Everything you always wanted to ask...

[John Howling Mouse]

I've often read the phrase "synthetic aperture" with regards to radar but have no idea what that really means in layman terms.

Anyone?

[Jschmus]

One of the questions that can keep me awake at night:

How do you pronounce cthulhu?

Most folks I know pronounce it as "K-Thu-lu", but I read somewhere that Mr. Lovecraft favored the pronunciation "Keth-Lu".

[GTX]

I've often read the phrase "synthetic aperture" with regards to radar but have no idea what that really means in layman terms.

Look here.

Regards,

Greg

[Shasper]

Whats the exact difference between passive & active IR? Is there a active laser seeker in use?

Shas B)

[retro_seventies]

Passive IR simply registers the emissions from the object being observed - night vison goggles for example are passive IR as they use detectors sensitive to the infrared energy given off by objects.  Active IR on the other hand projects an IR beam to "paint" the target and then reads the reflected energy (rather like a radar beam would).  This beam is "invisible" in that it is not visible to the naked eye, and it is not visible on radar warning receivers, but it IS visible to other passive IR devices.

put simply, there is no way that anybody knows if you are using passive IR, but use active IR and you risk being detected.

hope that helps?  

[Hobbes]

I've often read the phrase "synthetic aperture" with regards to radar but have no idea what that really means in layman terms.

Anyone?

Basically, you put a radar on a vehicle, and you scan a stationary object multiple times while the vehicle is moving. Combine the return images from all scans to get a single image with a higher resolution than the individual scans.  

[Shasper]

Active IR on the other hand projects an IR beam to "paint" the target and then reads the reflected energy (rather like a radar beam would).  put simply, there is no way that anybody knows if you are using passive IR, but use active IR and you risk being detected.

Thx Rally, I thought it worked the same way as the passive/active radar thing.

Next question is, Is a IRST passive, active or both?


Shas B)

[Hobbes]

Normally an IRST will use passive IR for the 'search' element, and a laser rangefinder (=active, may not be IR) for use during tracking.
I don't think active IR is very common. All military applications I know of use passive IR.  

[John Howling Mouse]

I've often read the phrase "synthetic aperture" with regards to radar but have no idea what that really means in layman terms.

Look here.

Regards,

Greg

Ahhhhhh, I see.  Thanks!

[nev]

Shas, search the Real Aviation forums on ARC, there was quite a long and detailed topic about the use of passive IR on aircraft, both from the fighter pilot side and the techincal side (engineery type who does that kind of thing for a living).  It was pretty technical at times, so I gave up on it.  But it was interesting for what IR can and can't do.

[lancer]

Again on the radar subject, but what is pulse doppler radar and how does that work?

[Leigh]

Again on the radar subject, but what is pulse doppler radar and how does that work?

Well it pulses then it dopples !

[rallymodeller]

Again on the radar subject, but what is pulse doppler radar and how does that work?

Pulse doppler radar works by not only measuring an object's location but also it's relative change in location and speed. THe explanation is a bit complex, but bear with and we'll get through it. WARNING: SCIENCE CONTENT!

Y'all know what the Doppler effect is, right? No? Okay, it's like this.

Here is my car:



My car has a really open exhaust system. YOu are standing by the side of the road, and I go just wailing by at full throttle. As I approach, my engine sounds much higher pitched, and as I go by, the engine note seems to drop. This is due to that same Doppler effect. Higher-energy sound waves (i.e. higher frequencies=higher pitch) move faster and lower-energy ones (lower=energy=lower pitch) are left behind.

OK?

Now we move it up a notch.

You get in a plane. I park my car and idle. You fly over where my car and I sit, and as you fly over you cut the engine and glide. At the same time, I rev my engine up to about 5000 RPM. You notice that as you approach overhead my engine sound is high-pitched, and as you pass by it gets lower pitched...

Still with me? Good.

So we do the same experiment again, but now I will wail by at 5000 RPM (in first gear that's about 60MPH) but this time at right angles to your direction of travel. As you pass overhead...you guessed it. As we approach, my engine note is still higher...

Which leads us to radar in a plane. Light waves behave a lot the same way as sound. And the theory of relativity tells us that everything observed is in the eye of the beholder, so to the radar the aircraft is standing still and it is the world that is moving. Since the radar is taking snapshots of it's surroundings all the time, anything moving -- either away from or towards it -- will reflect the radar waves either more energetically or less so depending on it's movement in relation to the radar itself. This is why Doppler is so handy for picking  moving targets out of the ground clutter. The pulse portion of it  is the "snapshots", and how energetically the pulses come back will tell the radar where the moving objects are, where they are moving to, and how fast they are moving there. THe idea of pulse-Doppler haas been around basically since radar itself but it's only in the last couple of decades that the computer processing power needed to keep these smapshots in memory and compare them has existed.

Too wordy?

[Hobbes]

Higher-energy sound waves (i.e. higher frequencies=higher pitch) move faster and lower-energy ones (lower=energy=lower pitch) are left behind.

Not quite. The speed of sound doesn't depend on its frequency. What happens is this:
imagine the sound as a series of waves. Your car emits these waves at a certain interval (=frequency). The wave leaves the car, and travels though the air at the speed of sound.

If the car and the listener are stationary, the situation is simple: the listener hears the same frequency as was produced by the car.

When the car moves, it emits a wave, gets a bit closer, and then emits another wave. So the waves end up being closer together than when the car was stationary. Waves closer together means the frequency is higher.

Same happens with Doppler radar. The target moves, and this influences the distance between reflected pulses. The change in frequency tells you the speed of the target, or rather the rate of closure. If the target flies in a circle around the radar, you wouldn't be able to measure its speed using the Doppler effect.    

[lancer]

Rally and Harro,

Many thanks for the explinations. It's brought understanding where there was ignorance and confusion.