a method and apparatus for determining a position of an external transceiver relative to an implanted transceiver comprising means for measuring the strength of a magnetic field proximal to thc external transceiver and means for determining a position of the external transceiver relative to the implanted transceiver from said measured magnetic field strength . furthermore there is disclosed is a method and apparatus for determining a skin flap thickness of a recipient of a prosthesis including a transcutaneous link between the external transceiver and the implanted transceiver . a skin - flap thickness meter is also provided .