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https://earth.esa.int/web/guest/missions/esa-operational-eo-missions/ers/instruments/sar/applications/radar-courses/content-3/-/asset_publisher/mQ9R7ZVkKg5P/content/radar-course-3-the-radar-equation

"The geometry of scattering from an isolated radar target (scatterer) is shown in the figure, along with the parameters that are involved in the radar equation."

enter image description here

I'm a bit confused about usage of the words "scatterer" and "point scatterer" in radar. The above makes it sound like 'scatterer' is exactly synonymous with target. That can't be right- then we would just say "target." There must be some connotation with the above that makes us use the word scattterer.

My googling efforts:

I searched "scattering" on wikipedia but am confused how it relates to the above https://en.wikipedia.org/wiki/Scattering "The types of non-uniformities which can cause scattering, sometimes known as scatterers or scattering centers, are too numerous to list, but a small sample includes particles, bubbles, droplets, density" <- this doesn't sound like anything related to radar. A scatterer in wikipedia seems to be anything that can deflect radiation

There is also the term "backscatter" (something to do with bouncing according to this site http://www.radartutorial.eu/07.waves/wa51.en.html "“bouncing” or “backscattering” radio waves off the ionosphere" ) but I can't tell if it is related to "scatterer"

NoTechBackground
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  • Photons are also waves (quantum mechanics) and scattering occurs when photons encounter a lot of tiny isolated small bits which are much smaller, say, than the wavelength under discussion. The waves shake up/oscillate the tiny charges in each tiny bit of matter and these shaken charges radiate in *all* directions -- we call this "scattering." The intensity of the scattering is related to how many of these tiny bits of matter can oscillate. – jonk Jun 18 '19 at 21:56
  • SCattering, diffusion , absorption and reflection occurs in the optical region just as in RF with dielectrics and conductors and depends on losses, dielectric constant, reflections, time delay and the 4 S-parameters for a 2 port. And more for each time delay in the medium which of course depends on v=c/sqrt D for (Dk effective). Antennae are point sources like targets are point reflectors. Rice Fading is the mixing of equal amplitude variable inverse phase cancellation properties which also occurs in optics. THe rules are the same but the medium is different. – Tony Stewart EE75 Jun 18 '19 at 22:46
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    Partial bending of light on the horizon is from moisture and heat effects on Dk with diffraction or stretching of the horizon view, while RF bending of waves depends on the different properties of the ionosphere where VHF allows people to skip around the world or moon bounce to 5 continents depending on the conditions. VLF bends around the globe due to the < 30kHz wavelengths which reflect easily off the ionosphere and shift during sunset and sunrise due to path length . Doppler Radar measures the changes in propagation velocity to detect rain content which when saturated is 80 x air. – Tony Stewart EE75 Jun 18 '19 at 22:53
  • How about the reflection? of a dipole antenna, versus the reflection of a flat metal sheet? These differences underly the stealth fighter methods. I've concluded a flat metal sheet presents a horrid radiation resistance, because the inductance of myriad parallel "wires" is very low and the capacitance of myriad parallel "wire segments" is very high. With Zo = sqrt( L / C), the flat sheets would make bad re-radiators. Thoughts? – analogsystemsrf Jun 19 '19 at 03:24
  • Something I'm still confused about is the quote at the top of the OP makes it sound like a scatterer is just a target, a macroscopic object "The geometry of scattering from an isolated radar target (scatterer) "

    whereas a comment below mentions "scattering occurs when photons encounter a lot of tiny isolated small bits which are much smaller, say, than the wavelength under discussion".

    If you had to explain a "scatterer" to a 13 year old, what would you say?

     – NoTechBackground Jun 20 '19 at 20:41

3 Answers3

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There are both technical and semantic reasons that can make a "target" and a "scatterer" mean the same thing, or refer to different, but potentially related, things.

For example, one can think of a "target" as a generic object that will in someway or another reflect energy we are interested in. In the case of a radar, this object will reflect some of the power that we have transmitted.

Here, the target can be thought of as "a scatterer". This is the most appropriate when the target is modeled as a point-target.

A target with a complex geometry can me modeled as being composed of multiple scatterers. Here, the scatterers themselves produce a return but together make up the entire "target" return.

In practice, the term "scatterer" is used to refer to point or near-point objects that will produce some kind of return. They are treated in many cases as isotropic to simplify modeling. You can then use multiple scatterers to model a more geometrically complex object.

Using these terms really comes down to context:

  1. A target can be modeled as a single scatterering point. Here a "target" and a "scatterer" are one in the same.
  2. A target can be composed of multiple scatterers. Here, the two terms refer to two theoretically different things, but are obviously related since the scatterers compose the target.
  3. The term "target" usually refers to an object of interest. There can be many scatterers in the environment, but none of them is your "target".
Envidia
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I found this in your link:

The radar equations (eq. 8 and eq. 10) are general equations for both point and area targets. That is, the scattering cross-section s is not defined in terms of any characteristic of a target type, but rather is the scattering cross-section of a particular target.

Aaron
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  • Right so I guess what confuses me now is that if there are both point and area targets, then are both points and areas "scatterers"? Doesn't 'point and area' cover all of matter? So what is not a 'scatterer' ? – NoTechBackground Jun 20 '19 at 20:44
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I'm a bit confused about usage of the words "scatterer" and "point scatterer" in radar.

The terminology here has a specific meaning. A "point" scatterer is one that is much smaller than the wavelength, and so scatters isotropically in all directions. A non-point scatterer is approximately a wavelength or larger, and so has high anisotropic scattering (usually forward and backward peaked). As the object becomes many wavelengths wide it begins to reflect (redirect the beam without scattering). In the extreme case of a large, smooth object you have a mirror, which does not scatter at all (ideally).

The distinction is relevant here because they are looking at the object with widely separated TX and RX angles. A point scatterer will look the same for all angles, while a larger scatterer will look much dimmer when the RX channel is at a high angle (as depicted).

There is also the term "backscatter" (something to do with bouncing according to this site

All scatterers are backscatters. Backscatter is relevant if you have a combined antenna for RX and TX, since that is the only type of signal you will receive. Backscatter tends to be highly efficient when the object is about a wavelength wide. Too small and energy is lost to side scattering. Too large and it is lost to reflection.

I searched "scattering" on wikipedia but am confused how it relates to the above

The page on Mie scattering (object about 1 wavelength wide) has a lot of helpful background, including plots of the radar cross-section of an object vs. size:

https://en.m.wikipedia.org/wiki/Mie_scattering

user1850479
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