Ground clutter

Clutter is a term used for unwanted echoes in electronic systems, particularly in reference to radars. Such echoes are typically returned from ground, sea, rain, animals/insects, chaff and atmospheric turbulences, and can cause serious performance issues with radar systems.

What one person considers to be clutter, another may consider to be a target. However, targets usually refer to point scatterers and clutter to extended scatterers (covering many range, angle, and Doppler cells). The clutter may fill a volume (such as rain) or be confined to a surface (like land). In principle, all that is required to estimate the radar return (backscatter) from a region of clutter is a knowledge of the volume or surface illuminated and the echo per unit volume, η, or per unit surface area, σ° (the backscatter coefficient).

In addition to any possible clutter there will also always be noise. The total signal competing with the target return is thus clutter plus noise. In practice there is often either no clutter or clutter dominates and the noise can be ignored. In the first case the radar is said to be Noise Limited, in the second it is Clutter Limited.

Rain, hail, snow and chaff are examples of volume clutter. For example, suppose an airborne target, at range ${\displaystyle R}$, is within a rainstorm. What is the effect on the detectability of the target?

First find the magnitude of the clutter return. Assume that the clutter fills the cell containing the target, that scatterers are statistically independent and that the scatterers are uniformly distributed through the volume. The clutter volume illuminated by a pulse can be calculated from the beam widths and the pulse duration, Figure 1. If c is the speed of light and ${\displaystyle \tau }$ is the time duration of the transmitted pulse then the pulse returning from a target is equivalent to a physical extent of c${\displaystyle \tau }$, as is the return from any individual element of the clutter. The azimuth and elevation beamwidths, at a range ${\displaystyle R}$, are ${\displaystyle \theta /2}$ and ${\displaystyle \phi /2}$ respectively if the illuminated cell is assumed to have an elliptical cross section.

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