DIRECTION FINDER

A method of determining a reference direction for an angular measurement device, comprising: providing a rigid structure having an antenna for a global navigation satellite system (GNSS) fixed at a first point thereof; fixing the angular measurement device to a second point on the rigid structure, separated from the first point by at least 0.5 metres; while rotating the rigid structure so as to cause rotational movement of the antenna around the sensitive axis, acquiring velocity measurement data from the GNSS and angular velocity measurement data from the angular measurement device; and using the velocity measurement data and the angular velocity measurement data to determine a reference direction for the angular measurement device. By rotating the antenna around the angular measurement device, the antenna moves relative to the frame of reference of the GNSS and thus has a velocity in that frame of reference. This velocity can be associated with a current angular position of the angular measurement device, making it possible to provide a reference direction to which all further angular measurements of the angular measurement device can be related (i.e. an absolute orientation). The reference direction is obtained by making use of the lever arm effect created by the separation of the antenna from the angular measurement device and the rotation of the antenna around the angular measurement device. This process makes use of the fact that the motion of the antenna is different from that of the angular measurement device. The difference between these motions is then used to calculate a reference direction.