Abstract: The invention provides a method and a corresponding instrument for measuring the direction of a remote source of radiation, the instrument comprising a support, a detector head rotatably mounted thereon for rotation about a fixed axis, and a position-indicator means which provides an output signal indicating the rotational position of the detector head, the detector head being provided with a detector sensitive to radiation emitted by the remote source and with optics for forming an image of the source which transits across the detector as the detector head rotates, the image-forming optics being such that the image comprises two line-image components which extend transversely to the transit direction and which are oppositely inclined to that direction, and the detector being arranged, in response to the transit across it of each line-image component, to cause the instantaneous angular position of the detector head as indicated by the position indicator to be recorded, there being further provided computer wh

Abstract: Position sensing apparatus comprises generating means (12) for generating a beam of radiation; rotation means (24) for rotating the beam about a vertical axis to define a horizontal reference plane, the beam having a sectional shape (for example "V" shape or "X" shape) such that a leading edge and a trailing edge are straight and diverge away from the reference plane; two radiation sensors (38, 40) spaced a known vertical distance apart; and means (30, 32, 44, 46) for determining the distances between the leading and trailing edges at the positions along those edges which pass across each sensor. The vertical distance of each sensor from the horizontal reference place and the distance of the sensors from the axis of rotation can then be determined.