled. Its been a pleasure.


[ Parent ]
start (none / 0) (#59)
by adiffer on Fri Jul 04, 2003 at 01:23:00 AM EST

I usually hunt through amateur radio sites starting at the ARRL for some of the newest things going on in the field that people can build themselves.  If you know what you are looking for, Google is a lot faster, of course.  It is the hunt for the unknown new stuff that I reserve for this approach.  8)

http://www.remote.arrl.org/qst/worldabove/
--BE The Alien!
[ Parent ]

TX/RX (none / 0) (#48)
by Bossk on Thu Jul 03, 2003 at 01:49:54 PM EST

You may want to add some thoughts on "maximum unambiguous range" to your TX/RX switching section. Also, what is the TX/RX sequence of events. Is one pulse transmitted, then received, then the next pulse it transmitted, or are multiple pulses sent out in rapid succession? Is this also true for continuous wave? Are CW transmissions ever literally a "continuous pulse" or do they just have high prf? Looking forward to the future discussions...

This article is rather stale (2.00 / 2) (#54)
by Spencer Perceval on Thu Jul 03, 2003 at 08:05:40 PM EST

This is how to mold it into a front page quality article: write an article concerning the history of stealth technology, along with techniques and technology which have been developed to thwart it. Include perhaps two or three shorter paragraphs at the beginning of the article describing the basic technology behind radar.

I for one would love to read a well written article about the current workings of stealth and anti-stealth technology. Front page material, friend.


All the animals come out at night - whores, skunk pussies, buggers, queens, fairies, dopers, junkies, sick, venal. Someday a real rain will come and wash all this scum off the streets.

I meant for this article to be an intro... (4.00 / 2) (#62)
by StormShadow on Fri Jul 04, 2003 at 05:39:19 PM EST

...and at the bottom of the article I did mention that I would write a separate article about some of the radar requirements of steath airplanes. Unfortunately, it was prematurely kicked out of the queue before I could expand this article with additional content.

I'm writing these articles not so much for K5 but for two other reasons: as a review for myself and something I may use for a work project. I figured I could use the K5 audience as a test case and a yard-stick to measure the readibility and style of the articles.


-----------------
oderint dum metuant - Cicero
We aren't killing enough of our [America's] enemies. Re-elect Bush in 2004 - Me
12/2003: This account is now closed. Password scrambled. Its been a pleasure.


[ Parent ]
Magnetrons were not the earliest radar tubes... (5.00 / 1) (#65)
by hughk on Fri Jul 04, 2003 at 06:58:59 PM EST

It was just straight forward high power tubes as the earliest Radar was transmitting at around the border of HF and VHF. Magnetrons were invented later during the hunt for higher power, better efficiency and much higher frequencies.

I must admit... (none / 0) (#66)
by StormShadow on Fri Jul 04, 2003 at 09:27:50 PM EST

...you are right. When I was learning about the technical aspects of radar the texts I used began by discussing the magnetron as the technological breakthrough of the early World War II years. That was as far back as I bothered to learn about the history of radar since it was not directly related to my work. I'll try to bring this into version 1.5 of my article that I will post only in my diary.

Thank you for the information.


-----------------
oderint dum metuant - Cicero
We aren't killing enough of our [America's] enemies. Re-elect Bush in 2004 - Me
12/2003: This account is now closed. Password scrambled. Its been a pleasure.


[ Parent ]
If tx period raises, minimmum distance too (none / 0) (#69)
by jmones on Sun Jul 06, 2003 at 02:44:41 PM EST

For a given frequency, the average emitted power can be increased by transmitting for a longer period (i.e. increasing the length of the TX pulse) but this will make it impossible to resolve closely separated targets because their echoes will overlap.

I'd would that, since we can't switch to Rx since we have tx everything, raising power by increasing length also limits the ability to detect very close targets, since we would receive the echo BEFORE we can switch to Rx mode.



Comparison with Eye Misleading (none / 0) (#70)
by OldCoder on Sun Jul 06, 2003 at 03:13:55 PM EST

Comparisons with vision and the images displayed on radar scopes give people the impression that the detection of radar reflection is directional, or focused, like the image formed on the retina by a lens. The radar antenna does not focus an image, and it does not provide a location beyond that of the orientation of the antenna itself. The receiving antenna is more of an ear than an eye.

Let me explain what I mean. When a reflection comes back, the electronics know the direction the antenna was pointing when the signal was sent out, and that is used as the direction. An airplane 20 miles away coming in from due north would bounce that signal back about 100 microseconds after the trasmitter had sent a signal out due north. By the timing of the signal (100 usecs each way yield 200 usecs round trip), the electronics knows that the airplane is due north and 20 miles away. If there were no airplane, but a fake signal were broadcast by somebody standing next to the radar antenna at the right instant (200 microseconds after transmission due north) the radar system would "see" an airplane due north at 20 miles distance. The directional nature of the rx antenna merely serves to weaken any spurious signals coming in from the side, to prevent this sort of false image.

For this false imaging system to work well, the shape, frequencies, and intensity of the fake reflection would have to match that of real reflected radar signals.

Timing and echo is different from focusing an image. There is no way of timing a light flashing on your eye to create a false image.

--
By reading this signature, you have agreed.
Copyright © 2003 OldCoder

Not quite correct (none / 0) (#71)
by scart on Wed Jul 09, 2003 at 04:09:02 AM EST

It's possible to 'paint' an image onto the retina using a laser. This technology is already being tested in head mounted displays.

[ Parent ]
You are quite correct in what you speak (none / 0) (#74)
by StormShadow on Wed Jul 09, 2003 at 08:11:37 AM EST

but perhaps you misunderstood my analogy. A radar is very much like a flashlight that revolves on a pedastal and illuminates a given section of space. When it paints a target and detects a target, this is very similar to a flashlight lighting up something by bouncing visible light off its surface. I never did say anything about focusing the light or radiation.

Further down my article I did mention how range and angle are computed and how there exists an angular ambiguity in the calculated azimuth (i.e. angle) of the target. I didn't mention side-lobes but I meant to.

This article was spam'md out of the queue and posted prematurely. I plan to post version 1.5 in my diary in a few days before beginning work on article 2.


-----------------
oderint dum metuant - Cicero
We aren't killing enough of our [America's] enemies. Re-elect Bush in 2004 - Me
12/2003: This account is now closed. Password scrambled. Its been a pleasure.


[ Parent ]
Interesting (none / 0) (#77)
by ascension on Thu Jul 10, 2003 at 11:45:20 AM EST

Too bad about the premature post; nevertheless, I enjoyed the article, it was well written, and I learned something. I think you did an excellent job of providing a metaphor for the subject and highlighting some key points (clarity of image based on bandwidth, range resolution and sensitivity), and even though the Tx and Rx details (modulation and phase shifting) were way over my head the sense was clear. Look forward to the next installment.

sincerely, ascension

Version 1.5 posted in my diary... (none / 0) (#81)
by StormShadow on Sat Jul 26, 2003 at 04:31:38 PM EST

... right here.


-----------------
oderint dum metuant - Cicero
We aren't killing enough of our [America's] enemies. Re-elect Bush in 2004 - Me
12/2003: This account is now closed. Password scrambled. Its been a pleasure.


Introduction to Airborne Radar | 81 comments (48 topical, 33 editorial, 0 hidden)
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