An apparatus or an add-on module for a, in particular mobile, device, is disclosed. In an embodiment, the device to be localized or the add-on module uses a measuring unit to measure, via suitable sensors, a local electromagnetic field distribution generated by a given infrastructure. An instantaneous position of the add-on module or of the device equipped therewith is then determined by comparing the measured field distribution with a specified map. In order to facilitate tracking of the add-on module or of the device, the measured field distribution and/or the determined position can be sent via a wireless data connection to a server unit.