1. A device for determining a coarse component of a latitude or longitude position datum of a moving craft, said position datum being reconstructed cyclically in accordance with cycles of reconstruction of the position datum, by combining said coarse component acquired cyclically in accordance with cycles of acquisition of the coarse component, with a corresponding fine component acquired cyclically in accordance with cycles of acquisition of the fine component, comprising: a first determination module configured to determine a sign of a difference between the fine component for a given cycle of acquisition of the fine component and the fine component for the preceding cycle of acquisition of the fine component; a second determination module configured to determine a sign and a value of the speed of the craft; a computer configured to compute the coarse component corresponding to said given cycle of acquisition of the fine component, as a function of said sign of the difference between the fine component for said given cycle of the fine component and the fine component for said preceding cycle of the fine component, and as a function of said sign and said value of the speed of the craft.