Abstract: A method an arrangement to reduce amount data to be sent in a tracking system of a mobile station (MTT) having a positioning device to obtain positioning data, and the mobile station (MTT) and the server (S) perform the following steps: the mobile station (MTT) sends its dynamic state parameters including at least the position and velocity, which are derived from the positioning measurements, to server (S), the mobile station (MTT) computes an error criterion based on said sent dynamic state and current dynamic state, which is derived from new positioning measurements, such that the error criterion is calculated based on at least sent and current velocities, the mobile station (MTT) sends a set of new dynamic state parameters, when the said error criterion is over a predefined limit.