GPS guidance can not work in the metro because satellite signals do not go underground.
The only way to guide yourself, without making any physical installations in the environment, is to use inertial navigation.
For this we use the sensors of the mobile phone: accelerometers, gyrometers, magnetometers to deduce the displacement of the user. The latter must declare his starting point, for example headend station Denfert-Rochereau and it is then guided by voice. When the device finds that the requested action is correctly executed, it validates it by saying "OK" and proceeds to the next instruction.
There are two demo videos on two line changes by a visually impaired user and another blind.
These demonstrations are experimental, to extend them to the whole of the subway RER, it is necessary to have the complete cartography of the lines and to redo the routes once to check the veracity of the plans and the relevance of the edited messages.
We hope to find partners or means to carry out this work, the magnitude of which exceeds our current resources.