A portable GPS personal navigation device (51) is characterised in that the device operating system (OS) has no Graphical User Interface (GUI); a navigation application instead providing its own User Interface (UI). The operating system may be installed into RAM (54) upon insertion of an SD card (52), which also contains the navigation application. Once installation is complete, control is passed to the application which can access map data from the SD card (52). RAM (54) contents is preserved on switch-off so that the boot up procedure and operating system installation occur once only.
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