Abstract: A frequency-and-direction-finding system (10) employing a circular array (12) performs a two-dimensional Fourier transformation in space and time on the signals from the individual elements. The results of the transformation are fed to multipliers (22) that multiply them by correction factors that are derived from antenna patterns of the array but do not include the bearing-dependent factors in those systems. The result is an ensemble of values whose relative phases represent a spatial frequency proportional to the bearing angle of the source. Accordingly, a second fast-Fourier-transform circuit (26) operating on this ensemble of values produces an output only on an output port corresponding to the bearing angle of the source.

Abstract: An interferometer type DF system uses an array of five antennas (A,B,C,D,E) arranged at the apices of a regular pentagon to define five wide apertures along the sides of the pentagon and a further five apertures along the diagonals. The phases of the signals received by each antenna, are measured modulo 2.pi. and processed to give a unique bearing of the radio source to the accuracy of the widest aperture defined by the array. One method of processing the phases is to calculate from them the Fourier coefficients of the Fourier series representing the spatial phase distribution. By comparing the difference between each calculated coefficient and a corresponding order coefficient of a set of imaginary antenna phases expressed as integral multiples of 2.pi., the complete 2.pi. phase differences between the measured phases modulo 2.pi. can be found.

Abstract: A system for indicating the direction of a source of radio waves received by a circular antenna array (12) includes a Butler matrix (18) that receives the array outputs and feeds the resultant matrix outputs to correction circuits (22) whose transfer functions are the inverses of the direction-independent factors of antenna patterns generated by antenna elements driven at relative phases that advance around the array at rates that complete an integral number of cycles in one circuit of the array. The resultant corrected signals are fed to a compressive receiver (26), which accordingly generates an output on an output port whose position indicates the direction of the source of the signal.

Abstract: A DF system using a circular array of three to eight antennas, analyses the received signals digitally by first calculating at (8) the Fourier transform of the received signals to obtain frequency information and deriving at (10) from the Fourier transform the relative phases of the received signals at each of a number of spaced sample frequencies. This phase information is then fed to a stage (12) which takes the spatial Fourier series of the phases from which the required bearing information is derived as .pi./2 minus the arctan of the ratio of the real and imaginary parts of the Fourier series taken to suitable moduli. For a four antenna array the diameter of the circular array is constrained to be less than half of the wavelength at the highest frequency of interest. For a three antenna array, the diameter is constrained to be less than one third the wavelength at the highest frequency of interest in order that the analysis should yield accurate and unambiguous bearing outputs.