According to an example aspect of the present invention, there is provided an apparatus, such as a user equipment, configured to receive from a wireless network node, or determine autonomously, a sidelink positioning reference signal configuration comprising a constraint set, the constraint set comprising at least one constraint, each of the at least one constraint being based on at least one of: a location of the apparatus, a time instant, a time duration, a radio measurement, a physical characteristic of surroundings of the apparatus, a state of motion of the apparatus and a connection status of the apparatus to a network, determine whether at least one constraint in the constraint set is satisfied, and responsive to the at least one constraint in the constraint set being satisfied, transmit to a mobile device a first sidelink positioning reference signal and/or receive a second sidelink positioning reference signal from the mobile device.