Abstract: A vertical radio-interferometer sector-definition system, using an array of aerial elements which are arranged in a vertical straight line and are symmetrically disposed about the center of the array, which is unaffected by ground-reflected indirect-path signals.If .theta. is the angle of arrival, by a direct path, of a signal and p is the ground reflection coefficient, then from a symmetrical pair of aerial elements is derived a product of two functions f.sub.1 (.theta.).f(p) and from another symmetrical pair is derived another product f.sub.2 (.theta.).f(p): division of one product by the other "cancels out" f(p) to provide a quotient signal which is a function of .theta. but not of p. Either or both product functions may be derived with or without the aid of a central element.