7.a. Clutter Flashcards
Define Clutter
Returns from objects which are of no interest to the mission.
Noise vs. Clutter
Noise: Amplitude independant of transmitted signal, environment, frequency.
Returns vary randomely between pulses.
Clutter:
Amplitude proportional to transmitted signal.
Return may be correlated between pulses.
Varies with changing frequency.
Varies proportional to Amplitude, frequency, spacial component, etc
What are the two types of clutter scattering coeficients?
Surface reflectivity and volume reflectivity.
Autocorrelation vs. Integration
Autocorrelation is comparing multiple samples within a CPI to see which returns would correspond to clutter (stationary) and which would correspond to a target (moving) via time shift (not Doppler).
Integration is summing all the samples in a CPI so that the zero-mean noise will cancel.
Decorrelation Time
The amount of time required for autocorrelation to complete for a set of clutter samples.
Each type of clutter has its own decorrelation time.
A long decorrelation makes integration difficult because more time must be spent collecting samples down any given bearing.
Grazing angle
angle at which the illumination energy strikes a clutter surface
How does high vs low grazing angles affect the surface reflectivity?
Describe the variation in Sea Reflectivity
A smooth sea appears like a flat conductive plate that scatters energy in the forward direction (with small grazing angle)
As the wave weight increases, the sea appears rough, increasing the returns off the sea.
There is a strong dependency in the return for:
Grazing angle
Frequency
Look Direction
Attenuation vs clutter or noise?
Attenuation is the absorption of the signal, meaning there is less power on return.
What are the 3 mechanisms involved in autorcorrelation of sea returns?
Returns from sea spray and white caps
Specular returns (spikes) from wavefronts
Bragg scattering
Bragg Scattering
The return from all the peaks of the waves constructively interfere and appear as a very large return at the receiver.
Factors affecting RCS
Target geometry
Target material
Position of transmitter
Position of receiver
Frequency - surface will appear rougher to a higher freq.
Transmitter/receiver polarization
Phasor Addition
Multiple scattering centers are likely to exist on the body of an object. Net scattering from all different centres is the phasor sum of all individual centers.
It dictates the shape of the return.
Specular Scattering
Occurs where the angle of reflection is equal to the angle of incidence. Gives maximum return.
This explains why modern, low RCS objects are angular, not round.
End Region Scattering
Occurs when surface currents do not taper smoothly to zero at an edge. The abrupt change in value creates a scattering center.
This causes sidelobes I guess.
Edge Diffraction
Caused by induced localized wire-like currents at edges.
Edge currents generate specular scattering in a Keller cone of reflected waves.
Multiple Bounce Scattering
Occurs when two or more specular bounces act in combination to reflect incident energy back to a receiver.
