The invention relates to a network element comprising a controlling element for forming assistance data relating to two or more signals transmitted by a reference station of at least one navigation system; and a transmitter for transmitting the assistance data via a communications network to a device. The device comprises a positioning receiver for performing positioning on the basis of two or more signals transmitted by a reference station of the at least one satellite navigation system; a receiver for receiving the assistance data from the network element; and an examining element configured to examine the received assistance data to find out information relating to the status of the two or more signals. The information comprises indication on the reference station the signal relates to, and the status indicates the usability of the signal.