www.radartutorial.eu www.radartutorial.eu Radar Basics

Scattering Mechanisms

Figure 1: Mechanisms of scattering using the example of an A320

Figure 1: Mechanisms of scattering using the example of an Airbus A320

Scattering Mechanisms

There are three basic physical scattering mechanisms and contributors to compose the Radar Cross Section (RCS) of a target. These are either specular, i.e., mirror-like reflections that satisfy Snell’s law, or caused by scattered waves that originate at abrupt discontinuities (e.g., diffraction by peaks, edges, and corners ), or surface waves – the body acts like a transmission line guiding waves along its surface. The fundamental echo sources on a typical airborne target are:[1][2]

  1. Tip diffraction
  2. Specular surface return
  3. Creeping wave return
  4. Edge diffraction
  5. Corner diffraction
  6. Traveling wave echo
  7. Interaction echo (multiple reflections)
  8. Seam echo
  9. Cavity return
  10. Curvature discontinuity return
Wave Front

Figure 2: The scattered wave is a sum of contributions from a collection of scatterers with varying degrees of phase shift

Wave Front

Figure 2: The scattered wave is a sum of contributions from a collection of scatterers with varying degrees of phase shift

Specular surface return

A specular scatterer is any target surface oriented perpendicular to the line of sight to the radar. Flat surfaces provide particularly large echoes in the specular direction, but the echoes fall off sharply away from that direction. Mirror echoes from single and double curved surfaces (cylindrical and spherical surfaces) are slightly weaker than those from flat surfaces but are more consistent with changes in aspect angle.

Interaction echo

Relatively strong echoes can occur when two target surfaces are aligned to bounce from one surface to the other and then back to the radar, as in the interaction between the hull and the trailing edge of the left wing shown in Figure 1, example ⑦. Similar interactions occur with ship targets when bulkheads, railings, masts, and other topside features are reflected in the mean sea surface.

Creeping wave return

A creeping wave is a wave that is bound to a smooth, shaded surface, passes around the rear of a smooth body, and then returns to the radar when it reappears at the shadow boundary on the opposite side. Through Mie- Scattering, the creeping wave causes echoes from small spheres to vary with sphere size. The creeping wave mechanism is significant for military and civilian targets only at low frequencies.

Traveling wave echoes

When the angle of incidence is a small grazing angle to the surface, a surface traveling wave can be induced. The surface wave tends to build up toward the back of the body and is usually reflected forward from any discontinuity at the back. Traveling wave echoes at low grazing angles are almost as significant as mirror echoes at normal incidence.

Diffraction at peaks, edges, and corners

Scattering from peaks, edges, and corners is less significant than specular echoes and of concern to the designer only when most other echo sources have been suppressed. The echoes from tips and corners are localized and tend to increase with the square of the wavelength, not the size of any surface feature. Therefore, they become increasingly less important as the transmitter’s carrier frequency increases.

Figure 3: Complex reflection pattern of an Airbus A320 depending on the aspect angle.

Figure 3: Complex reflection pattern of an Airbus A320 depending on the aspect angle.

Cavity and curvature discontinuity return

Most aircraft have slots or gaps where the control surfaces (e.g. ailerons, stabilizers, canards, flaps) meet the stationary airframe. Slots, gaps, and even rivet heads can reflect energy back to the illuminating radar. To minimize this type of returns, modern stealth aircrafts use extremely smooth surfaces, avoiding discontinuities and resorting to specific dielectric compounds and sealants to fill in gaps and slots and to treat those portions of the external skin where an imperfection could result in a scattering hot spot. Within openings, for example the engine intakes or cockpit windows, strong echo signals can again be generated by multiple reflections. For this reason, the glass surfaces of the cockpit canopy of military aircraft are coated with a thin layer of metal.

 

Not all of these mechanisms show up in the features of simple and complex target selection but all of these echo sources overlap with varying degrees of phase shift. In some directions, all scattering sources may add in phase and result in a large RCS. In other directions, some sources may cancel other sources resulting in a very low RCS. In sum, they form a complex reflection pattern depending on the aspect angle. The RCS can therefore fluctuate by more than 30 dB, which is noticeable as a fluctuation loss.

Sources:

  1. Eugene F. Knott, “Radar observables,” in Tactical Missile Aerodynamics: General Topics, Vol. 141, M. J. Hemsch, ed., Washington, DC: American Institute of Aeronautics and Astronautics, 1992, Chap. 4.
  2. Eugene F. Knott, “Radar Cross Section,” in M. Skolnik: “Radar Handbook”, Third Edition, McGraw-Hill Education, 2008, ISBN 9780071485470, page 14.2f