HF direction finding with a vertical array of collocated loop antennas

The use of antenna diversity and (or) space diversity in HF (3 to 30 MHz) system has been investigated in numerous studies, a synthesis about some methods and applications is described in [1]. The purpose of this paper is to present a particular application in the HF radio wave direction finding. The knowledge of propagation effects and antenna effects allowed particular array configurations to provide DOA estimation. The effect of introducing a small vertical diversity in a collocated antenna array (regarding to the wavelength) is to improve the elevation measurements. The paper describes the collocated array and presents the first experimental results of direction finding related to this new array structure. INTRODUCTION In the HF band, the propagation channel is related to the presence of several ionospheric layers, which act as reflectors for sky waves propagating in one or more hops. It generates signals issued from multipath propagation. In addition, the anisotropic nature of the ionosphere imposes a multimode propagation. This complex propagation channel needs the use of a high resolution algorithm such as MUSIC to discriminate angles of arrival. In a such method of signal processing, the contribution of antenna effect and propagation effects on the incoming wave have to be known. Depending on the type of antenna array, AOA estimation can be affected. A GENERAL VECTORIAL EXPRESSION OF THE ACQUIRED DATA The vectorial expression of the data at the output of the antennas can be written as: ) ( ) ( ). , ( ) ( t N t S El Az F t X + = where : X(t) denotes the observation vector obtained on the collocated antennas, S(t) is the signal vector corresponding to the different modes, F(Az,El) represents the steering vector matrix, it is the complex responses of each antenna for the several paths of the signal. This function includes the spatial diversity. N(t) is an additive noise vector. The components of this vector are assumed to be temporally and spatially white. MUSIC ALGORITHM OPERATING ON A HETEROGENEOUS ARRAY. The MUSIC parametric method [2] aims to estimate parameters which are bound by incident sources such as angles of arrival, signal strength and by computation of complex responses F(Az,El). An original method operating on an array, which is made up different sensors, has been proposed [3]. The different antenna patterns need computation of each antenna complex response in respect to the array configuration. Therefore, antenna characteristic functions are assumed to be known. Antenna complex responses can be obtained from computational method of moment (NEC 2 D). The responses take into account the following parameters: • Angle of arrival(azimuth and elevation) • Location of the received station, • Frequency of the carrier • Type of antenna • Height of the antenna, • Characteristics of the ground, • Electrical characteristics of amplifiers and cables. The two first sets of parameters include the polarization characteristics (function of the geomagnetic field) of the incoming waves. In the HF band and for ionospheric propagation, the polarization for the two possible incoming modes can be determined by the limit conditions of Budden [4]. The type and the height of antenna are two interesting parameters because the system designer can change them depending on ambiguities of collocated array. When the complex antenna responses are obtained, the pseudo spectrum can be computed. The method takes into account the two possible polarization modes, so we have to compute two pseudo spectrum (PSSPx et PSSPo).