GPS units have come down in price considerably in recent years and as such many mobile phones now come with thee technology built in. Mobile phones also come with video and audio recording ability as well as just about every other function you can never find a use for.
I suggest the construction of a small bluetooth device which can act as a rape alarm. The device could be a bracelet, ring, key fob or any other small item which may be kept at hand. In the unfortunate event that the device would need to be used it would send a bluetooth signal to the mobile phone will activate the GPS unit and send the co-ordinates to the local emergency services. The Microphone and camera will activate to provide evidence which may be used later to secure a conviction. If the phones speaker is rated as loud enough then that will sound a terrible shrieking noise to alert passers by.