Backscatter

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For information Backscatter technologies to search people, vehicles and buildings, see Backscatter X-ray

Backscatter is the reflection of waves, particles, or signals back to the direction they came from. The term is used in several fields of physics, as well as in photography, telecommunication, and e-mail.

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[edit] Backscatter of waves in physical space

Backscattering occurs in quite different physical situations. The incoming waves or particles can be deflected from their original direction by quite different mechanisms:

Sometimes, the scattering is more or less isotropic, i. e. the incoming particles are scattered randomly in various directions, with no particular preference for backward scattering. In theses cases, the term "backscattering" just designates the detector location chosen for some practical reasons:

  • In X-ray imaging, backscattering means just the opposite of transmission imaging;
  • in optical fibers, light can only propagate forward or backward. Forward Brillouin or Raman scattering would violate momentum conservation, so inelastic scattering in optical fibers cannot be anything else but backscattering;
  • in inelastic neutron or X-ray spectroscopy, backscattering geometry is chosen because it optimizes the energy resolution.

In other cases, the scattering intensity is enhanced in backward direction. This can have different reasons:

[edit] Radar, especially weather radar

Backscattering is the principle behind all radar systems.

In weather radar, the strongest backscatter comes from graupel (solid ice). This causes sleet and hail to often show up as much higher rates of precipitation than are actually occurring. Rain has moderate backscatter, being stronger with large drops (such as from a thunderstorm) and much weaker with small droplets (such as mist or drizzle). Snow has rather weak backscatter, because the spiky crystals tend to scatter in all directions, rather than straight back.

[edit] Backscatter in waveguides

The backscattering method is also employed in fiber optics applications to detect optical faults. Light propagating through a fiber optic cable gradually attenuates due to Rayleigh scattering. Faults are thus detected by monitoring the variation of part of the Rayleigh backscattered light. Since the backscattered light attenuates exponentially as it travels along the optical fiber cable, the attenuation characteristic is represented in a logarithmic scale graph. If the slope of the graph is steep, then power loss is high. If the slope is gentle, then optical fiber has a satisfactory loss characteristic.

The loss measurement by the backscattering method allows measurement of a fiber optic cable at one end without cutting the optical fiber hence it can be conveniently used for the construction and maintenance of optical fibers.

[edit] Backscatter in photography

The term backscatter in photography refers to light from a flash or strobe reflecting back from particles in the lens' field of view causing specks of light to appear in the photo. Causes can result from snowflakes, rain or mist, and airborn dust. Backscatter is particularly a problem in underwater photography, where particulate matter can be very dense and include plankton which would otherwise be near transparent.

Backscatter can be reduced by offsetting the direction of the photo strobe as far from the angle of the lens as possible. This is normally done by placing the light source high and to one side by placing the strobe on an extendable strobe arm. By having the light come from the side, the reflected light is primarily in the direction of the strobe instead of the camera lens. This is similar to comparing a full moon to a half moon. The full moon is when the moon is lit from almost behind the earth, creating reflection off the whole surface facing the earth. A half moon is when the moon is being lit from one side, making the reflection half the size and the light intensity much less. In photography, the side lighting makes the backscatter less pronounced.

Backscatter can often also be removed digitally after the photo is taken with photo editing software using digital filters or cloning of areas of the photo near the backscatter spots.

[edit] Backscatter from denial-of-service attacks

In computer network security, backscatter is a side-effect of a spoofed denial of service (DoS) attack. In this kind of attack, the attacker spoofs (or forges) the source address in IP packets sent to the victim. In general, the victim machine can not distinguish between the spoofed packets and legitimate packets, so the victim responds to the spoofed packets as it normally would. These response packets are known as backscatter.

If the attacker is spoofing source addresses randomly, the backscatter response packets from the victim will be sent back to random destinations.

The term "backscatter analysis" refers to observing backscatter packets arriving at a statistically significant portion of the IP address space to determine characteristics of DoS attacks and victims.

An educational animation describing backscatter can be found on the animations page maintained by CAIDA.

[edit] Backscatter of email spam

This term is also used to describe the side-effect of E-mail spam and email viruses, where an existing address from someone else is used as the sender address and large quantities of email is sent using that forged address. All bounce messages, vacation/out-of-office notices, challenge-responses, autoresponders, etc., end up with the forged sender address. The result is often hundreds or thousands of mails in the inbox of the innocent owner of that address.

Since these messages were not solicited by the recipients, are substantially similar to each other and are delivered in bulk quantities, they qualify as unsolicited bulk email or E-mail spam. As such, systems that generate e-mail backscatter can end up being listed on various DNSBLs and be in violation of ISPs Terms-of-Service for being abusive. Email servers are now beginning to provide solutions to mitigate against the effects of the email backscatter that they receive.

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