Backscatter
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 telecommunication and E-mail.
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:
- Rayleigh scattering of electromagnetic waves from small particles, diffuse reflection from large particles, or Mie scattering in the intermediate case, causing alpenglow and gegenschein, and showing up in weather radar;
- inelastic collisions between electromagnetic waves and the transmitting medium (Brillouin scattering and Raman scattering, important in fiber optics, see below;
- inelastic collisions between accelerated ions and a sample (Rutherford backscattering)
- Bragg diffraction from crystals, used in inelastic scattering experiments (neutron backscattering, X-ray backscattering spectroscopy);
- Compton scattering, used in Backscatter X-ray imaging.
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 fiber optics, the one-dimensional implies that scattered 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:
- In alpenglow, red light prevails because the blue part of the spectrum is depleted by Rayleigh scattering;
- in gegenschein, constructive interference might play a role (this needs verification);
- in multiple scattering from suspensions like milk, the enhancement of backscattering is connected with weak localization.
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.
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.
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. The backscatter analysis technique was recently featured in the May 5, 2006 episode of NUMB3RS titled "Backscatter".
Backscatter of email spam
This term is also used to describe the side-effect of email 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 non-delivery reports, 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. Email servers are now beginning to provide solutions to mitigate against the effects of the email backscatter that they receive: