The invention relates to navigation systems and elements. A network element (M) includes a controlling element (M.1) for forming assistance data relating to one or more reference stations (S1, S2) of at least one navigation system; and a transmitter (M.3.1) for transmitting the assistance data via a communications network (P) to a device (R). The device (R) includes a positioning receiver (R.3) for performing positioning on the basis of one or more signals transmitted by reference stations (S1, S2) of the at least one satellite navigation system; a receiver (R.2.2) for receiving the assistance data relating to at least one navigation system from the network element (M); and an examining element (R.1.1) adapted to examine the received assistance data to find out information relating to the status of the one or more signals of the reference stations (S1, S2) including indication on the reference station (S1, S2) the signal relates to, and the status indicating the usability of the signal. Therefore, the device (R) is adapted not to use in the positioning such a signal which is indicated not to be usable.