Polarisation behaviour zyxw of ground clutter during dwell time zyxwv Prof. D. Giuli Prof. M. Fossi M. Gherardelli, PhD zyxwvutsrqp Indexing terms: zyxwvutsrqponmlkj Radar, Ground clutter Abstract: zyxwvutsrqpon The paper analyses the behaviour of ground clutter during dwell time in a low- resolution radar. The analysis is made with refer- ence to measurements made through a pulse radar using right-handed circular polarisation at trans- mit and two orthogonally circularly polarised channels on reception. Symmetric and asymmetric distribution of polarisation with respect to its average is pointed out for two different types of ground clutter, the ‘clustered clutter’ and the ‘dis- tributed clutter’. Suitable parameters are defined and evaluated for statistical analysis of the experi- mental data to characterise and classify the observed phenomena. Frequent scan-to-scan sta- tionarity of ground clutter polarisation is high- lighted, which makes the statistical analysis meaningful for the application of suitable adaptive polarisation filtering techniques for ground clutter rejection. 1 Introduction In general, conventional radar systems operate with fixed single-polarisation antennas for transmission as well as for reception. This functional feature is simple to imple- ment, but does not permit the utilisation of information derived from the acquisition of the echo’s polarisation state for the purposes of radar targets signatures. The measurement of the polarisation state of the radar echo allows enhanced discrimination between targets [ 11. The acquisition of the radar echo’s polarisation state can also increase the possibility of resolving interfering targets, when exploited together with signal observations in additional domains, such as the Doppler frequency domain [2]. The analysis of polarimetric behaviour of the radar echo becomes relevant in order to use the informa- tion on the polarisation state to improve radar per- formance in the presence of echoes which interfere with each other. It is known [3] that, in general, the polarimetric behaviour of a radar target can be completely described by its scattering matrix, through which, once the polari- Paper 7907F (E15), first received 25th March and in revised form 26th September 1990 Prof. Giuli and M. Gherardelli are with the Dipartimento di Ingegneria Eletuonica, Universita di Frienze, Via di S. Marta 3, 50139 Florence, Italy Prof. Fossi is with the Istituto di Teona e Tecnica delle Onde Elettro- magnetiche, Istituto Universitario Navale, Via A. Acton 38, 80133 Naples, Italy IEE PROCEEDINGS-F, Vol. 138, No. 3, JUNE 1991 sation of the transmitted signal is known, it is possible to obtain the polarisation of the received signal. The measurement of the target’s scattering matrix is a complex process since it requires the alternate transmis- sion of orthogonally polarised signals. However, for the purpose of using polarisation as a radar echo signature, the limited acquisition of the echo’s polarisation state obtained with single polarisation transmission can still be profitably exploited through suitable adaptive polarisa- tion processing techniques [I]. The performance evaluation of adaptive polarisation techniques requires a deeper knowledge as well as proper modelling of short-term polarisation behaviour of targets during the dwell time zyxwv of a scanning antenna on the target. On the basis of these considerations, this paper analyses the polarisation behaviour of ground clutter and suggests the use of some useful models capable of simple polarisation signature models fitting experimental data of a low-resolution radar operating in the S-band, with a single circular polarisation in transmission and orthog- onally double circularly polarised channels on reception. 2 It is well known that during propagation the end point of an electric (and magnetic) field vector of a monochro- matic plane completely polarised electromagnetic wave traces, on the transverse plane, an ellipse. In special cases this may reduce to a straight line (linearly polarised wave) or to a circle (circularly polarised wave). Conse- quently, when representing time-harmonic, plane electro- magnetic (EM) waves, the relative polarisation state is described in terms of such a polarisation ellipse. The parameters which completely characterise the polarisa- tion ellipse are (Fig. 1) the amplitude E, the angle of Brief recall of analytical tools y=v I I Fig. 1 Polarisation ellipse 21 1