I need a mobile app that runs off of the phones gps

i need an mobile application. I will try to explain how it works. Lets say you have im assuming some sort of transmitter in a vehicle.(maybe the new PING technology) once the vehicle comes withing 5 blocks of second person, it will show in second persons phone. (maybe the second person would see a small cartoony map of there currennt area and see a tiny car moving closer and closer to them. How it would work: When said vehicle enters the 10 block radius, the second person holding the phone would get a notification. That is really the whole thing in one sentence. Notification via a beep and then if they open it they see small map of the area and the car/vehicle seen as a red dot comming closer and closer. I would like the app to say in words somthing like, vehicle is 2 blocks away.