Positioning Reference Signal (PRS) is taken into consideration in one of the LTE release 9 features to determine the location of User Equipment (UE) based on radio access network information. Now you might be thinking that what is the necessity of PRS, if we have a GPS technology already built in smartphones and in other cellular equipment. Follow the below link for more details .
http://lte-question.blogspot.in/2014/09/PRSLTE.html