Small-aperture array processing for passive multi-target angle of arrival estimation

Small aperture arrays (size less than a wavelength) can be used for passive angle of arrival (AOA) estimation of both broadband and narrowband signals in the frequency domain. Phase differences across the array are measured in the frequency domain and can be spectrally averaged if desired for stationary source AOA and frequencies. The array is capable of measuring multiple target AOAs so long as each target produces unique frequencies or radiates in unique frequency bandwidths. Broadband and narrowband signals from targets can be collected from the fast Fourier transform (FFT) bins and summed by arrival angle in elevation and azimuth using a bearing histogram where the amplitude is displayed in dB. This provides a very useful and intuitive display of received signal levels. The Cramer-Rao lower bound (CRLB) for bearing accuracy is presented as a function of frequency, aperture, and signal-to-noise ratio (SNR). The small aperture background noise includes any other coherent or incoherent signal other than the source of interest. Analysis of phase calibration requirements, nonstationary frequencies, and channel cross-talk are also presented, as well as a technique for generating broadband source signals with the fractional delays needed to simulate the correct AOA.