After the radar altimeter came into use at the end of the second world war, the combination with a pressure altimeter made it possible to measure horizontal pressure gradients, which enabled the navigator to estimate the geostrophic wind. The pressure gradient is directly related to the component of the wind at right angles to the track by means of the geostrophic relation and the navigator is thus provided with a method of evaluating one component of the drift. A position line of pressure combined with another position line (Loran, astro, &c.) enables the position of the aircraft to be determined.