how to distance measure (RTL system )from moving object using RFID

First of all, we want to excuse us for my bad English..
I want to make project on "prevention anti collision system using RFID ". I'm facing many problem in select RF reader and tag . because ,I think according for this project required UHF generator ,tags ,reader all things are expansive. If any idea about "distance measure in RTL(real time location system ) from moving object using RFID" this can help better in my project ..... thanks...
please reply OR you can send any information and suggestion related this project in my id ramdinesh1989@gmail.com