| Charles Elachi |
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[Slide 1]
[Slide 2]
[Slide 3]
[Slide 4]
[Slide 5]
[Slide 6]
[Slide 7]
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[Slide 1]
Good morning, I guess I should say "ohayou gozaimasu".
What I would like to do is give you a quick overview of the work that we have done in synthetic aperture radar over the last 25 years. But before I do that I'm going to give you a quick introduction on how does the technique work and how does it compare to visible imaging which is what we usually are accustomed to see with our eyes.
[Slide 2]
First, as the most basic principle, the microwave, or the signal that we use in microwave, is very similar to the signal of light. It's called electromagnetic waves and, basically, in the case of the light we use very short wavelength, and in the case of the microwaves we use very long wavelength. So the basic fundamental principles are exactly the same.
[Slide 3]
To illustrate for you, the kind of radio waves we use are similar to what you get on your radio stations and similar to what you use for your TV. The same electromagnetic wave, but with shorter wavelength - are used in infraredc they are the same that are used in visible that we see with our eyes, and the same that are used in X-ray that when you go to a doctor and use X-ray to see inside your body. The basic electromagnetic waves are the same. The difference being the wavelength that you use.
[Slide 4]
In the case of microwave for radars, we use basically about a millimeter to one-meter wavelength. If you go to the visible, that's about a half of micron, and that's what our eyes are sensible for. The camera technique that one uses for imaging, from satellite, is basically to scan the viewing angle on the surface as the satellite is moving, so we scan it back and forth and build an image. In some cases, we have an array of detectors like what you see in here, and this array covers an area on the ground, and as the satellite moves we generate, or build, an image.
[Slide 5]
In the case of radar, we use a different concept. What we do is to transmit a pulse of electromagnetic wave, and when the pulse hits an object, an echo comes back. Similar to if you shout in a valley and then you will hear an echo coming back. So objects which are far away, the echo comes later, objects which are close to you, the echo comes back earlier. That's a fundamental technique that we use in imaging radar.
[Slide 6]
From satellite or the shuttle, we send a pulse down to the surface and as the signal moves on the surface we get an echo back coming from here and then an echo coming from here (see figure). And by looking at these echoes then we build a signal from the surface. In the other dimension, we use Doppler technique which is the change in the frequency, to give us information along the track below the satellite. So that's a fundamental concept of how we generate radar images.
[Slide 7]
Now when the signal comes and interacts with the surface, depending on the wavelength, for long wavelength, the signal will penetrate through the surface. So if it's in a forest, it will actually penetrate through the trees. While the shorter wavelength, if you use 3 centimeter, the echo comes from the top of the trees. So by looking at different wavelength, we can have different penetration through the forest.
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