Radars - Detection of mobiles in the clutter

In this article, we focus on radar signal processing to detect mobiles in the clutter, its principles, the resulting reception techniques and their performance criteria.

The performance of these devices depends on the nature of the so-called "clutter" (echoes reflecting waves off the ground, sea or clouds) or "clutter" that surrounds them. An analysis of the characteristic properties of clutter (reflectivity, intensity of clutter echoes, spectral behavior), and their consequences on target visibility, is therefore proposed, with a focus on the case of clutter as seen by airborne pulse doppler radars.

Moving Target Visualization (MTV), which aims to detect the presence of moving targets in a fixed or slow echo environment, is discussed by examining the effect of target Doppler on a pulse train. The transmission/reception chains used to obtain these signals are presented. The different types of signal processing :

• single and double cancellation filters ;

• linear filters ;

• speed filtering ;

• pulse doppler ;

are described and their performance analyzed.

The particular case of echoes seen by airborne pulse doppler radars is analyzed and leads to an examination of different solutions:

• unambiguous distance radars (or BFRs);

• unambiguous speed radar (or HFR);

• radar with distance and speed ambiguity (or MFR) ;

and performance levels.

This is followed by a description of the principle and operating modes of continuous radars, the simplest solution for measuring the Doppler frequency of targets, if only their speed is of interest.

Their special features include:

• the influence of transmit-receive decoupling on radar range ;

• the behavior of continuous radars in front of fixed echoes.

are examined.

Finally, a presentation of the principles of over-the-horizon radars for the remote detection of low-altitude targets:

• Grounds-waves or surface-waves radars ;

• Skywave radars or OTH (over the horizon) radars.

as well as a description of their characteristic features