An Antichaff Technique For Tracking Radar Saidi,H., Askari,G.H., Akhlaghi,J., Mirmohammad Sadeghi,H. Isfahan University of Technology, Information & Communication Technology Institute, 8415683111, Isfahan, Iran Tel: +983113912480 Abstract — The critical interference in radar systems is unwanted echo named clutter. Some important clutter includes echoes from land, weather (particularly rain), sea and deliberated chaff. Therefore, a military radar designer is faced with the problem of how to reject interference. In this paper, chaff elimination methods such as Doppler processing and Log-FTC are discussed and then an antichaff algorithm suitable in transient and steady descending state is introduced. Also, this technique is applicable and independent on chaff type and central frequency. I. INTRODUCTION The electronic warfare technique known as chaff is usually considered as clutter since it is unwanted and similar to rain [1]. The use of chaff to confuse radar systems dates back to World War II [1]. Its popularity is primarily due to its simplicity and low cost relative to electronic countermeasures. Generally speaking, the chaff has two situations in the space, transient and steady state [2]. It has destructive effects on the radar systems that are [1, 3]: • Attempts to break the tracking loop of tracking radar in range or angle. • Forms a corridor or cloud which increases false alarm rate. • Obstructs the target acquisition. There are some general methods of chaff clutter rejection such as Doppler processing and Log-FTC [2]. In contrast, some algorithms are special methods one of which is discussed in this paper. his paper consists of five section. At first clutter modeling is considered in Sec.II. In Sec.III&IV, general chaff rejection methods such as Doppler processing and Log-FTC are explained and then a technique of clutter elimination especially chaff clutter in both situation (transient and steady state) is introduced that simulation and experimental results verify its performance. Finally, Sec.V concludes the paper. II. CHAFF CLUTTER MODELLING The chaff is a form of volumetric radar clutter composed of distributed metallized reflectors [1, 2]. The chaff dipoles are designed to resonate at the frequencies of the radar that they are attempting to confuse. Commonly chaff packages contain hundreds of thousands of aluminum or aluminum coasted glass, which, when dispensed into the atmosphere, reflected radar energy comparable to that of the largest aircraft [2]. The large number of randomly moving dipoles that make up the chaff cloud ideally satisfy the conditions for Gaussian distributed clutter [2]. Experimental analysis of chaff echo also show full agreement with the Gaussian clutter theory. This means that the chaff's envelope- detected amplitude probability density function follows a Rayleigh [2]: 0 ) exp( ) ( 2 2 2 > − = x x x x P σ σ (1) The power spectral density will also be approximated by a Gaussian-shaped function [2]: ) 2 ) ( exp( . 2 1 ) ( 2 2 0 f f f f f S σ σ π − − = (2) Where, 0 , f f σ are variance and central frequency, respectively. Doppler spectrum of chaff return can be split into four components in the same manner as for precipitation. It includes wind shear due to finite height of radar beams, beam broadening due to finite width the beams, turbulence and fall velocity distribution of the dipoles. In other words, the general equation for the total variance of the power spectral density for chaff is then: ) ( ) ( 2 2 1 2 2 2 2 Hz turb foll beam shear f σ σ σ σ λ σ + + + = (3) Where, fall trub beam shear σ σ σ σ , , , are related to mentioned cases respectively. Central frequency can be found from wind velocity. As shown in Fig.1, 0 , f f σ has been depicted for typical radar [2]. Figure 1: different clutters information