The disclosure relates to a method, performed by at least one apparatus. The method may include obtaining new opportunity signal data pertaining to an opportunity signal source and indicating opportunity signal values for particular locations at a site; obtaining previous opportunity signal data pertaining to said opportunity signal source and indicating opportunity signal values for particular locations at said site; comparing at least a part of said opportunity signal values of said new opportunity signal data pertaining to said opportunity signal source of said site and said opportunity signal values of said previous opportunity signal data pertaining to said opportunity signal source of said site for corresponding locations at said site; and classifying said opportunity signal source based at least in part on said comparing.