Random antenna array interferometer for radio location

A system for locating one or more radio transmitters within a search area, the transmitters transmitting signals at a carrier frequency, the system comprising:

• (a) a grid of possible grid point locations covering the search area, adjacent grid point locations being a distance equal to a predetermined fraction of the wavelength of the carrier frequency apart, said predetermined fraction being less than or equal to ⅛;
• (b) an array of antennas positioned at infrequent intervals around the search area at known locations relative to the grid and operating on a common clock;
• (c) a library of manifold vectors, one for each grid point location, describing the phase interrelationships between a hypothetical signal at the carrier frequency emanating from said grid point location and said signal as received by all antennas;
• (d) means for detecting and processing the transmitter signals to extract a snapshot vector containing phase angle of arrival information of the transmitter signals at each antenna;
• (e) means for computing a covariance matrix for the phase angle of arrival of the transmitter signals and for combining the signal covariance matrix with each manifold vector in the library to produce a virtual interference pattern;
• (f) means for selecting one or more points in the virtual interference pattern that exceed a threshold as the locations for the signals of interest.

A method for locating one or more transmitters within a search area using an array of antennas, the one or more transmitters transmitting signals having the same carrier frequency, and the array of antennas operating on a common clock signal, the method comprising the following steps:

• (a) specification of a grid of possible grid point locations covering the search area, adjacent grid point locations being a distance equal to a predetermined fraction of the wavelength of the carrier frequency apart, said predetermined fraction being less than or equal to ⅛;
• (b) establishment of a library of manifold vectors, one for each grid point location, describing the phase interrelationships between a hypothetical signal at the carrier frequency emanating from said grid point location and the signal as received by all antennas;
• (c) selection of a signature of known grid point locations positioned relative to a grid point in question;
• (d) establishment of a library of hypothesized signature vectors, one for each grid point location, the elements of the signature vector corresponding to the signature of known grid point locations, and the value of each element of the signature vector being a combination of the manifold vector at the grid point location in question to the manifold vector at the grid point location corresponding to said element;
• (e) reception and digitization of one or more signals by the array of antennas to determine snapshot vectors containing phase angles of arrival information for the one or more signals at each antenna;
• (f) computation of a covariance matrix for the snapshot vectors over all antennas;
• (g) combination of the covariance matrix or a submatrix thereof with each manifold vector in the library of manifold vectors to create a virtual interference pattern for the one or more signals;
• (h) comparison of the library of signature vectors with corresponding values of the virtual interference pattern and selecting as the possible locations for the transmitters those matches that exhibit a maximum value, and are greater than a threshold representing a null hypothesis;
• whereby the possible locations for the transmitters have a precision of ¼ wavelength.

An antenna array for locating one or more transmitters transmitting at a carrier frequency within a search area at a resolution of ¼ of the wavelength of the carrier frequency, the search area identified by a grid of possible grid point locations wherein adjacent grid point locations are a distance less than or equal to ⅛ of the wavelength of the carrier frequency apart, the antenna array comprising:

• (a) a number of antennas positioned about the search area, at known locations relative to the grid, and operating on a common clock;
• (b) means for computing a library of manifold vectors, one for each grid point location, describing the phase interrelationships between a hypothetical transmitter signal emanating from each grid point location and said signal as it would be hypothetically received by all antennas;
• (c) means for computing one or more calibration covariance matrices, each calibration covariance matrix representing a modeled response of the antenna array to a modeled transmitter signal emanating from a calibration transmitter location within the search area;
• (d) the number of antennas sufficiently large that for all calibration transmitter locations in a set of calibration transmitter locations, a combination of the calibration covariance matrix corresponding to a particular calibration transmitter location in the set with the library of manifold vectors creates a virtual interference pattern for the antenna array having a unique maximum at the grid point location nearest to the particular calibration transmitter location.