  This paper presents LIPS, a Light Intensity based Positioning System for
indoor environments. The system uses off-the-shelf LED lamps as signal sources,
and uses light sensors as signal receivers. The design is inspired by the
observation that a light sensor has deterministic sensitivity to both distance
and incident angle of light signal, an under-utilized feature of photodiodes
now widely found on mobile devices. We develop a stable and accurate light
intensity model to capture the phenomenon, based on which a new positioning
principle, Multi-Face Light Positioning (MFLP), is established that uses three
collocated sensors to uniquely determine the receiver's position, assuming
merely a single source of light. We have implemented a prototype on both
dedicated embedded systems and smartphones. Experimental results show average
positioning accuracy within 0.4 meters across different environments, with high
stability against interferences from obstacles, ambient lights, temperature
variation, etc.
