Some good folks in Zurich are working on using solar cells to measure distance indoors. It's called Luxtrace: a wearable location tracking by solar cells. It's described in "Towards LuxTrace: Using Solar Cells to Measure Distance Indoors" by Julian Randall, Oliver Amft, and Gerhard Troster (Wearable Computing Laboratory, IfE, ETH Zurich, Switzerland)

Navigation for and tracking of humans within a building usually implies significant infrastructure investment and devices are usually too high in weight and volume to be integrated into garments. We propose a system that relies on existing infrastructure (so requires little infrastructure investment) and is based on a sensor that is low cost, low weight, low volume and can be manufactured to have similar characteristics to everyday clothing (flexible, range of colours). This proposed solution is based on solar modules. This paper investigates their theoretical and practical characteristics in a simplified scenario. Two models based on theory and on experimental results (empirical model) are developed and validated. First distance estimations indicate that an empirical model for a particular scenario achieves an accuracy of 18cm with a confidence of 83%.

The flexible solar module system on the shoulder (1) transmits one or more RF pulses only when there is sufficient energy to do so i.e. beneath a light source. This data is collected and processed by the belt worn computer (3):

Why do I blog this? it's a novel way of doing location-tracking I was not aware of, interesting, after using gps, wifi, ultrasound, tv-signals, there is this.