Method and apparatus for determining the focal axis (26) of an asymmetrical antenna (14) such as an offset paraboloid reflector whose physical rim (15) is not coincident with the boundary of the electrical aperture but whose focal point is known. A transmitting feed horn array (34) consisting of at least two feed horn elements (36, 38) is positioned asymmetrically on either side of an estimated focal axis (40) which is generally inclined with respect to the boresight axis (12) of the antenna. The feed horn array (34) is aligned with the estimated focal axis (40) so that the phase centers (CP.sub.1, CP.sub.2) of the two feed horn elements (36, 38) are located on a common line (42) running through the focal point (F) orthogonally with respect to the estimated focal axis. RF circuit means (44) are coupled to the feed horn elements for generating RF electric field components which are directed to the offset antenna (14) whereupon sum and difference radiation far field patterns (.SIGMA., .DELTA.) are generated and transmitted in a manner analogous to systems of the monopulse angle tracking type. The far field radiation patterns (.SIGMA., .DELTA.) are sensed by a far field detector (43) and the transmitting array (34) is rotated in discrete angular (.beta.) incremental steps in at least one plane about an axis through the focal point whereupon the amplitude of the minimum value of the difference radiation pattern (.DELTA.) as a function of angular rotation (.beta.) is detected and measured with a determination being made of the angular orientation of the array where extreme magnitude of the minimum difference pattern signal strength occurs, which location is indicative of the focal axis (26) being determined. Alternatively, the relative phases (.psi..sub.93, .psi..sub.66) of the sum and difference radiation patterns (.SIGMA., .DELTA.) are detected and measured as a function of angular rotation (.beta.) and their difference value (.vertline..psi..sub.93 -.psi..sub.66 .vertline.) determined. The maximum difference value provides an indication of the location of the focal axis (26) being determined.